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Farm-level assessment of the nutrient balance in northern Nigeria
Agriculture, Ecosystems and Environment 71 (1998) 201±214
Farm-level assessment of the nutrient balance in northern Nigeria F.M.A. Harris* Centre for Overseas Research and Development, University of Durham, South Road, Durham, DH1 3LE, UK
Abstract The farming system of the Kano close-settled zone, northern Nigeria, is known for its longevity and continued productivity in spite of population increases in the area. Mounting circumstantial evidence has led to its reputation of being `sustainable'. Rural population density in this region now exceeds 300 kmÿ2, and almost all land (86.4%) is under annual cultivation. It is generally believed that such an intensive farming system is only possible under high input levels. This study set out to determine the nutrient balance to see if the system really was sustainable. Agronomic and soil fertility management practices were monitored on three farms over a two year period. Inputs and outputs measured at the ®eld level included fertilizer use (manure and inorganic), dry deposition, biological N ®xation, and the harvest of crops and hedgerow products. The results indicate great variability among ®elds, farmers, and years, and showed that the N balance was strongly negative, whereas P, K, and Mg balances were close to zero. The Ca balance was quite positive as a result of dry deposition. The study also quanti®ed nutrient dynamics within the farming system, highlighting, (1) the role of leguminous crops in bringing in N through ®xation, and as a source of fodder for small ruminants, (2) the role of small ruminants in converting cop residues into manure, and (3) the input of nutrients through dry deposition. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Kano close-settled zone; Nutrient cycling; Semi-arid tropics; Farming systems; Nigeria
1. Introduction Semi-arid west Africa has been the focus of a debate concerning the ability of the environment to support increasing populations primarily dependent on farming and livestock for their livelihoods. This debate has centered on the sustainability of farming practices in the light of competition for use of resources between farmers and herders, and the need to increase agricultural output to provide for the increasing populations. It is feared that as present population densities increase, land degradation will result. Soil fertility declines as a result of cultivation, and land cannot *Corresponding author. Tel./Fax: +44 191 374 2495; e-mail: [email protected]
support extended periods of agriculture unless it is left fallow for long periods of time, or large inputs of nutrients are added to the soil to compensate for the nutrients removed through cultivation. Traditionally, small-holder farmers in west Africa have improved soil fertility through fallowing of land, or the use of manure and small amounts of fertilizer. However it is feared that these methods may not be suf®cient to maintain soil fertility in the light of recent population increases. Studies by van der Pol (1992) and Stoorvogel and Smaling (1990) have drawn attention to the problem. van der Pol (1992) acquired data to calculate the nutrient balance of small-holder farming in southern Mali from literature and 1988±1989 production statistics for the region. From his calculations he con-
0167-8809/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0167-8809(98)00141-8
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cluded that the nutrient balances per hectare per year in southern Mali would be ÿ25 kg N haÿ1, 0 kg P haÿ1, ÿ20 kg K haÿ1, 3 kg Ca haÿ1 and ÿ5 kg Mg haÿ1. He then converted these amounts of nutrients into cost values, based on the price of inorganic fertilizers, and considered the cost of the nutrient de®cit in relation to the value of the harvested crop. The result was that the value of the de®ciency of nutrients was equal to 40% of the value of harvested products. van der Pol concluded ``permanent traditional cropping systems typically offer low returns, and will probably continue to be entirely dependent on soil mining, so that long term sustainability seems quite unlikely.'' Stoorvogel and Smaling (1990) calculated nutrient balances by countries, using national statistics, FAO yearbooks etc. For heterogeneous countries such as Nigeria, the overall ®gure is hard to relate to a speci®c farming system, and it was admitted that ``the supranational scale inevitably encompassed numerous assumptions, estimates, simpli®cations and aggregations'' (Smaling, 1993). However, the results of their study also showed that nutrient mining was occurring. Pieri's Pieri (1989) review of 30 years of research in francophone west Africa concluded that traditional farming systems, using little or no fertilizer but involving fallow, seem to have kept soils at low or stable fertility for the last 30±50 years, at low yields (500± 600 kg haÿ1 of millet and groundnut). As populations increase, it is necessary to determine how farmers can increase yields while maintaining soil fertility. It has been noted that some areas of west Africa support farming in areas of higher population density. In semi-arid west Africa, only a few areas supported rural population densities greater than 60 kmÿ2 in 1960 (Snrech, 1994). These included the Mossi Plateau (Burkina Faso) and the Kano close-settled zone (Nigeria). Since then rural populations have increased all over west Africa, and concern has mounted over the sustainability of these developing farming systems. The areas of high population density which have supported farming since the 1960's or earlier merit further investigation, to determine if they represent sustainable farming practices, and what can be learned from them. The Kano close-settled zone is an area around Kano city which supports a population density of more than 300 people per square kilometer (Snrech, 1994) in a
semi-arid environment. The city of Kano has been a major trading centre for centuries. Around the city is an area of intensive rural farming, which has expanded and increased in population density over the years. This area was ®rst noted by Clapperton in 1829, and later by 1851 by Barth, who noted `farms without interruption' from Bichi to Kano, and that agriculture was practised from Kano to Gezawa with little uncultivated bush (Amerena, 1982). Later visitors also commented on the annual cultivation system, and Morel commented ``Instead of wasting money with the deluded notion of `teaching modern methods' to the northern Nigerian farmers we should be better employed in endeavoring to ®nd an answer to the puzzling question of how it is that land which for centuries has been yielding enormous crops of grain, which in the spring of one carpet of green, and in November one huge corn®eld `white unto harvest', can continue to do so'' (Morel, 1911 in Hill, 1982). More recent work using air photographs con®rms that cultivation around Tumbau, a village 35 km from Kano, close to Gezawa, was 77.6% of land in 1950, and had reached 86.4% of the land by 1971, with all the remaining land occupied by fadama, roads or cattle tracks (Turner, 1994). There is no grazing land. The Kano close-settled zone was the focus of detailed studies by Hendy (1977); Hill (1977) and Mortimore (Mortimore and Wilson, 1965; Mortimore, 1970; Mortimore, 1975; Mortimore, 1993; Mortimore et al., 1990), during the 1960±1980's, but the issue of soil fertility was not addressed directly until 1990. In the 1970's, the Land Resources Division had carried out a resource inventory of north central Nigeria, and as part of that study they had taken soil samples along catenary sequences at locations representative of major land systems (Wall, 1979). In 1990 Mortimore et al. took soil samples at the same sites and analyzed them, to see if there had been any noticeable change in soil fertility. Their results showed that no signi®cant change had occurred in soil physical properties, whereas organic carbon levels had decreased, and total N levels had, surprisingly, increased. They concluded that the soils had a low fertility but had not been seriously degraded during these 20-odd years. They went on to say that the `maintenance of soil fertility was a prime objective of small-holder management' and that the farming system of the Kano close-settled zone, from the evidence of their study,
F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
was sustainable in the short and medium term (30 years). Unfortunately these statements were not supported by crop production trends, which would have strengthened the case considerably. In the light of these anecdotal and pedological ®ndings that point into the direction of sustainable systems in the Kano close-settled zone, it was decided to study the area's biophysical sustainability, through the quanti®cation of farm and ®eld nutrient ¯ows and balances, and the farm management that determines their levels. 2. Description of the study area As already mentioned, the cultivation intensity in the study village Tumbau had reached 86.4% by 1971, and did not increase further in the period to 1981 (Turner, 1994). It appears to have reached its upper limit. The Kano CSZ has brown to reddish-brown soils, formed by the deposition of dust carried by the Harmattan wind during the Quaternary (McTainsh, 1982). The result is a basement complex covered by a mantle of `drift', giving soils whose character is not related to that of the underlying rock. The eolian mantle is approximately 150 cm deep. There are laterite mesas present, and occasional outcrops of granite, gneiss and schist. The soils of the region are generally well drained, and rooting depth is adequate for most crops (Wall, 1979). Slopes are approximately 2%, with distance between high and low positions not exceeding 25 m. Erosion from rainfall is minimal. The sandy topsoils (0±15 cm) have low organic carbon (0.24%) and total N (0.06%). Available P is, by comparison, relatively high (27.8 mg kgÿ1 Bray 1), and pH is, on average, 6.1. The mean long-term annual rainfall (1906±1985) is 822 mm, which falls during a 5 month rainy season (May±September). Next to considerable year-to-year variability, there has been a downward trend since 1960. The farming of the Kano CSZ is based on crop, livestock and tree production. Dryland cultivation of millet (Pennisetum typhoides), sorghum (Sorghum bicolor), cowpea (Vigna unguiculata (L) Walp.) and groundnut (Arachis hypogaea), usually intercropped, provides cereals for consumption, cash crops (cowpeas and groundnuts are usually sold) and fodder for
203
livestock (groundnut and cowpea residues are more nutritious, sorghum leaves and stalks are also eaten). No space is wasted, and ®eld boundaries are marked with useful plants, such as andropogon (Andropogon gayanus) and vetiver (Vetiveria nigritans) grasses for thatching, and henna (Lawsonia inermis), whose leaves are used for dyeing hands and feet at ceremonial occasions. Other crops grown on upland soils include cassava (Manihot esculenta), maize (Zea mays), sesame (Sesamum indicum), peppers (Capsicum annum), okra (Hibiscus esculentus), roselle (Hibiscus sabdariffa), and sweet potatoes (Ipomoea batatas). The Kano CSZ supports a high density of livestock, most of which are small ruminants (Hendy, 1977; Bourn and Wint, 1994). Fulani herds of cattle only occasionally pass through, but do not contribute signi®cantly to the manure supply (Forde and Scott, 1946). Few farmers own cattle, with the exception of ox plough teams owned by some of the farmers in the villages. Because of the high farming intensity, there is no grazing land for livestock in the Kano CSZ. Fodder production from the farmland is a necessity. Residues not consumed as fodder are used in construction, and for fuelwood. During the growing season, animals are tethered in the compounds, and fed on residues, weeds, and grasses cut from ®eld boundaries, cattle tracks or the occasional uncultivated plot of land. Animals may roam ®elds during the dry season, but always return to the compound at night. Thus, most of the manure produced by animals is deposited in the compound, and farmers are able to transport this to the ®elds to fertilize next season's crops. Farmers allow trees to grow within their ®elds which provide fruit, fuelwood, and browse for animals. Aerial photographs indicate that the density of trees with a circumference greater than 1 m at breast height is about 12 haÿ1 (Mortimore et al., 1990; ClineCole et al., 1990). There was no signi®cant change in the number of trees of this size between 1956±66 and 1981 (Mortimore et al., 1990). 3. Materials and methods 3.1. Research protocol A farm-scale nutrient ¯ow measurement approach was chosen, which relates directly to management
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Fig. 1. Location and distribution of the fields of three farmers involved in this study.
decisions, allowing the constraints affecting those decisions (knowledge, labour and capital) to be investigated too. The farming systems in the area are quite heterogenous, with intercropping, interplanting (mixing trees and shrubs with crops), and variable land quality (sunny and shady patches, raised areas and moist depressions) all contributing. The hamlet of Gamji Tara, near Tumbau village, was chosen for in-depth studies (Fig. 1). This village is composed of a farming community in a rural setting, and is located about 10 km from any tarred road, approximately 30 km east of Kano. Gamji Tara contains approximately 20 households, whose main economic activity is farming. A small weekly market in Tumbau trades locally-made products and foodstuffs. Three households were selected for the present study, all having different, mostly non-contiguous ®elds for crop production (Fig. 1). Each farmer's land was surveyed to measure ®eld and landholding size, and soil samples were taken. Inputs and outputs measured were those that farmers can see and manipulate. Using the Stoorvogel and Smaling (1990) nutrient ¯ow terminology, these include inorganic fertilizers (IN1), manure input from
grazing animals (IN2) and manure recycled within the farm, Harmattan dust (IN3), symbiotic N ®xation (IN4), nutrients in harvested crops (OUT1) and crop residues and weeds (OUT2) and leaching (OUT3). The procedures for measuring these components are outlined below. Not measured were sedimentation (IN5), gaseous losses (OUT4), erosion (OUT5), and human waste (OUT6), as they were considered of minor importance or beyond the scope of the study. Observation and monitoring was done on a ®eld-by®eld basis. Inputs, outputs and yields were determined for the whole of each ®eld, and later the results of ®elds within the same landholding were combined. Boundary and hedgerow crops, being important components of the farming system, were included too. Samples of inputs and harvested crops were analysed according to the following procedures: total N: semimicro Kjeldahl (Hesse, 1971); total P: tri-acid mixture (nitric: perchloric: suphuric, 650:80:20) digest followed by determination of P concentration using the ammonium molybdate/ammonium vanadate method (Allen, 1974); soil available P: Bray l (Bray and Kurtz, 1945); organic carbon of soils and kraal manure: Walkley-Black method (Anderson and
F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
Ingram, 1989); soil K, Ca and Mg were extracted with ammonium acetate (Anderson and Ingram, 1989); cations in plant and manure samples were determined after wet-digestion (Allen, 1974); Mg and Ca were determined using atomic absorption spectrophotometry, and K concentration was determined by ¯ame photometry; soil nitrate: extraction in K chloride solution followed by reduction to nitrite with copperized cadmium (modi®ed Griess-Ilovsay) (Page et al., 1982). The data derived from monitoring and chemical analysis was combined to calculate a nutrient balance to indicate whether the system was losing or gaining nutrients, or remaining stable. The balance was calculated as kg of nutrients gained or lost both on a landholding basis, and on a standard area basis (ie. kg per hectare). The results were entered into a nutrient ¯ow diagram, which shows how farmers maintain soil fertility. 3.2. Measurement of nutrient inputs 3.2.1. Inorganic fertilizers Most farmers use inorganic fertilizers, although the amount used by each farmer is variable and depends on price and availability. During this study, the three farmers had access to three fertilizers. These were NPK 27:13:13, 20:10:10 and 20:10:5. Farmers distributed fertilizer on their ®elds by carrying mudu sized bowls of fertilizer to the ®elds and sprinkling the fertilizer by hand at the base of cereal plants. In some cases legumes were also fertilized. A mudu is a standard measure, equivalent to 4 kg of inorganic fertilizer. The farmers reported how many mudus of fertilizer were applied to each ®eld, and what type of fertilizer it was. The quantities of N, P and K contributed to the ®eld as inorganic fertilizer were calculated. In inter-cropped ®elds, farmers apply more inorganic fertilizer to cereals than legumes. 3.2.2. Manure Manure produced and recycled on the farm The farmers of the Kano CSZ apply taki, a term they translate as `local fertilizer', to their ®elds. The precise nature and composition of taki varies as it contains animal manure, refused fodder, ash, compound sweepings and kitchen waste. During the dry season livestock roam the farmland, and feed on whatever they
205
can ®nd. During the rainy season the lack of grazing land forces farmers to tether their animals in their compounds and feed them crop residues stored since the previous season, as well as weeds which are collected from the ®elds, and grass from roadsides. The livestock produce manure, which together with litter, is trampled underfoot and produces taki. During the dry season this is taken to the ®elds. Taki is usually transported to the ®elds in panniers on donkeys. The pannier-sized load is referred to as a mangala. Because of the variability in taki composition and mangala size it was necessary to quantify each to assess the contribution of taki to the nutrient budget of the farming system. At the beginning of the 1993 farming season the mangalas of taki on each ®eld were counted, weighed and described according to their composition. They were classi®ed into 8 groups according to the farmer's description of the contents. 37 Samples of different types of taki were collected for analysis for total N, total P and exchangeable cations. During the 1994 farming season mangalas were counted, and a subsample weighed, to con®rm the average weight of a mangala. In view of the detailed analytical work done the previous year, no further samples were analysed in 1994. Table 1 shows the average nutrient composition of the taki samples taken from farmers' ®elds. There was considerable variation in taki nutrient content from the variation in its production and composition. Samples had low N concentrations compared with other studies (Landon, 1991; Jones and Wild, 1975; Williams et al., 1995; Bationo and Mokwunye, 1991; Mortimore et al., 1990) which may be the result of the manure being mixed with grass and ash, or to losses by volatilization during storage within the compound or in the ®eld. In this study samples of taki were taken for analysis at the end of the dry season, just before incorporation into the soil. Application of taki per ®eld varied from 0±17.5 tons per hectare during the 1993 and 1994 growing seasons. The average application (total amount of manure applied per landholding) was 4.3 tons per hectare (Table 2). On the whole, more taki was applied to ®elds in 1994 than in 1993. Manure deposited by grazing animals Three ®elds were selected to study manure deposition in ®elds by grazing animals, according to their distance from Tumbau, their accessibility and their ownership. Once
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Table 1 Average nutrient content (% dry matter) of inputs and harvest Crop
Part
Taki Harmattan dusta Early millet
Grain Chaff Sticks Stalks Beans Pods Straw Grain Chaff Sticks Stalks Nuts Pods Haulms Grain Chaff Sticks Stalks
Cowpea Sorghum
Groundnut Late millet
Weeds a
n
N
P
n
K
Mg
Ca
37
0.3 0 1.22 0.88 0.43 0.50 3.13 1.34 1.29 1.19 0.59 0.42 0.32 3.48 0.88 1.49 1.54 0.86 0.51 0.35 1.32
0.19 0.09 0.15 0.27 0.05 0.12 0.43 0.15 0.23 0.35 0.40 0.24 0.21 0.68 0.12 0.15 0.27 0.37 0.23 0.33 0.18
37
0.8 2.03 1.67 1.76 1.22 1.51 1.29 1.17 1.38 1.00 0.57 0.86 0.79 1.06 0.88 1.76 0.84 0.53 1.72
0.24 0.39 0.19 0.14 0.19 0.24 0.21 0.21 0.33 0.23 0.15 0.20 0.15 0.54 0.07 0.32 0.17 0.15 0.13
0.84 1.13 0.20 0.38 0.53 0.42 0.26 0.18 0.89 0.21 0.12 0.21 0.29 0.48 0.11 0.84 0.54 0.14 0.22
1.65
0.30
0.58
15 17 17 16 25 25 31 34 35 32 18 23 30 23 9 10 10 7 8
3 5 5 3 4 4 3 4 4 4 3 1 3 3 3 3 3 0 2
From McTainsh, 1982
Table 2 Comparison of inputs and yields in 1993 and 1994 Inputs ÿ 1
Farm Farmer I 1 2 3 Farmer Y 1 2 3 4 Farmer S 1 2 3 4 5
Outputs ÿ1
Taki (ton ha
)
Inorganic Fertilizer (kg ha )
N Fixation (kg ha )
Biomass (ton haÿ1)
1993
1994
1993
1994
1993
1994
1993
1994
0 10.6 7.2
11.9 0 0
75 75 20
57 58 70
9 19 5
37 6 8
4.4 7.4 3.8
8.7 7.2 12.1
1.9 1 7.2 0
7.5 4.6 4.1 2
47 0 0 24
39 20 27 37
11 15 16 2
29 19 23 28
2.6 3.8 4.6 3.4
3.7 5.1 8 4.6
8.9 5.5 1.7 2.4 0
8.9 17.5 0 0 0
0 0 0 0 0
0 0 0 90 58
8 3 8 0 8
17 48 20 35 4
3.7 6 3.6 2.6 3.2
10.5 9.6 4.8 4.7 3.3
a month during the dry season a 1.0 m2 quadrat was thrown randomly on each ®eld ®ve times. The manure observed within the quadrat was collected and
ÿ1
weighed. The presence and average weight of manure found on each ®eld over the months of the dry season were recorded. Average deposition was only
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17 kg haÿ1 so obviously most manure was deposited within the compounds. Thus manure (with a mean composition of 0.3% N) deposited by grazing and browsing animals appears to contribute only 51 g N per hectare of land, a negligible contribution to soil fertility when compared with the inputs from taki brought to the ®elds from compounds, or N ®xation by legumes. 3.2.3. Harmattan dust A ground level wet dust trap was used to measure dust deposition on ®elds. The trap consisted of a straight-sided glass bowl containing distilled water, enclosed in a cage of mosquito screen to prevent birds, animals and insects from reaching the water, positioned in a farmer's ®eld. Dust deposition was monitored weekly through the dry season, starting from 19 October 1993 at Tumbau. Weekly deposition varied from 1±17 g mÿ2, with the higher values being noted at the end of the season. High winds before the onset of the rainy season cause so much local dust movement that the use of the dust trap to measure only Harmattan dust deposition becomes impossible at this time. The soil and dust movement in such winds is believed to cause only local soil movement, and so such wind erosion does not remove nutrients from the area (Mortimore, 1989). Total dust deposition measured in the ®eld at Tumbau was 158.13 g mÿ2 during the dry season (October± April). Using McTainsh's (1982) method to correct for secondary local re-deposition, 92.2 g mÿ2 (922 kg haÿ1) dust was deposited on the ®eld from far away. Nutrient content of dust has been measured by a number of researchers, i.e., Wilke et al. (1984); Beavington and Cawse (1979); McTainsh (1982). The composition data given by McTainsh (1982), who worked in northern Nigeria, were used to estimate its contribution to the nutrient balance. This study used the results from the ®eld measurements made during the 1993±1994 Harmattan season to calculate inputs to the system for the 1993 and 1994 growing seasons. Assuming that Harmattan dust is distributed evenly over all the ®elds, nutrient input to each ®eld is calculated from the overall deposition ®gure (922 kg haÿ1) and the area of each ®eld. Table 1 shows that the Harmattan wind contributes large amounts of K and Ca to the soils of the area.
207
3.2.4. N fixation Traditionally N ®xation trials are carried out on monocrops, to determine ®xed N per hectare of crop. However in the Kano CSZ farmers vary planting patterns within each ®eld, therefore the planting pattern and legume density in each ®eld were taken into account in order to calculate N ®xation inputs to each ®eld. The planting patterns within 10 metre squared quadrats on farmers ®elds were noted. Field experiments to quantify symbiotic N ®xation were carried out in 1994. N ®xation in the system was studied under local farming practices, using the N difference method. This consists of detailed monitoring of N uptake during the growth of a N-®xing crop in comparison with that of a similar non-®xing crop. The difference in N uptake is then assumed to come from ®xation (Giller and Wilson, 1991). Groundnuts and cowpeas were planted on a trial plot within a farmers' ®eld, in a pattern typical of local farmers' practice. Maize was planted as the reference crop. The number of seeds planted and weeding were carried out according to farmers' practices. From 60 days after planting, plant samples were harvested every 15 days. The initial samples included roots, which were examined for nodule number and colour. Later only shoot samples were collected. The entire shoot of 5±10 plants was removed, dried, weighed, and ground, and subsamples of ®ve plants were taken for analysis. Shoot weight multiplied by N content gives the N uptake as grammes of N per plant. The estimate of symbiotically ®xed N was the difference between the amount of N in the shoot of the legume at the date of maximum N accumulation and the amount of N in the shoot of the reference crop (maize) (Hauser, 1992) (Fig. 2). Groundnuts reached maximum N uptake at 105 days after planting, at which time they had accumulated 1.227 g N, and cowpeas reached maximum N uptake at 120 days after planting, at which time they had accumulated 2.32 g N. The maximum accumulation of N from the soil by maize was 0.616 g. Thus assuming 0.616 g N came from the soil, groundnuts ®xed 0.611 g N per plant, and cowpeas ®xed 1.706 g N per plant on the trial plot. From the N ®xation trial it was calculated that ®xed N was equivalent to 1.18% of harvested biomass of groundnut, and 1.48% of
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Fig. 2. N uptake (gN/plant) by maize, groundnut and cowpea in the N fixation trial.
harvested biomass of cowpea. As the experiment was conducted in 1994 only, this relationship was used to assess N ®xation during the 1993 season (Table 2). 3.3. Measurement of nutrient outputs 3.3.1. Harvested products and residues The harvest from each ®eld was recorded. Threshing trials determined what percentage of the harvest weight was grain, chaff, pods, or the remaining waste (sticks). Samples of the harvest were collected, threshed and taken for chemical analysis. Table 1 gives the average nutrient content of the main crops. The micro-management practised by farmers means that fertilization (with taki or inorganic fertilizer), cultivar and intercropping may differ from plant to plant within one ®eld, as well as across the landholding. Field by ®eld ®gures were used to calculate the nutrient balance (Harris, 1995). Combining harvest data with the results of chemical analysis of plant material, the nutrients removed in harvested crops, weeds and boundary plants from each ®eld in each year were calculated (Table 2).
3.3.2. Leaching The sandy soils are freely draining, and rainfall percolates through the soil unimpeded. Evaporation ®gures from the Bayero University meteorological station were combined with rainfall data to calculate the water balance for the 1993 and 1994 seasons. In 1993 precipitation was greater than evaporation for only two weeks, and in 1994 for four weeks, both events in the month of August, when the ®elds are full of rapidly growing crops approaching maturity. Conditions of excessive moisture were only experienced for a short part of the rainy season, when N uptake by the crop could be expected to be high. Measurements of soil nitrate-N were made during both seasons. At the end of July, Farmer I's ®eld 2 had approximately 1.0 mg nitrate-N/kg soil. Given a bulk density of the soil of 1.4 g cmÿ3 this corresponds to 2.8 kg of nitrate-N per hectare in the top 20 cm of soil. In years with a positive water balance, such as 1994, there may have been leaching of nitrate-N from the soil. It would seem that 2.8 kg haÿ1 is the maximum amount of N lost by leaching if all the nitrate was leached, and none was taken up by plants. In fact, it is probably less, as some would be taken up by the growing crop. Because of the variability from year
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to year, and the uncertainty concerning the amount of nitrate leached or taken up by the crop, the above ®gure of 2.8 was not included in the balance. 4. Calculating the nutrient balance 4.1. Farm balances Fig. 3 shows the nutrient balances of the three landholdings, and shows considerable variability among ®elds, among farms, and between years. The landholding balances of 1993 varied from ÿ11 to 1 kg N/ha, and 0±2 kg P/ha. In 1994 these ®gures were ÿ28 to 3 kg N haÿ1, and ÿ3 to 3 kg P haÿ1. The N balance was negative on all holdings except for Farmer S's holding in 1993 and Farmer Y's holding in 1994. The P balances were positive in 1993, but negative on Farmer I and Farmer S's holdings in 1994. The results suggest that cations are not in short supply, thus the farming system may be sustainable with respect to K, Mg and Ca. 4.2. Field to field variability Three quarters of the ®elds had negative N balances in 1993 and 1994. About half of the ®elds had negative
209
P or K balances in 1993. More ®elds had negative P balances in 1994. Four ®elds had negative Mg balances, and one had a negative Ca balance in 1993 (Table 3). The nutrient balances varied largely from ®eld to ®eld: in 1993 the N balances ranged from ÿ16 to 8 kg/ha, and for P from ÿ4 to 10 kg haÿ1 on the ®eld, yet in 1994 the balances of these nutrients ranged from ÿ56 to 24 kg haÿ1 and ÿ19 to 15 kg haÿ1 respectively. The cation balances also ranged a lot, with K and Ca balances reaching 45 and 69 kg haÿ1 respectively in 1993. The extreme variability from ®eld to ®eld is because of farmers' rotation of taki and inorganic fertilizer inputs, as well as the change in cropping patterns on each ®eld. Table 2 clearly shows these differences in ®eld management. Taki does not decompose completely within the ®rst season after its application, so that a large input in one year will supply nutrients over several years. However, division of the nutrient input in taki over several years could only be carried out if long-term monitoring of inputs was possible. Therefore, the whole nutrient input of taki is attributed to the year of application only, rather than dividing it over the subsequent years. Crop choice also affects the nutrient balance. Legumes contribute N through ®xation, and those ®elds with
Fig. 3. Nutrient balances for the three landholdings, expressed as kg haÿ1 of nutrient. N, P, K, Mg and Ca for 1993 and N and P for 1994.
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Table 3 Nutrient balance of fields (kg haÿ1) Year and nutrient Field
N
1993 P
K
I1 I2 I3 Y1 Y2 Y3 Y4 S1 S2 S3 S4 S5
ÿ8.25 ÿ1.01 ÿ3.73 ÿ12.14 ÿ15.52 ÿ2.80 ÿ11.06 8.29 ÿ11.29 ÿ10.16 5.61 0.34
ÿ2.56 7.36 9.58 1.64 ÿ3.45 8.41 ÿ4.39 6.31 ÿ0.29 ÿ3.68 0.30 ÿ2.68
ÿ43.90 45.38 24.38 5.34 ÿ16.81 32.62 ÿ14.25 39.70 ÿ17.7 ÿ5.19 7.73 ÿ10.08
high legume yields also had high N inputs from ®xation. 4.3. Year to year variability
Mg ÿ6.81 10.23 8.88 1.10 ÿ1.13 12.71 ÿ1.96 16.94 14.86 0.65 4.92 ÿ1.46
Ca
1994 N
P
ÿ7.59 66.48 49.25 14.56 9.90 62.51 1.54 69.08 33.86 8.76 21.44 0.49
15.64 ÿ33.11 ÿ71.37 24.95 ÿ10.93 ÿ27.00 13.26 ÿ38.34 ÿ48.71 ÿ56.11 ÿ0.98 ÿ18.06
9.90 ÿ9.08 ÿ19.23 9.65 2.15 ÿ2.28 ÿ1.42 ÿ6.99 14.71 ÿ10.81 ÿ3.64 ÿ1.80
structures become worn down and are replaced, the old material is usually burned. Stalks are also used as fuelwood, although it is hard to estimate to what extent this is practised. N is lost when stalks are burned. The
The N and P balances in 1994 were lower than those of 1993. This can be attributed to differences in inputs from N ®xation, manure and inorganic fertilizer: these were higher in 1994 than in 1993. As rainfall was also higher in 1994, the amount of biomass collected from ®elds in 1994 almost doubled that of 1993 (Table 2). 5. Nutrient management Farmers in the Kano CSZ recycle many nutrients within the farming system. Only a small amount of the material that is harvested from the ®elds leaves the area. Fig. 4 provides a pie chart of the types of biomass that are generated. Approximately 24% of dry matter harvested is food grains (5% is leguminous grains). Leguminous grains are usually sold. The cereal grain is stored in the compound to supply the household's food needs during the year. If supply exceeds the family's food needs, the excess may be sold to the market. The amount sold will vary, depending on the harvest and household size. 23% of biomass harvested is animal fodder. Unpalatable stalks and gamba grass are used to construct compound walls and huts, and also storage granaries. When these
Fig. 4. The uses made of harvested material (% of total harvested biomass).
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remaining nutrients in residues are returned to the ®elds when ash is combined with taki. None of the farmers in Tumbau sell haulms to Kano city residents who keep livestock in town, although some sell some sorghum stalks if they are in excess of their own needs. Farmers grow considerable quantities of groundnuts and cowpeas on their land, despite the cereals millet and sorghum being the staple components of their diet. N ®xation by these legumes contributed up to 48 kg N/ ha within individual ®elds, depending on the density of legumes in the ®eld. This is less than that often quoted in the literature (Giller and Wilson, 1991), because studies on N ®xation usually involve either inoculation of seed by rhizobia, high basal doses of P, or monocrops. In addition to contributing N, the residues of the legume crops are a nutritious and valued form of animal fodder for the livestock. In the Kano CSZ, where grazing land is unavailable and cut-and-carry feeding a necessity, crop residues are the main source of animal fodder. The majority of smallholder farmers in the Kano CSZ keep livestock for their value as producers of manure, for animal traction (donkeys and ox teams), for slaughter at times of religious and ceremonial occasions, and as ®nancial investment. Livestock are not kept with a view to maximizing pro®t from breeding and meat production. The fodder harvested is used to apply `maintenance' rations to the compound's livestock (Hendy, 1977). Farmers try to keep as many animals as they can feed, rather than reducing the number of animals so that a smaller number can be raised for `production', i.e. with an aim to maximizing live weight gain, reproduction and milk production. The farmers gained 167 and 549 kg haÿ1 of groundnut and 217 and 248 kg haÿ1 of cowpea haulms in 1993 and 1994 respectively (dry matter). Hendy (1977) calculated that groundnut and cowpea haulms are particularly important in the livestock diet, contributing 70% of the digestible crude protein and 20% of the total metabolizable energy (ME) harvested, whereas sorghum residues provided 30%, and grasses from waste and unused land provided 25% of all the ME. Their use as fodder permits the farmers of the Kano CSZ to keep the high population density of livestock noted by Hendy (1977); de Leeuw et al. (1995) and Bourn and Wint (1994).
211
Livestock are able to convert the crop residues into manure, thus rendering the nutrients more readily available to plants. Manure production by farmholding ranged from 2.3±15.3 tons over the two years of the study. The N ®xed in the leguminous crop residues passes through the animals before being applied to the ®elds as manure. Manure accounts for approximately one quarter of N inputs, and one ®fth of P inputs. It also contains other nutrients, supplies organic carbon, and along with the Harmattan dust supplies many micronutrients which would not be supplied in inorganic fertilizers or through N ®xation. The use of taki contributes approximately 4 ton haÿ1 organic matter to the ®elds, which breaks down slowly. This organic matter contributes to soil fertility in providing nutrients and by improving soil physical properties. The soils of the Kano CSZ are very sandy, with low clay contents. The addition of taki improves the water holding capacity and cation exchange capacity of the soil. Fig. 5 depicts the N nutrient ¯ows through the farming system. To conserve soil nutrients farmers must marshall the nutrients in such a way that maintains a ¯ow through the farming system, limiting loss. In this integrated farming system, ef®cient use is made of almost all the biomass produced on the farmers' ®elds. Integration is made possible by the incorporation of high-protein fodder crops in the rotation, and livestock husbandry. Recycling reduces nutrient losses. Some nutrients are taken in by growing livestock, or lost by volatilization during burning (especially N), but most are returned to the soil in a form much more appropriate for use a as fertilizer. 6. Conclusions The study presented in this paper identi®ed the Kano CSZ as an area thought to be sustainable, and then investigated how farmers had achieved this, through nutrient cycling and nutrient management. The results of two years of detailed monitoring indicate that the system is sustainable with respect to K, magenesium, Ca and P, but the results concerning N did not agree with the results of soil testing over a 13-year interval, or the evidence as seen on the ground of repeated harvests from annually cropped land.
Fig. 5. Amounts of N (kgN haÿ1) cycled within the integrated crop-livestock farming system, for 1993 and 1994.
212 F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
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The contradiction between the measured N balance and the historical evidence which suggests that the farming system must be sustainable can only be resolved by further detailed study over a longer period of time and a wider range of farmers. Although nutrient balances are known to vary, the system was clearly limited by N during the 2-years of this study. The model of nutrient dynamics that has been developed by this study has identi®ed nutrient ¯ows and allows the quanti®cation of some of the factors contributing to the success of this farming system, such as the crop-livestock integration, the inclusion of N ®xing leguminous species in the cropping system, high levels of manure use, and the Harmattan dust which imports cations. This has enabled a farming system to develop in the face of increasing population densities and continues to provide livelihoods for the community it supports. The high population density of people and livestock and the dispersed settlement pattern has created an environment of integrated crop and livestock production (Harris, 1996). The example of the Kano CSZ shows that reduction in grazing land may be compensated through the use of crop residues as animal fodder (especially residues of leguminous crops), and that the integration of crop and livestock production results in a more ef®cient recycling of nutrients within the farming system. 7. Unlinked reference Hermann et al., 1994 Acknowledgements The research was funded under the Semi-Arid Production Systems Programme of the Natural Resources Systems Programme of the UK's Department for International Development (EMCX 216). The work was carried out at the University of Cambridge, UK, and Bayero University, Kano, Nigeria. References Allen, S.E., 1974. Chemical Analysis of Ecological Materials. Blackwell Scientific Publications, Oxford, UK, pp. 88±89. Amerena, P.M.J., 1982. Farmers' participation in the cash economy: case studies of two settlements in the Kano close-
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settled zone of Nigeria. Ph.D. Thesis, University of London, London, UK. Anderson, J.M., Ingram, J.S.I., 1989. Tropical Soil Biology and Fertility: A Handbook of Methods. CAB. International, Wallingford, USA, pp. 35±37. Bationo, A., Mokwunye, A.U., 1991. Role of manure and crop residue in alleviating soil fertility constraints to crop production: with special reference to the Sahelian and Sudanian zones of west Africa. Fertilizer Research 29, 117±125. Beavington, F., Cawse, P.A., 1979. The deposition of trace elements and major nutrients in dust and rainwater in northern Nigeria. Sci. Total Environ. 13, 263±274. Bourn, D., Wint, W., 1994 Livestock, land use and agricultural intensification in sub-Saharan Africa. Pastoral development network discussion paper, ODI. Bray, R.H., Kurtz, L.T., .1945. Soil Sci. 59, 44. Cline-Cole, R.A., Falola, J.A., Main, H.A.C., Mortimore, M.J., Nichol, J.E., O'Reilly, F.D., 1990. Woodfuel in Kano. United Nations University Press, Tokyo. de Leeuw, P.N., Reynolds, L., Rey, B., 1995. Nutrient transfers from livestock in west-African production systems. In: Powell, J.M., FernaÂndez-Rivera, S., Williams, T.O., Renard, C. (Eds.), In: Proc. Int. Conf., Addis Ababa, Ethiopia, 22±26 November 1993. International Livestock Centre for Africa, Addis Ababa, Ethiopia. Livestock and sustainable nutrient cycling in mixed farming systems of sub-Saharan Africa, vol. 11, Technical papers. Forde, D., Scott, R., 1946. The Native Economies of Nigeria. Faber and Faber. Giller, K.E., Wilson, K.J., 1991. N Fixation in Tropical Cropping Systems. CAB International, Wallingford, USA. Harris, F., 1996. Intensification of agriculture in semi-arid areas: lessons from the Kano close-settled zone, Nigeria. Gatekeeper Series No. 59, Sustainable Agriculture Programme. International Institute for Environment and Development. Harris, F., 1995. Nutrient dynamics of the farming system of the Kano close-settled zone, Nigeria. Ph.D. Thesis, University of Cambridge, Cambridge, UK. Hauser, S., 1992. Estimation of symbiotically fixed N using extended N-difference method. In: Mulongoy, M., Gueye, K., Spencer, D.S.C. (Eds.), Biological N fixation and Sustainability of Tropical Agriculture. IITA / Wiley-Sayce, Chichester, UK. Hendy, C.R.C., 1977. Animal production in Kano state and the requirements for further study in the Kano close-settled zone. Land Resources Report 21, Land Resources Division, Ministry of Overseas Development. Hermann, L., Bationo, A., Stahr, K., 1994. Chemical composition of dusts from the Sahara to the Gulf of Guinea and the influence on rainwater chemistry. Atmospheric Environment, in connection with the CACGP, IGAC Symposium, 4±9 September, 1994, Japan. Hesse, P.R., 1971. A Textbook of Soil Chemical Analysis. William Clowes, London, UK, pp. 197±199. Hill, P., 1982. Dry Grain Farming Families. Cambridge University Press, Cambridge, UK. Hill, 1977. Population, Prosperity and Poverty. Rural Kano 1900 and 1970. Cambridge University Press, Cambridge, UK.
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Jones, M.J., Wild, A., 1975. Soils of the west African savanna. Technical communication No. 55, Commonwealth Bureau of Soils, Commonwealth Agricultural Bureau, Harpenden, UK. Landon, R.J. (Ed.), 1991. Booker Tropical Soil Manual. Longman, New York, USA. McTainsh, G., 1982. Harmattan dust, aeolian mantles and dune sands of central northern Nigeria. Ph.D. Thesis, Macquarrie University, Australia. Mortimore, M., 1993. The Intensification or Peri-Urban Agriculture: The Kano Close-Settled Zone, 1964±1986. In: Turner, B.L., Kates, R.W., Hyden, G. (Eds.), Population growth and Agricultural Change in Africa. University Press of Florida, FL, USA. Mortimore, M., 1989. The causes, nature and rate of soil degradation in the northernmost states of Nigeria and an assessment of the role of fertilizer in counteracting the problems of degradation. Environment Department Working Paper No. 17, The World Bank, Policy planning and research staff. Mortimore, M.J., 1975. Peri-urban pressures. In: Moss, P.R., Rathbone, R.J.A.R. (Eds.), The population factor in African studies. University of London Press, London, UK. Mortimore, M.J., 1970. Population densities and rural economies in the Kano close-settled zone, Nigeria. In: Zelinsky, W., Kosinski, L.A., Prothero, R.M. (Eds.), Symp. on Geography in a Crowding World : Population Pressures Upon Physical and Social Resources in the Developing Lands. Oxford University Press, New York, USA. Mortimore, M., Essiet, E.U., Patrick, S., 1990. The nature, rate and effective limits of intensification in the small holder farming system of the Kano close-settled zone. Federal agriculture coordinating unit. Nigeria. Mortimore, M., Wilson, J., 1965. Land and people in the Kano close-settled zone. Occasional Paper 1, Department of Geography, Ahmadu Bello University, Zaria. Morel, E.D., 1911. Nigeria: Its peoples and its problems. London, 1911, reprinted Cass, London 1968. Page, A.L., Miller, R.H., Keeney, D.R., 1982. Methods of soil
analysis. Part 2 - Chemical and microbiological properties, 2nd ed. Agronomy No. 9, Part 2. American Society of Agronomy, Soil Science Society of America, pp. 649±685. Pieri, C., 1989. Fertilite des terres et savanes. Bilan de trente ans de recherche et de deÂveloppement agricole au sud du Sahara. Ministere de la coopeÂration et du deÂveloppement, CIRAD. Smaling, E., 1993. An agro-ecological framework for integrated nutrient management with special reference to Kenya. Ph.D. Thesis, Agricultural University, Wageningen, Wagenigen, Netherlands. Snrech, S., 1994. West Africa long term perspective study. CILLS, Club du Sahel, CINERGIE. Stoorvogel, J.J., Smaling, E.M.A., 1990. Assessment of soil nutrient depletion in Sub-Saharan Africa: 1980±2000. Report 28, The Winand Staring Centre for Integrated Land, Soil and Water Research (SC-DLO), Wageningen, Netherlands. Turner, B., 1994 Agropastoral adaptation to environmental change in north east Nigeria. Land use changes in four study villages. Project Document. Department of Geography, University of Cambridge, Cambridge, UK. van der Pol, F., 1992. Soil mining. An unseen contributor to farm income in southern Mali. Royal Tropical Institute Bulletin 325. Amsterdam Netherlands. Wall, J.R.D. (Ed), 1979 Land resources of central Nigeria. Agricultural development possibilities. Vol 6B. The Kano plains. Land Resources study 29, Land Resources Development Centre. Wilke, B.M., Duke, B.J., Jimoh, W.M.O., 1984. Mineralogy and geochemistry of Harmattan dust in northern Nigeria. Catena 11, 91±96. Williams, T.O., Powell, J.M., FernaÂndez-Rivera, S., 1995. Manure utilization, drought cycles and herd dynamics in the Sahel: implications for cropland productivity. In: Powell, J.M., FernaÂndez-Rivera, S., Williams, T.O., Renard, C. (Eds.), In: Proc. Int. Conf., 22±26 November 1993, International livestock centre for Africa, Addis Ababa, Ethiopia. Livestock and sustainable nutrient cycling in mixed farming systems of subSaharan Africa. vol. ll, Technical papers.
Farm-level assessment of the nutrient balance in northern Nigeria F.M.A. Harris* Centre for Overseas Research and Development, University of Durham, South Road, Durham, DH1 3LE, UK
Abstract The farming system of the Kano close-settled zone, northern Nigeria, is known for its longevity and continued productivity in spite of population increases in the area. Mounting circumstantial evidence has led to its reputation of being `sustainable'. Rural population density in this region now exceeds 300 kmÿ2, and almost all land (86.4%) is under annual cultivation. It is generally believed that such an intensive farming system is only possible under high input levels. This study set out to determine the nutrient balance to see if the system really was sustainable. Agronomic and soil fertility management practices were monitored on three farms over a two year period. Inputs and outputs measured at the ®eld level included fertilizer use (manure and inorganic), dry deposition, biological N ®xation, and the harvest of crops and hedgerow products. The results indicate great variability among ®elds, farmers, and years, and showed that the N balance was strongly negative, whereas P, K, and Mg balances were close to zero. The Ca balance was quite positive as a result of dry deposition. The study also quanti®ed nutrient dynamics within the farming system, highlighting, (1) the role of leguminous crops in bringing in N through ®xation, and as a source of fodder for small ruminants, (2) the role of small ruminants in converting cop residues into manure, and (3) the input of nutrients through dry deposition. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Kano close-settled zone; Nutrient cycling; Semi-arid tropics; Farming systems; Nigeria
1. Introduction Semi-arid west Africa has been the focus of a debate concerning the ability of the environment to support increasing populations primarily dependent on farming and livestock for their livelihoods. This debate has centered on the sustainability of farming practices in the light of competition for use of resources between farmers and herders, and the need to increase agricultural output to provide for the increasing populations. It is feared that as present population densities increase, land degradation will result. Soil fertility declines as a result of cultivation, and land cannot *Corresponding author. Tel./Fax: +44 191 374 2495; e-mail: [email protected]
support extended periods of agriculture unless it is left fallow for long periods of time, or large inputs of nutrients are added to the soil to compensate for the nutrients removed through cultivation. Traditionally, small-holder farmers in west Africa have improved soil fertility through fallowing of land, or the use of manure and small amounts of fertilizer. However it is feared that these methods may not be suf®cient to maintain soil fertility in the light of recent population increases. Studies by van der Pol (1992) and Stoorvogel and Smaling (1990) have drawn attention to the problem. van der Pol (1992) acquired data to calculate the nutrient balance of small-holder farming in southern Mali from literature and 1988±1989 production statistics for the region. From his calculations he con-
0167-8809/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0167-8809(98)00141-8
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cluded that the nutrient balances per hectare per year in southern Mali would be ÿ25 kg N haÿ1, 0 kg P haÿ1, ÿ20 kg K haÿ1, 3 kg Ca haÿ1 and ÿ5 kg Mg haÿ1. He then converted these amounts of nutrients into cost values, based on the price of inorganic fertilizers, and considered the cost of the nutrient de®cit in relation to the value of the harvested crop. The result was that the value of the de®ciency of nutrients was equal to 40% of the value of harvested products. van der Pol concluded ``permanent traditional cropping systems typically offer low returns, and will probably continue to be entirely dependent on soil mining, so that long term sustainability seems quite unlikely.'' Stoorvogel and Smaling (1990) calculated nutrient balances by countries, using national statistics, FAO yearbooks etc. For heterogeneous countries such as Nigeria, the overall ®gure is hard to relate to a speci®c farming system, and it was admitted that ``the supranational scale inevitably encompassed numerous assumptions, estimates, simpli®cations and aggregations'' (Smaling, 1993). However, the results of their study also showed that nutrient mining was occurring. Pieri's Pieri (1989) review of 30 years of research in francophone west Africa concluded that traditional farming systems, using little or no fertilizer but involving fallow, seem to have kept soils at low or stable fertility for the last 30±50 years, at low yields (500± 600 kg haÿ1 of millet and groundnut). As populations increase, it is necessary to determine how farmers can increase yields while maintaining soil fertility. It has been noted that some areas of west Africa support farming in areas of higher population density. In semi-arid west Africa, only a few areas supported rural population densities greater than 60 kmÿ2 in 1960 (Snrech, 1994). These included the Mossi Plateau (Burkina Faso) and the Kano close-settled zone (Nigeria). Since then rural populations have increased all over west Africa, and concern has mounted over the sustainability of these developing farming systems. The areas of high population density which have supported farming since the 1960's or earlier merit further investigation, to determine if they represent sustainable farming practices, and what can be learned from them. The Kano close-settled zone is an area around Kano city which supports a population density of more than 300 people per square kilometer (Snrech, 1994) in a
semi-arid environment. The city of Kano has been a major trading centre for centuries. Around the city is an area of intensive rural farming, which has expanded and increased in population density over the years. This area was ®rst noted by Clapperton in 1829, and later by 1851 by Barth, who noted `farms without interruption' from Bichi to Kano, and that agriculture was practised from Kano to Gezawa with little uncultivated bush (Amerena, 1982). Later visitors also commented on the annual cultivation system, and Morel commented ``Instead of wasting money with the deluded notion of `teaching modern methods' to the northern Nigerian farmers we should be better employed in endeavoring to ®nd an answer to the puzzling question of how it is that land which for centuries has been yielding enormous crops of grain, which in the spring of one carpet of green, and in November one huge corn®eld `white unto harvest', can continue to do so'' (Morel, 1911 in Hill, 1982). More recent work using air photographs con®rms that cultivation around Tumbau, a village 35 km from Kano, close to Gezawa, was 77.6% of land in 1950, and had reached 86.4% of the land by 1971, with all the remaining land occupied by fadama, roads or cattle tracks (Turner, 1994). There is no grazing land. The Kano close-settled zone was the focus of detailed studies by Hendy (1977); Hill (1977) and Mortimore (Mortimore and Wilson, 1965; Mortimore, 1970; Mortimore, 1975; Mortimore, 1993; Mortimore et al., 1990), during the 1960±1980's, but the issue of soil fertility was not addressed directly until 1990. In the 1970's, the Land Resources Division had carried out a resource inventory of north central Nigeria, and as part of that study they had taken soil samples along catenary sequences at locations representative of major land systems (Wall, 1979). In 1990 Mortimore et al. took soil samples at the same sites and analyzed them, to see if there had been any noticeable change in soil fertility. Their results showed that no signi®cant change had occurred in soil physical properties, whereas organic carbon levels had decreased, and total N levels had, surprisingly, increased. They concluded that the soils had a low fertility but had not been seriously degraded during these 20-odd years. They went on to say that the `maintenance of soil fertility was a prime objective of small-holder management' and that the farming system of the Kano close-settled zone, from the evidence of their study,
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was sustainable in the short and medium term (30 years). Unfortunately these statements were not supported by crop production trends, which would have strengthened the case considerably. In the light of these anecdotal and pedological ®ndings that point into the direction of sustainable systems in the Kano close-settled zone, it was decided to study the area's biophysical sustainability, through the quanti®cation of farm and ®eld nutrient ¯ows and balances, and the farm management that determines their levels. 2. Description of the study area As already mentioned, the cultivation intensity in the study village Tumbau had reached 86.4% by 1971, and did not increase further in the period to 1981 (Turner, 1994). It appears to have reached its upper limit. The Kano CSZ has brown to reddish-brown soils, formed by the deposition of dust carried by the Harmattan wind during the Quaternary (McTainsh, 1982). The result is a basement complex covered by a mantle of `drift', giving soils whose character is not related to that of the underlying rock. The eolian mantle is approximately 150 cm deep. There are laterite mesas present, and occasional outcrops of granite, gneiss and schist. The soils of the region are generally well drained, and rooting depth is adequate for most crops (Wall, 1979). Slopes are approximately 2%, with distance between high and low positions not exceeding 25 m. Erosion from rainfall is minimal. The sandy topsoils (0±15 cm) have low organic carbon (0.24%) and total N (0.06%). Available P is, by comparison, relatively high (27.8 mg kgÿ1 Bray 1), and pH is, on average, 6.1. The mean long-term annual rainfall (1906±1985) is 822 mm, which falls during a 5 month rainy season (May±September). Next to considerable year-to-year variability, there has been a downward trend since 1960. The farming of the Kano CSZ is based on crop, livestock and tree production. Dryland cultivation of millet (Pennisetum typhoides), sorghum (Sorghum bicolor), cowpea (Vigna unguiculata (L) Walp.) and groundnut (Arachis hypogaea), usually intercropped, provides cereals for consumption, cash crops (cowpeas and groundnuts are usually sold) and fodder for
203
livestock (groundnut and cowpea residues are more nutritious, sorghum leaves and stalks are also eaten). No space is wasted, and ®eld boundaries are marked with useful plants, such as andropogon (Andropogon gayanus) and vetiver (Vetiveria nigritans) grasses for thatching, and henna (Lawsonia inermis), whose leaves are used for dyeing hands and feet at ceremonial occasions. Other crops grown on upland soils include cassava (Manihot esculenta), maize (Zea mays), sesame (Sesamum indicum), peppers (Capsicum annum), okra (Hibiscus esculentus), roselle (Hibiscus sabdariffa), and sweet potatoes (Ipomoea batatas). The Kano CSZ supports a high density of livestock, most of which are small ruminants (Hendy, 1977; Bourn and Wint, 1994). Fulani herds of cattle only occasionally pass through, but do not contribute signi®cantly to the manure supply (Forde and Scott, 1946). Few farmers own cattle, with the exception of ox plough teams owned by some of the farmers in the villages. Because of the high farming intensity, there is no grazing land for livestock in the Kano CSZ. Fodder production from the farmland is a necessity. Residues not consumed as fodder are used in construction, and for fuelwood. During the growing season, animals are tethered in the compounds, and fed on residues, weeds, and grasses cut from ®eld boundaries, cattle tracks or the occasional uncultivated plot of land. Animals may roam ®elds during the dry season, but always return to the compound at night. Thus, most of the manure produced by animals is deposited in the compound, and farmers are able to transport this to the ®elds to fertilize next season's crops. Farmers allow trees to grow within their ®elds which provide fruit, fuelwood, and browse for animals. Aerial photographs indicate that the density of trees with a circumference greater than 1 m at breast height is about 12 haÿ1 (Mortimore et al., 1990; ClineCole et al., 1990). There was no signi®cant change in the number of trees of this size between 1956±66 and 1981 (Mortimore et al., 1990). 3. Materials and methods 3.1. Research protocol A farm-scale nutrient ¯ow measurement approach was chosen, which relates directly to management
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Fig. 1. Location and distribution of the fields of three farmers involved in this study.
decisions, allowing the constraints affecting those decisions (knowledge, labour and capital) to be investigated too. The farming systems in the area are quite heterogenous, with intercropping, interplanting (mixing trees and shrubs with crops), and variable land quality (sunny and shady patches, raised areas and moist depressions) all contributing. The hamlet of Gamji Tara, near Tumbau village, was chosen for in-depth studies (Fig. 1). This village is composed of a farming community in a rural setting, and is located about 10 km from any tarred road, approximately 30 km east of Kano. Gamji Tara contains approximately 20 households, whose main economic activity is farming. A small weekly market in Tumbau trades locally-made products and foodstuffs. Three households were selected for the present study, all having different, mostly non-contiguous ®elds for crop production (Fig. 1). Each farmer's land was surveyed to measure ®eld and landholding size, and soil samples were taken. Inputs and outputs measured were those that farmers can see and manipulate. Using the Stoorvogel and Smaling (1990) nutrient ¯ow terminology, these include inorganic fertilizers (IN1), manure input from
grazing animals (IN2) and manure recycled within the farm, Harmattan dust (IN3), symbiotic N ®xation (IN4), nutrients in harvested crops (OUT1) and crop residues and weeds (OUT2) and leaching (OUT3). The procedures for measuring these components are outlined below. Not measured were sedimentation (IN5), gaseous losses (OUT4), erosion (OUT5), and human waste (OUT6), as they were considered of minor importance or beyond the scope of the study. Observation and monitoring was done on a ®eld-by®eld basis. Inputs, outputs and yields were determined for the whole of each ®eld, and later the results of ®elds within the same landholding were combined. Boundary and hedgerow crops, being important components of the farming system, were included too. Samples of inputs and harvested crops were analysed according to the following procedures: total N: semimicro Kjeldahl (Hesse, 1971); total P: tri-acid mixture (nitric: perchloric: suphuric, 650:80:20) digest followed by determination of P concentration using the ammonium molybdate/ammonium vanadate method (Allen, 1974); soil available P: Bray l (Bray and Kurtz, 1945); organic carbon of soils and kraal manure: Walkley-Black method (Anderson and
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Ingram, 1989); soil K, Ca and Mg were extracted with ammonium acetate (Anderson and Ingram, 1989); cations in plant and manure samples were determined after wet-digestion (Allen, 1974); Mg and Ca were determined using atomic absorption spectrophotometry, and K concentration was determined by ¯ame photometry; soil nitrate: extraction in K chloride solution followed by reduction to nitrite with copperized cadmium (modi®ed Griess-Ilovsay) (Page et al., 1982). The data derived from monitoring and chemical analysis was combined to calculate a nutrient balance to indicate whether the system was losing or gaining nutrients, or remaining stable. The balance was calculated as kg of nutrients gained or lost both on a landholding basis, and on a standard area basis (ie. kg per hectare). The results were entered into a nutrient ¯ow diagram, which shows how farmers maintain soil fertility. 3.2. Measurement of nutrient inputs 3.2.1. Inorganic fertilizers Most farmers use inorganic fertilizers, although the amount used by each farmer is variable and depends on price and availability. During this study, the three farmers had access to three fertilizers. These were NPK 27:13:13, 20:10:10 and 20:10:5. Farmers distributed fertilizer on their ®elds by carrying mudu sized bowls of fertilizer to the ®elds and sprinkling the fertilizer by hand at the base of cereal plants. In some cases legumes were also fertilized. A mudu is a standard measure, equivalent to 4 kg of inorganic fertilizer. The farmers reported how many mudus of fertilizer were applied to each ®eld, and what type of fertilizer it was. The quantities of N, P and K contributed to the ®eld as inorganic fertilizer were calculated. In inter-cropped ®elds, farmers apply more inorganic fertilizer to cereals than legumes. 3.2.2. Manure Manure produced and recycled on the farm The farmers of the Kano CSZ apply taki, a term they translate as `local fertilizer', to their ®elds. The precise nature and composition of taki varies as it contains animal manure, refused fodder, ash, compound sweepings and kitchen waste. During the dry season livestock roam the farmland, and feed on whatever they
205
can ®nd. During the rainy season the lack of grazing land forces farmers to tether their animals in their compounds and feed them crop residues stored since the previous season, as well as weeds which are collected from the ®elds, and grass from roadsides. The livestock produce manure, which together with litter, is trampled underfoot and produces taki. During the dry season this is taken to the ®elds. Taki is usually transported to the ®elds in panniers on donkeys. The pannier-sized load is referred to as a mangala. Because of the variability in taki composition and mangala size it was necessary to quantify each to assess the contribution of taki to the nutrient budget of the farming system. At the beginning of the 1993 farming season the mangalas of taki on each ®eld were counted, weighed and described according to their composition. They were classi®ed into 8 groups according to the farmer's description of the contents. 37 Samples of different types of taki were collected for analysis for total N, total P and exchangeable cations. During the 1994 farming season mangalas were counted, and a subsample weighed, to con®rm the average weight of a mangala. In view of the detailed analytical work done the previous year, no further samples were analysed in 1994. Table 1 shows the average nutrient composition of the taki samples taken from farmers' ®elds. There was considerable variation in taki nutrient content from the variation in its production and composition. Samples had low N concentrations compared with other studies (Landon, 1991; Jones and Wild, 1975; Williams et al., 1995; Bationo and Mokwunye, 1991; Mortimore et al., 1990) which may be the result of the manure being mixed with grass and ash, or to losses by volatilization during storage within the compound or in the ®eld. In this study samples of taki were taken for analysis at the end of the dry season, just before incorporation into the soil. Application of taki per ®eld varied from 0±17.5 tons per hectare during the 1993 and 1994 growing seasons. The average application (total amount of manure applied per landholding) was 4.3 tons per hectare (Table 2). On the whole, more taki was applied to ®elds in 1994 than in 1993. Manure deposited by grazing animals Three ®elds were selected to study manure deposition in ®elds by grazing animals, according to their distance from Tumbau, their accessibility and their ownership. Once
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Table 1 Average nutrient content (% dry matter) of inputs and harvest Crop
Part
Taki Harmattan dusta Early millet
Grain Chaff Sticks Stalks Beans Pods Straw Grain Chaff Sticks Stalks Nuts Pods Haulms Grain Chaff Sticks Stalks
Cowpea Sorghum
Groundnut Late millet
Weeds a
n
N
P
n
K
Mg
Ca
37
0.3 0 1.22 0.88 0.43 0.50 3.13 1.34 1.29 1.19 0.59 0.42 0.32 3.48 0.88 1.49 1.54 0.86 0.51 0.35 1.32
0.19 0.09 0.15 0.27 0.05 0.12 0.43 0.15 0.23 0.35 0.40 0.24 0.21 0.68 0.12 0.15 0.27 0.37 0.23 0.33 0.18
37
0.8 2.03 1.67 1.76 1.22 1.51 1.29 1.17 1.38 1.00 0.57 0.86 0.79 1.06 0.88 1.76 0.84 0.53 1.72
0.24 0.39 0.19 0.14 0.19 0.24 0.21 0.21 0.33 0.23 0.15 0.20 0.15 0.54 0.07 0.32 0.17 0.15 0.13
0.84 1.13 0.20 0.38 0.53 0.42 0.26 0.18 0.89 0.21 0.12 0.21 0.29 0.48 0.11 0.84 0.54 0.14 0.22
1.65
0.30
0.58
15 17 17 16 25 25 31 34 35 32 18 23 30 23 9 10 10 7 8
3 5 5 3 4 4 3 4 4 4 3 1 3 3 3 3 3 0 2
From McTainsh, 1982
Table 2 Comparison of inputs and yields in 1993 and 1994 Inputs ÿ 1
Farm Farmer I 1 2 3 Farmer Y 1 2 3 4 Farmer S 1 2 3 4 5
Outputs ÿ1
Taki (ton ha
)
Inorganic Fertilizer (kg ha )
N Fixation (kg ha )
Biomass (ton haÿ1)
1993
1994
1993
1994
1993
1994
1993
1994
0 10.6 7.2
11.9 0 0
75 75 20
57 58 70
9 19 5
37 6 8
4.4 7.4 3.8
8.7 7.2 12.1
1.9 1 7.2 0
7.5 4.6 4.1 2
47 0 0 24
39 20 27 37
11 15 16 2
29 19 23 28
2.6 3.8 4.6 3.4
3.7 5.1 8 4.6
8.9 5.5 1.7 2.4 0
8.9 17.5 0 0 0
0 0 0 0 0
0 0 0 90 58
8 3 8 0 8
17 48 20 35 4
3.7 6 3.6 2.6 3.2
10.5 9.6 4.8 4.7 3.3
a month during the dry season a 1.0 m2 quadrat was thrown randomly on each ®eld ®ve times. The manure observed within the quadrat was collected and
ÿ1
weighed. The presence and average weight of manure found on each ®eld over the months of the dry season were recorded. Average deposition was only
F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
17 kg haÿ1 so obviously most manure was deposited within the compounds. Thus manure (with a mean composition of 0.3% N) deposited by grazing and browsing animals appears to contribute only 51 g N per hectare of land, a negligible contribution to soil fertility when compared with the inputs from taki brought to the ®elds from compounds, or N ®xation by legumes. 3.2.3. Harmattan dust A ground level wet dust trap was used to measure dust deposition on ®elds. The trap consisted of a straight-sided glass bowl containing distilled water, enclosed in a cage of mosquito screen to prevent birds, animals and insects from reaching the water, positioned in a farmer's ®eld. Dust deposition was monitored weekly through the dry season, starting from 19 October 1993 at Tumbau. Weekly deposition varied from 1±17 g mÿ2, with the higher values being noted at the end of the season. High winds before the onset of the rainy season cause so much local dust movement that the use of the dust trap to measure only Harmattan dust deposition becomes impossible at this time. The soil and dust movement in such winds is believed to cause only local soil movement, and so such wind erosion does not remove nutrients from the area (Mortimore, 1989). Total dust deposition measured in the ®eld at Tumbau was 158.13 g mÿ2 during the dry season (October± April). Using McTainsh's (1982) method to correct for secondary local re-deposition, 92.2 g mÿ2 (922 kg haÿ1) dust was deposited on the ®eld from far away. Nutrient content of dust has been measured by a number of researchers, i.e., Wilke et al. (1984); Beavington and Cawse (1979); McTainsh (1982). The composition data given by McTainsh (1982), who worked in northern Nigeria, were used to estimate its contribution to the nutrient balance. This study used the results from the ®eld measurements made during the 1993±1994 Harmattan season to calculate inputs to the system for the 1993 and 1994 growing seasons. Assuming that Harmattan dust is distributed evenly over all the ®elds, nutrient input to each ®eld is calculated from the overall deposition ®gure (922 kg haÿ1) and the area of each ®eld. Table 1 shows that the Harmattan wind contributes large amounts of K and Ca to the soils of the area.
207
3.2.4. N fixation Traditionally N ®xation trials are carried out on monocrops, to determine ®xed N per hectare of crop. However in the Kano CSZ farmers vary planting patterns within each ®eld, therefore the planting pattern and legume density in each ®eld were taken into account in order to calculate N ®xation inputs to each ®eld. The planting patterns within 10 metre squared quadrats on farmers ®elds were noted. Field experiments to quantify symbiotic N ®xation were carried out in 1994. N ®xation in the system was studied under local farming practices, using the N difference method. This consists of detailed monitoring of N uptake during the growth of a N-®xing crop in comparison with that of a similar non-®xing crop. The difference in N uptake is then assumed to come from ®xation (Giller and Wilson, 1991). Groundnuts and cowpeas were planted on a trial plot within a farmers' ®eld, in a pattern typical of local farmers' practice. Maize was planted as the reference crop. The number of seeds planted and weeding were carried out according to farmers' practices. From 60 days after planting, plant samples were harvested every 15 days. The initial samples included roots, which were examined for nodule number and colour. Later only shoot samples were collected. The entire shoot of 5±10 plants was removed, dried, weighed, and ground, and subsamples of ®ve plants were taken for analysis. Shoot weight multiplied by N content gives the N uptake as grammes of N per plant. The estimate of symbiotically ®xed N was the difference between the amount of N in the shoot of the legume at the date of maximum N accumulation and the amount of N in the shoot of the reference crop (maize) (Hauser, 1992) (Fig. 2). Groundnuts reached maximum N uptake at 105 days after planting, at which time they had accumulated 1.227 g N, and cowpeas reached maximum N uptake at 120 days after planting, at which time they had accumulated 2.32 g N. The maximum accumulation of N from the soil by maize was 0.616 g. Thus assuming 0.616 g N came from the soil, groundnuts ®xed 0.611 g N per plant, and cowpeas ®xed 1.706 g N per plant on the trial plot. From the N ®xation trial it was calculated that ®xed N was equivalent to 1.18% of harvested biomass of groundnut, and 1.48% of
208
F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
Fig. 2. N uptake (gN/plant) by maize, groundnut and cowpea in the N fixation trial.
harvested biomass of cowpea. As the experiment was conducted in 1994 only, this relationship was used to assess N ®xation during the 1993 season (Table 2). 3.3. Measurement of nutrient outputs 3.3.1. Harvested products and residues The harvest from each ®eld was recorded. Threshing trials determined what percentage of the harvest weight was grain, chaff, pods, or the remaining waste (sticks). Samples of the harvest were collected, threshed and taken for chemical analysis. Table 1 gives the average nutrient content of the main crops. The micro-management practised by farmers means that fertilization (with taki or inorganic fertilizer), cultivar and intercropping may differ from plant to plant within one ®eld, as well as across the landholding. Field by ®eld ®gures were used to calculate the nutrient balance (Harris, 1995). Combining harvest data with the results of chemical analysis of plant material, the nutrients removed in harvested crops, weeds and boundary plants from each ®eld in each year were calculated (Table 2).
3.3.2. Leaching The sandy soils are freely draining, and rainfall percolates through the soil unimpeded. Evaporation ®gures from the Bayero University meteorological station were combined with rainfall data to calculate the water balance for the 1993 and 1994 seasons. In 1993 precipitation was greater than evaporation for only two weeks, and in 1994 for four weeks, both events in the month of August, when the ®elds are full of rapidly growing crops approaching maturity. Conditions of excessive moisture were only experienced for a short part of the rainy season, when N uptake by the crop could be expected to be high. Measurements of soil nitrate-N were made during both seasons. At the end of July, Farmer I's ®eld 2 had approximately 1.0 mg nitrate-N/kg soil. Given a bulk density of the soil of 1.4 g cmÿ3 this corresponds to 2.8 kg of nitrate-N per hectare in the top 20 cm of soil. In years with a positive water balance, such as 1994, there may have been leaching of nitrate-N from the soil. It would seem that 2.8 kg haÿ1 is the maximum amount of N lost by leaching if all the nitrate was leached, and none was taken up by plants. In fact, it is probably less, as some would be taken up by the growing crop. Because of the variability from year
F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
to year, and the uncertainty concerning the amount of nitrate leached or taken up by the crop, the above ®gure of 2.8 was not included in the balance. 4. Calculating the nutrient balance 4.1. Farm balances Fig. 3 shows the nutrient balances of the three landholdings, and shows considerable variability among ®elds, among farms, and between years. The landholding balances of 1993 varied from ÿ11 to 1 kg N/ha, and 0±2 kg P/ha. In 1994 these ®gures were ÿ28 to 3 kg N haÿ1, and ÿ3 to 3 kg P haÿ1. The N balance was negative on all holdings except for Farmer S's holding in 1993 and Farmer Y's holding in 1994. The P balances were positive in 1993, but negative on Farmer I and Farmer S's holdings in 1994. The results suggest that cations are not in short supply, thus the farming system may be sustainable with respect to K, Mg and Ca. 4.2. Field to field variability Three quarters of the ®elds had negative N balances in 1993 and 1994. About half of the ®elds had negative
209
P or K balances in 1993. More ®elds had negative P balances in 1994. Four ®elds had negative Mg balances, and one had a negative Ca balance in 1993 (Table 3). The nutrient balances varied largely from ®eld to ®eld: in 1993 the N balances ranged from ÿ16 to 8 kg/ha, and for P from ÿ4 to 10 kg haÿ1 on the ®eld, yet in 1994 the balances of these nutrients ranged from ÿ56 to 24 kg haÿ1 and ÿ19 to 15 kg haÿ1 respectively. The cation balances also ranged a lot, with K and Ca balances reaching 45 and 69 kg haÿ1 respectively in 1993. The extreme variability from ®eld to ®eld is because of farmers' rotation of taki and inorganic fertilizer inputs, as well as the change in cropping patterns on each ®eld. Table 2 clearly shows these differences in ®eld management. Taki does not decompose completely within the ®rst season after its application, so that a large input in one year will supply nutrients over several years. However, division of the nutrient input in taki over several years could only be carried out if long-term monitoring of inputs was possible. Therefore, the whole nutrient input of taki is attributed to the year of application only, rather than dividing it over the subsequent years. Crop choice also affects the nutrient balance. Legumes contribute N through ®xation, and those ®elds with
Fig. 3. Nutrient balances for the three landholdings, expressed as kg haÿ1 of nutrient. N, P, K, Mg and Ca for 1993 and N and P for 1994.
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F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
Table 3 Nutrient balance of fields (kg haÿ1) Year and nutrient Field
N
1993 P
K
I1 I2 I3 Y1 Y2 Y3 Y4 S1 S2 S3 S4 S5
ÿ8.25 ÿ1.01 ÿ3.73 ÿ12.14 ÿ15.52 ÿ2.80 ÿ11.06 8.29 ÿ11.29 ÿ10.16 5.61 0.34
ÿ2.56 7.36 9.58 1.64 ÿ3.45 8.41 ÿ4.39 6.31 ÿ0.29 ÿ3.68 0.30 ÿ2.68
ÿ43.90 45.38 24.38 5.34 ÿ16.81 32.62 ÿ14.25 39.70 ÿ17.7 ÿ5.19 7.73 ÿ10.08
high legume yields also had high N inputs from ®xation. 4.3. Year to year variability
Mg ÿ6.81 10.23 8.88 1.10 ÿ1.13 12.71 ÿ1.96 16.94 14.86 0.65 4.92 ÿ1.46
Ca
1994 N
P
ÿ7.59 66.48 49.25 14.56 9.90 62.51 1.54 69.08 33.86 8.76 21.44 0.49
15.64 ÿ33.11 ÿ71.37 24.95 ÿ10.93 ÿ27.00 13.26 ÿ38.34 ÿ48.71 ÿ56.11 ÿ0.98 ÿ18.06
9.90 ÿ9.08 ÿ19.23 9.65 2.15 ÿ2.28 ÿ1.42 ÿ6.99 14.71 ÿ10.81 ÿ3.64 ÿ1.80
structures become worn down and are replaced, the old material is usually burned. Stalks are also used as fuelwood, although it is hard to estimate to what extent this is practised. N is lost when stalks are burned. The
The N and P balances in 1994 were lower than those of 1993. This can be attributed to differences in inputs from N ®xation, manure and inorganic fertilizer: these were higher in 1994 than in 1993. As rainfall was also higher in 1994, the amount of biomass collected from ®elds in 1994 almost doubled that of 1993 (Table 2). 5. Nutrient management Farmers in the Kano CSZ recycle many nutrients within the farming system. Only a small amount of the material that is harvested from the ®elds leaves the area. Fig. 4 provides a pie chart of the types of biomass that are generated. Approximately 24% of dry matter harvested is food grains (5% is leguminous grains). Leguminous grains are usually sold. The cereal grain is stored in the compound to supply the household's food needs during the year. If supply exceeds the family's food needs, the excess may be sold to the market. The amount sold will vary, depending on the harvest and household size. 23% of biomass harvested is animal fodder. Unpalatable stalks and gamba grass are used to construct compound walls and huts, and also storage granaries. When these
Fig. 4. The uses made of harvested material (% of total harvested biomass).
F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
remaining nutrients in residues are returned to the ®elds when ash is combined with taki. None of the farmers in Tumbau sell haulms to Kano city residents who keep livestock in town, although some sell some sorghum stalks if they are in excess of their own needs. Farmers grow considerable quantities of groundnuts and cowpeas on their land, despite the cereals millet and sorghum being the staple components of their diet. N ®xation by these legumes contributed up to 48 kg N/ ha within individual ®elds, depending on the density of legumes in the ®eld. This is less than that often quoted in the literature (Giller and Wilson, 1991), because studies on N ®xation usually involve either inoculation of seed by rhizobia, high basal doses of P, or monocrops. In addition to contributing N, the residues of the legume crops are a nutritious and valued form of animal fodder for the livestock. In the Kano CSZ, where grazing land is unavailable and cut-and-carry feeding a necessity, crop residues are the main source of animal fodder. The majority of smallholder farmers in the Kano CSZ keep livestock for their value as producers of manure, for animal traction (donkeys and ox teams), for slaughter at times of religious and ceremonial occasions, and as ®nancial investment. Livestock are not kept with a view to maximizing pro®t from breeding and meat production. The fodder harvested is used to apply `maintenance' rations to the compound's livestock (Hendy, 1977). Farmers try to keep as many animals as they can feed, rather than reducing the number of animals so that a smaller number can be raised for `production', i.e. with an aim to maximizing live weight gain, reproduction and milk production. The farmers gained 167 and 549 kg haÿ1 of groundnut and 217 and 248 kg haÿ1 of cowpea haulms in 1993 and 1994 respectively (dry matter). Hendy (1977) calculated that groundnut and cowpea haulms are particularly important in the livestock diet, contributing 70% of the digestible crude protein and 20% of the total metabolizable energy (ME) harvested, whereas sorghum residues provided 30%, and grasses from waste and unused land provided 25% of all the ME. Their use as fodder permits the farmers of the Kano CSZ to keep the high population density of livestock noted by Hendy (1977); de Leeuw et al. (1995) and Bourn and Wint (1994).
211
Livestock are able to convert the crop residues into manure, thus rendering the nutrients more readily available to plants. Manure production by farmholding ranged from 2.3±15.3 tons over the two years of the study. The N ®xed in the leguminous crop residues passes through the animals before being applied to the ®elds as manure. Manure accounts for approximately one quarter of N inputs, and one ®fth of P inputs. It also contains other nutrients, supplies organic carbon, and along with the Harmattan dust supplies many micronutrients which would not be supplied in inorganic fertilizers or through N ®xation. The use of taki contributes approximately 4 ton haÿ1 organic matter to the ®elds, which breaks down slowly. This organic matter contributes to soil fertility in providing nutrients and by improving soil physical properties. The soils of the Kano CSZ are very sandy, with low clay contents. The addition of taki improves the water holding capacity and cation exchange capacity of the soil. Fig. 5 depicts the N nutrient ¯ows through the farming system. To conserve soil nutrients farmers must marshall the nutrients in such a way that maintains a ¯ow through the farming system, limiting loss. In this integrated farming system, ef®cient use is made of almost all the biomass produced on the farmers' ®elds. Integration is made possible by the incorporation of high-protein fodder crops in the rotation, and livestock husbandry. Recycling reduces nutrient losses. Some nutrients are taken in by growing livestock, or lost by volatilization during burning (especially N), but most are returned to the soil in a form much more appropriate for use a as fertilizer. 6. Conclusions The study presented in this paper identi®ed the Kano CSZ as an area thought to be sustainable, and then investigated how farmers had achieved this, through nutrient cycling and nutrient management. The results of two years of detailed monitoring indicate that the system is sustainable with respect to K, magenesium, Ca and P, but the results concerning N did not agree with the results of soil testing over a 13-year interval, or the evidence as seen on the ground of repeated harvests from annually cropped land.
Fig. 5. Amounts of N (kgN haÿ1) cycled within the integrated crop-livestock farming system, for 1993 and 1994.
212 F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
F.M.A. Harris / Agriculture, Ecosystems and Environment 71 (1998) 201±214
The contradiction between the measured N balance and the historical evidence which suggests that the farming system must be sustainable can only be resolved by further detailed study over a longer period of time and a wider range of farmers. Although nutrient balances are known to vary, the system was clearly limited by N during the 2-years of this study. The model of nutrient dynamics that has been developed by this study has identi®ed nutrient ¯ows and allows the quanti®cation of some of the factors contributing to the success of this farming system, such as the crop-livestock integration, the inclusion of N ®xing leguminous species in the cropping system, high levels of manure use, and the Harmattan dust which imports cations. This has enabled a farming system to develop in the face of increasing population densities and continues to provide livelihoods for the community it supports. The high population density of people and livestock and the dispersed settlement pattern has created an environment of integrated crop and livestock production (Harris, 1996). The example of the Kano CSZ shows that reduction in grazing land may be compensated through the use of crop residues as animal fodder (especially residues of leguminous crops), and that the integration of crop and livestock production results in a more ef®cient recycling of nutrients within the farming system. 7. Unlinked reference Hermann et al., 1994 Acknowledgements The research was funded under the Semi-Arid Production Systems Programme of the Natural Resources Systems Programme of the UK's Department for International Development (EMCX 216). The work was carried out at the University of Cambridge, UK, and Bayero University, Kano, Nigeria. References Allen, S.E., 1974. Chemical Analysis of Ecological Materials. Blackwell Scientific Publications, Oxford, UK, pp. 88±89. Amerena, P.M.J., 1982. Farmers' participation in the cash economy: case studies of two settlements in the Kano close-
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settled zone of Nigeria. Ph.D. Thesis, University of London, London, UK. Anderson, J.M., Ingram, J.S.I., 1989. Tropical Soil Biology and Fertility: A Handbook of Methods. CAB. International, Wallingford, USA, pp. 35±37. Bationo, A., Mokwunye, A.U., 1991. Role of manure and crop residue in alleviating soil fertility constraints to crop production: with special reference to the Sahelian and Sudanian zones of west Africa. Fertilizer Research 29, 117±125. Beavington, F., Cawse, P.A., 1979. The deposition of trace elements and major nutrients in dust and rainwater in northern Nigeria. Sci. Total Environ. 13, 263±274. Bourn, D., Wint, W., 1994 Livestock, land use and agricultural intensification in sub-Saharan Africa. Pastoral development network discussion paper, ODI. Bray, R.H., Kurtz, L.T., .1945. Soil Sci. 59, 44. Cline-Cole, R.A., Falola, J.A., Main, H.A.C., Mortimore, M.J., Nichol, J.E., O'Reilly, F.D., 1990. Woodfuel in Kano. United Nations University Press, Tokyo. de Leeuw, P.N., Reynolds, L., Rey, B., 1995. Nutrient transfers from livestock in west-African production systems. In: Powell, J.M., FernaÂndez-Rivera, S., Williams, T.O., Renard, C. (Eds.), In: Proc. Int. Conf., Addis Ababa, Ethiopia, 22±26 November 1993. International Livestock Centre for Africa, Addis Ababa, Ethiopia. Livestock and sustainable nutrient cycling in mixed farming systems of sub-Saharan Africa, vol. 11, Technical papers. Forde, D., Scott, R., 1946. The Native Economies of Nigeria. Faber and Faber. Giller, K.E., Wilson, K.J., 1991. N Fixation in Tropical Cropping Systems. CAB International, Wallingford, USA. Harris, F., 1996. Intensification of agriculture in semi-arid areas: lessons from the Kano close-settled zone, Nigeria. Gatekeeper Series No. 59, Sustainable Agriculture Programme. International Institute for Environment and Development. Harris, F., 1995. Nutrient dynamics of the farming system of the Kano close-settled zone, Nigeria. Ph.D. Thesis, University of Cambridge, Cambridge, UK. Hauser, S., 1992. Estimation of symbiotically fixed N using extended N-difference method. In: Mulongoy, M., Gueye, K., Spencer, D.S.C. (Eds.), Biological N fixation and Sustainability of Tropical Agriculture. IITA / Wiley-Sayce, Chichester, UK. Hendy, C.R.C., 1977. Animal production in Kano state and the requirements for further study in the Kano close-settled zone. Land Resources Report 21, Land Resources Division, Ministry of Overseas Development. Hermann, L., Bationo, A., Stahr, K., 1994. Chemical composition of dusts from the Sahara to the Gulf of Guinea and the influence on rainwater chemistry. Atmospheric Environment, in connection with the CACGP, IGAC Symposium, 4±9 September, 1994, Japan. Hesse, P.R., 1971. A Textbook of Soil Chemical Analysis. William Clowes, London, UK, pp. 197±199. Hill, P., 1982. Dry Grain Farming Families. Cambridge University Press, Cambridge, UK. Hill, 1977. Population, Prosperity and Poverty. Rural Kano 1900 and 1970. Cambridge University Press, Cambridge, UK.
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