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Future land utilization and management for sustainable crop production
EI.SEVIER
Soil & TillageResearch 30 (1994) 345-357
Soil & Tillage . Researcn
Future land utilization and management for sustainable crop production A r n o r Njos JORDFORSK, Centre for Soil and Environmental Research, N-1432 .~s, Norway
(Accepted 21 December 1993)
Abstract Soil cultivation caused a great change in the ecological state, from a situation with the soil surface protected by vegetation to a sitation with bare soil exposed to climatic forces during part of the year. Annual crops require annual soil cultivation, and the increased mineralization of organic matter and the loss of soil productivity by erosion, leaching and other degradation processes remain a problem for sustainable food production. The challenge for the future is to manage the agricultural landscape in units which are catchments. These units would contain intensively drained and cultivated areas, permanent vegetation zones, as well as natural and constructed wetlands working as condensors and self-purifying units. The management would be differentiated according to a land suitability approach with intensity of use based on productivity and requirements for maintenance of environmental quality. Key words: Catchment;Management;Productivity;Sustainability;Unit
1. Introduction In agricultural history there have been several technological breakthroughs, sometimes called agricultural revolutions (Blaxter, 1972 ). According to Thomas (1989) the first agricultural revolution occurred between about 12 BC and 7000 BC and involved several steps towards settled agriculture, such as the introduction of tillage to create a seedbed for cultivable plants. Thomas (1989) used the wording "robust historians" about authors who spoke of revolution, when the change might have taken such a long time. The tillage was very simple, scratching the surface for placing the seeds. Early ploughs were little more than hoes. In: Egypt a two-man plough, with one man pulling a rope in front and a second m a n 0167-1987/94/$07.00 © 1994 ElsevierScienceB.V. All fightsreserved SSDIO167-1987(94)O8029-Y
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pressing the plough's point into the ground, was used around 7000 BC. It became evident to the early farmers that the competition from weeds was serious for the survival of the cultivated plants, and tillage was necessary to create a seedbed and to eliminate or decrease the competition from weeds. The preparation of a seedbed for crops is probably the major break with a natural or ecological balance, if it ever existed. If the cultivation started on a steppe or river plain, clearing was simpler and less laborious than in a forest. Energy was now spent on a small area to increase its productivity. Previously, energy had been used for hunting. According to Stout et al. (1979), hunters and gatherers would need an area of 1.5 km 2 per person for survival under good conditions, while under poor conditions 60-80 km 2 would be needed. Clark and Haswell (1970), after collecting material from several authors, state that in cold climate the Eskimos and Indians of north western Canada would need about 140 km 2 per head, while on the American prairies 25 km 2 would suffice. On the shores of rivers rich in salmon the figure might fall to 1.5 km 2. Pastoral people in East Africa would need 0.5 to 1.5 km 2 per person, and settled agricultarists with shifting cultivation around 0.15 km 2 per person. Generally, from the introduction of soil cultivation and seeding, agriculture might be defined as a planned and implemented ecological imbalance to increase the productivity per unit area by introducing more energy into the system. In millenia after the first agricultural revolution, cultivated fields were generally permanent in the better agroecological regions, and grain yields varied around two to four fold (two to four times the quantity of seeds sown). This would correspond to a yield of about 0.4-0.8 t h a - 1 (Hannerberg 1971; Blaxter, 1972 ). In many parts of Europe, with the three field system (2 years of grain, 1 year of rest) the yields on the average soils were not above this. On the large river plains of Asia and Europe and in the Nile valley of Northern Africa yields were higher, for example, 1.0-1.5 t ha-1. The same applied to volcanic soils. For the river valley soils in ancient Egypt a figure of 2 t h a - 1 has been estimated (Clark and Haswell, 1970). The supply of plant nutrients was the limiting factor in the subhumid and humid areas; water in the semiarid and arid areas.
2. The crop rotation system in Europe The setting for the second agricultural revolution was the industrial revolution and the growth in Europe's population. With increasing demand for food, a larger area had to be cultivated or the productivity per unit area had to be increased. In this case the result was increased productivity per unit area. Legumes, such as clover, were grown for forage and to supply nitrogen for next year's crop. A new agricultural technology developed, based on the turning effect of the steel moldboard plough, and crops grown in a time sequence in the same field. Crops, such as cereals and root crops were grown in a sequence with a legume-grassland ley. In Europe, England is considered the home of crop rotation agriculture, because of the Norfolk crop rotation. A good description of this agricultural revolution is
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given by Chambers and Mingay (1970). The steel moldboard plough has become more or less the symbol of the crop rotation agricultural system. Grain yields increased to 1.5-2.0 t h a - ~ (Blaxter, 1972). Looking at this period in retrospect, many people feel that this was the era of a sustainable agriculture and of an ecologically balanced system. However, the system was not entirely balanced. Soil cultivation, and the turning of the sod, contributed to mineralization of the organic matter conserved during the sod years. This showed up as a gradual decrease in soil organic matter with time. The human labour was partly replaced with animal horse power. Fertilizers and lime were gradually added as inputs. Fields were drained, which further increased the mineralization of organic matter.
3. The development of agricultural technology since 1940 The third agricultural revolution, from about 1940 onwards (also called the Chemistry Revolution), was characterized by the tremendous rise in yields by chemical and other inputs, a strong decrease in human labour and a replacement of animal power with full mechanization. The input of energy by fertilizers, pesticides, mechanical power, drying, irrigation and drainage increased many times. The crop rotation of grain-rootcrops-grain-ley yielded to grain-grain or grassgrass. An important change to the ecological balance was the drainage of wetlands. The wetlands, open ditches and small ponds and lakes are, in a broad sense, the natural elements of self-purifying mechanisms in the agricultural landscape. The mineralization process was speeded up simultaneously by increased cultivation and drainage. The infiltration rate and the structural stability of the annually cultivated soils decreased and was followed by erosion. In the drier areas of the World, improper irrigation often resulted in rising water tables in depressions, giving rise to salinization and alkalinization (see, for instance, Israelsen, 1950 or Carter and Dale, 1974). The Chemistry Revolution has raised serious questions about environment, ecological balance and sustainability. This issue was brought forward through the Brundtland report "Our Common Future" (World Commission on Environment and Development, 1987). Before discussing the question and philosophy of sustainabillity, it should be mentioned that the present developments in agriculture might be expressed as the fourth agricultural revolution or the 'Biology Revolution'. The developments within biotechnology, genetic engineering and information technology are forceful tools for a new type of agriculture, where inputs of agrochemicals might decrease and thereby energy input per unit produced.
4. Sustainability----definitions The expression sustainability in many ways is similar to bearing capacity or to common expressions about good husbandry. In German, "Haltbarkeit" would
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provide a reasonable cover of the meaning. In French "durabititr" would probably be close to sustainability. There are several definitions of sustainability. (a) Conway (1985 ): The ability of a system to maintain productivity in spite of a major disturbance, such as caused by intensive stress or a large perturbation. (b) Oram (1988 ): A system's ability to maintain productivity when exposed to external stress or shock. (c) Consultative Group on International Agricultural Research, (1988). The successful management of resources for agriculture to satisfy the changing human needs, while maintaining or enhancing the quality of the environment and conserving natural resources. Some authors have stressed the ability to survive 'a shock' while others stress the environmental aspect. Should the system be strong enough to survive natural disasters (such as pests and large floods), volcanic eruptions (such as Lakagigar in Iceland in 1783 ) or human induced shocks (such as the 30-Years War in Europe)? It is easy to agree that a system cannot be expected to endure a major climatic change, such as an ice age. Thus, a reasonable requirement might be that the system should be able to sustain normal variations over, say, a generation or a century. Lynam and Herdt (1989) included the system's ability to maintain production at an average historical level within historical variability. Ruttan ( 1988 ) proposed to include the requirement that a technology is developed which maintains and/or enhances the quality of soil and water resources, improves plant and animal materials and paves the way for a transition from a chemical to a biological technology. The inclusion of the environmental requirements might be seen as a new addition to the old perception of good husbandry, which was summarized in the proverb "leave the farm to the next generation in a better shape than it was received". Consider what has happened in the Nile catchment through some thousand years. The erosion in Ethiopia was always considerable and the loss of soil was the base for the fertility of the river plain and the delta of the Nile. Similar examples are found in many river plains in Asia, such as Mesopotamia, the Indus and Ganges floodplains on the Indian Sub-continent and the large river plains in China. Taking the World as a whole, most agricultural civilizations have not survived more than 30 generations (Carter and Dale, 1974). However, the river plains together with some volcanic areas and the irrigated rice cultures in the East Asia have persisted for longer. The causes for persistance are generally a good and regular water supply and some mechanism of refreshing the nutrient supply, for instance through flooding or volcanic ashes. There are interesting long-term effects of ancient agriculture on soils. Sandor and Eash ( 1991 ) report lower organic C values for the upper layers of previously cultivated terrace soils than for uncultivated soils in New Mexico, although cultivation ceased more than 800 years ago. The authors also describe work on ancient terrace soils in Peru. In the Colca valley in Peru, agricultural terrace soils have properties indicating favourable tilth and fertility after 1500 years of cultivation. In this case the cultivated soils had a higher organic C content than their uncultivated counterparts. Traditional soil and crop management practices might
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have been responsible for this condition. The soils were mollisols in the study area in New Mexico as well as in the study area in Peru. In New Mexico the mollisols had a very strong argiUic horizon with a shallow upper boundary. In Peru the farming system actually was a crop rotation system with tillage being done with a chisel-like plough working shallowly and not inverting the soil (Sandot and Eash, 1991 ). Although Carter and Dale (1974) stated that most agricultural civilizations did not last more than 30 generations because of soil degradation, well-known exceptions took place on river plains and volcanic soils.
5. Sustainability and productivity Productivity is the base for sustainability. Productivity is the ratio of output to input in a given system, where inputs and outputs must be measured in the same units. Economic viability is the criterion used to compare the productivity of several systems at a given time. The time derivative of the productivity is a measure of sustainability. The basic idea behind the concept of a sustainable system is that of productivity which does not decrease with time. There are ways of solving the differential equation that results from the time derivative of the productivity. The solution is likely to contain the natural logarithms of outputs and inputs. Very often the difficulty is the handling of an aggregated index for products, as well as an aggregated index for inputs. Christensen (1975 ) used actual prices in different years for weighting. Ehui and Spencer (1990) calculated sustainabilities for tropical farming systems, including also the loss of soil fertility estimated by the decrease in soil nutrients. The latter would be a change in resource base. Similarly, if erosion were a problem, soil losses might be recalculated as losses of nutrients and available water storage. Dahlberg (1993) also includes ecology, ethics and equity in a concept which he describes as regenerative systems. If ethics and equity are included, the sustainability concept would not be described sufficiently by productivity.
6. The boundaries of the system Setting the boundaries at the farm fence, the system is the farm. If there were to be no pressure from the outside world, this inner world would continue to produce with regard to its own resources, such as the upper metre of the soil, the slopes and depressions and the small streams. The environment would be the local landscape, the air, the unpredictable weather with too little or too much o f everything. The ground water would be outside the range of interest except for water supply, and the volumes of gases leaving as carbon dioxide and ammonia would also be outside the system. The system might also be the catchment. Here, it is practical to think of a catchment of a lake, stream, canal or a tile drainage system of a few square kilometres, mainly with agricultural land and activities, with a mixture of forests, arable fields
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and pastures. The quality of the water, measured at the outlet of the catchment, would be a measure of the health of the system and its potential influence on the environment, which might be a larger watershed. The main difference from the farm as a system would be that the measures against soil and nutrient losses could be based on a clear hydrology or agrohydrology concept. Transport of water within the catchment would include surface runoff, drainage discharge through canals or tile systems, or discharge from ground water to a stream, the total discharge being accountable at the outlet of the system. The inputs would be accountable both for the farm and the catchment, but the total effect on the water leaving the area would not be easy to associate with the farm. Self-purifying processes, such as trapping sediments and nutrients in lakes, streams, ponds, natural and planted vegetation zones and constructed wetlands as well as denitrification in shallow waterways could operate both at the farm and catchment level, but the planning, implementing and monitoring would have to be based on a catchment approach. Constructed wetlands might be used for treating agricultural non-point sources of pollution (Hammer, 1992; Jenssen et al., 1994), as well as wastewater (Hammer, 1989). Generally the catchment would be a proper way for mastering nonpoint, as well as point sources in the agricultural landscape. For gases, the catchment offers about the same problem as for the farm, but the total of liquid phase and gaseous phase balances would be more easily accountable in a catchment.
7. Soil and water management Clearing the natural vegetation, land levelling, drainage and soil cultivation are important changes in the natural landscape. Drainage and tillage both increase the aeration and thereby the mineralization of organic matter. Drainage and the closing of natural waterways have removed many small lakes, ponds, streams, swamps and other natural wetlands. Their self-purifying processes, such as trapping suspended soil material with phosphorus and removing excess nitrogen through denitrification, as well as trapping nutrients through plant and algae growth and fish production, have been important for keeping surface waters relatively unpolluted. An actual measure would be to re-open closed waterways, construct ponds, dams (for irrigation water and sediment traps), construct artificial wetlands and establish border zones with permanent vegetation close to streams. The required area for such water conservation measures is not excessive. It is easier to limit the necessary area for a self-cleaning waterscape in a region with more relief than in a flat landscape. It seems rational to choose, as a management unit, the catchment of the nearest natural waterway for planning measures to conserve soil and water resources and prevent pollution of the larger waterways and rivers. A future agricultural landscape would consist of production units, based on natural production capacity, with a regulated intensity of hydrotechnical and agricultural inputs, based on the risk of soil and nutrient loss, and conservation units for trapping moving resources and for self-purification of surface and drainage waters. This calls for cooperation and organization of the
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farmers, such that they take responsibility for their catchment and waterway, and a government policy of incentives to maintain sustainability management. At farm level (and at catchment level) the 'farming by soil' concept outlined by Larson and Robert ( 1991 ) is a strong possibility for the future. This would involve the application of intensity according to digitized soil and land characteristics. The spatial variability of soil parameters is enormous, and the only way to handle it is by using the methods for information technology. Farming by soil can increase the profitability, reduce the potential for chemical leaching and reduce the potential for erosion and sedimentation on parts of the landscape. According to Larson and Robert (1991 ) the tools for farming by soil are digitized soil maps, a ground position system, fertilizer application implements with automatic adjustment to soil requirements and a microcomputer.
8. Soil management for sustainability on a World scale
Stewart et al. ( 1991 ) adapted a model by Hornick and Parr ( 1987 ), based on soil degradation processes, soil productivity and soil conservation processes to develop a "Difficulty of achieving sustainability" index for selected sites around the World. The calculated indices showed the greatest difficulty for hot and dry climates and the least for humid and cool regions. This seems to be an interesting approach. In arid and semi-add regions, lack of water is a serious limitation not only because of the importance of water in the production, but also because of the low half-life of organic matter and the potential problems of salinity and alkalinity with time, especially on the more fine-textured soils. In the Sudano-Sahel region up to 50% of the organic matter may be composed of plant residues at various stages of humification (Feller et al., 1983). The lack of fine particles tends to increase the rate of organic matter decomposition. In dry savannas of Senegal and Burkina Faso, Pied (1989) found an annual decomposition rate of 4% in sandy soils and 2% in loamy sands. Over-application of irrigation water is the main reason for salinity problems (and eventually alkalinity problems) developing in depressions where the ground water levels tend to be high. In the humid tropics the forested areas are often characterized by an accumulation of soil organic matter in the topsoil as a result of litter fall and the superficial nature of forest tree roots, and when soils are brought under cultivation the organic matter decreases rapidly (Sanchez, 1976). Greenland and Nye (1959) give an example from a maize-cassava rotation in Ghana, where the decomposition rate of organic matter was 4.7% of the total organic matter per year, and from a dean-weeded bare fallow in Zaire, where the decomposition rate was 12.8% per year. Weeds are a problem, and minimum tillage or no-till methods of soil cultivation, therefore, are correspondingly difficult to use successfully. As Lal ( 1982 ) has pointed out, these techniques may be feasible only under a high level of management. Trees, and in many cases legume trees, seem to be well adapted to the tropical environment. Scientists of IITA (Mulungoy and Akobundu, 1992 ) have
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calculated that leguminous woody species may fix between 130 and 270 kg N ha-~ and that in alley cropping, hedgerow prunings may contain even more. As B.T. Kang (personal communication, 1992) of IITA pointed out, farmers' willingness to adopt agroforestry may be a question of alternatives. In regions with subsistence agriculture, with no nearby markets and poor accessibility, farmers have shown greater interest in switching from shifting cultivation to agroforestry than in areas with good infrastructure and some markets. However, IITA ( 1993 ) maintains its push for alley farming as the most promising technology yet developed for stabilizing agriculture in the humid lowland tropics, and possibly in other zones. Bushy crops, such as cassava, are also interesting in the humid tropics. Cassava is especially important to resource-poor farmers in less well endowed areas, is tolerant to poor soils and drought and its low and flexible labour requirement make it an ideal component in mixed systems, mainly devoted to other more demanding crops (IITA, 1993 ). In the moist savannas with precipitation around 800-1500 mm year -~, conditions are often suitable for a maize-legume rotation. Under annual crops in savanna areas of Ghana, Senegal and Sudan, Greenland and Nye (1959) found a decomposition rate for soil organic matter of 2.5-6.6% of the total organic matter per year. In Nigeria the rotation maize-soybean in the Guinea savanna region has recently been rather successful according to IITA (1992). This region is the African equivalent of the American Mid-West. It is certainly well worth following up the potential use of this agroecological zone. However, it is important to underline that there are many caveats. If prices of inputs were to rise, it may well be that the system would not prove sustainable. It is to be expected that there would be an increasing need for further additions of inputs, such as micronutrients, lime, and other macronutrients in addition to ordinary NPK fertilizers. In semi-arid landscapes, the length of the growing season is generally limited by the amount and reliability of rainfall. Risk of losses are due to water and wind erosion, and even leaching after heavy, long-lasting rains. For Zimbabwe, FAO (1986) presented figures for the losses of nitrogen, organic C and phosphorus by different farming groups (Table 1 ). The annual soil losses varied from 3 to 75 t ha-~. The total annual cost for N and P losses of all soils was calculated at 2.5 billion US dollars. Over-grazing may very often be the main reason for soil erosion in semi-arid regions. The structure Table 1 Total losses of nitrogen, organic carbon and phosphorus in Zimbabwean soils as related to land use Group
Commercial grazing Commercial arable Communal arable Communal grazing
Annual loss in 1000 t Nitrogen
Organic carbon
Phosphorus
57 19 134 1425
454 1548 1324 13679
5 2 20 209
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of property rights (individual ownership of the animals and c o m m o n property rights to limited areas of land) is a likelyexplanation of over-grazing. In the US, Allmaxas et al. ( 1991 ) investigated the use of conservation tillage and planting. They observed that the surface cover of crop residue, shallow burial of residue and surface roughness wcrc all important factors for controlling water and wind erosion and improving soilwater relations.Furthcrmorc, it will bc important to control ground water contamination resulting from the use of agrochemicals. The types of plant disease and the interaction of disease treatment with residues arc important for the total effectof the tillagesystem on yield and the environment. It is expected that there will be an increased area of combined chemical and mechanical wccd control. Lal ct al. (1990) asserted that conservation tillageis a package of cultural practices that arc specificallydeveloped and adopted to conserve soil and water resources, sustain high and satisfactory returns, minimize degradation of soiland environment and maintain the resource base. For the World as a whole, the greatest threats to sustainability arc probably intensive cultivation of arabic fieldsand over-grazing of communal land. In the future it may bc necessary to return to a crop rotation system where perennial crops are interchanged with annuals on many areas used for cereals,cotton and soybeans. The measures for increased sustainabilitygiven by the U S Food SecurityAct (USDA, 1988) e.g.conservation rcscrvc,compliance soiltillage,etc.may bc a model to build on. There is a general nccd to develop a differentiatedsystem for soilmanagement, and a government incentive system that makes conservation measures attractive to farmers. The landscape/catchment planning must bc based on good information about agroecological factors,such as climate, terrain and soilsand information on socioeconomic factors, such as property rights and structure, education level, credits, markets, information on the public's interest of acccssibility for recreation, the nccd for conservation areas for water, plants and animals and the long-term effects of different management measures on the productivity and on the environment. Considering the long-term effectsof different soil management systems, these effectsmay show up quickly under humid tropical conditions, but more slowly in cool or temperate climates. One experiment on a clay soil in the cool climate of Norway, at 59 ° North, with autumn and spring ploughing, stubble treatment and straw treatment, showed no negative effectof spring ploughing during the first6 years. However, for the next 14 years there was a steady long-term yield decrease of the spring-ploughed treatments, especially on plots with straw and no stubble treatment in autumn (Njos and Borrcscn, 1991 ). In a ploughing depth experiment which lasted from 1940 to 1990 (Borrcscn and Njos, 1994), thcrc wcrc slightdifferences in yields during the first7-8 years. Over the whole period of the experiment, however, the shallow ploughing (12 cm) reduced yields considerably as compared with the 18 c m depth (considered normal, at the start of the experiment), and cvcn more so when related to the 24-cm ploughing depth. An interesting result was that the organic matter content down to 40 c m was not
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influenced by ploughing depth. For the shallowest ploughing depth ( 12 cm), the organic matter was highest in the upper layer (0-12 cm ); for the deepest ploughing (24 cm), the organic matter was highest in the 18-24 cm layer.
9. Other possibilities One major breakthrough for soil conservation and hence sustainability might come from genetic engineering methods, if these methods could be used to produce perennial grain crops giving high yields without annual soil cultivation and sowing of seeds. Dry periods, during the early establishment of annual crops with subsequent heavy rains, may cause uneven grain quality because of late tillers, less yields than fertilized for, and heavy leaching losses (resulting in eutrophication of water courses). A good agronomic practice, both for productivity and conservation, would be to add water at the time of tillering for small grain crops and to split the nitrogen application in portions according to water supply and crop development. The author observed a similar type of irrigation for winter wheat on the Ganges Plain. Lal et al. (1990) maintained that the population growth would lead to intensification of agriculture to produce sufficient food. Leach (1976) pointed out that the energy productivity in low input crop production generally is higher than in intensive agriculture. The latter productivity is still higher than unity. Transferring biological nitrogen fixation properties to major cereal crops might be a reasonable expectation for future plant improvement work, as would be perennial grain crops. The latter would certainly be a major contribution towards reduced soil erosion.
10. Long-term experiments--the base for planning a sustainable management system Sustainable agricultural production is difficult to envisage without proper sets of data for inputs in the productivity calculations. The classical experiments started by Sir John Lawes at Rothamsted (in cooperation with Sir Henry Gilbert) celebrated its 150th anniversary in 1993. As pointed out by Southwood (1993) the value of long-term experiments in agriculture is the ability to study: (i) Variation in crop yield with time, and the effects of new agricultural methods, as well as environmental changes. (ii) The way in which patterns of diseases and pests change with time. (iii) Changes in soil parameters, including pH, organic matter content and pollutants derived from contaminants in fertilizers or manures, or changed inputs from e.g. the atmosphere.
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(iv) Nutrient cycles in soils to identify the factors affecting the efficiency of nutrient use as well as the processes that allow nutrients to be lost to the environment. He also mentioned that long-term data sets have been used for statistical ecology based on the changes in relationships between species over time, and the development of many statistical methods by e.g. Fisher and Yates, who worked at Rothamsted. The only practical way to test the sustainability of an agricultural system, and the external and internal factors affecting sustainability, are continuous, longterm experiments (Johnston, 1993 ). He maintained that such experiments must (a) test modifications to enhance productivity or maintain sustainability; (b) provide a resource of soil and plant material, including archived samples, for research into soil and plant processes; (c) assess non-agricultural activities on soil fertility and crop quality; (d) provide sets of data for mathematical modelling. There are some important long-term experiments outside Rothamsted, such as the Morrow Plots in Illinois, USA from 1876, the Sanborn Field Experiment in Missouri, USA from 1888, the Magruder plots in Oklahoma, USA from 1892, the Permanent Rotation Trial at the Waite Agricultural Research Institute, Adelaide, Australia from 1926, Halle, Germany 1878, Askov, Denmark from 1894, Moscow, Russia from 1912 and Skierniewice, Poland from 1923. In developing countries, very few experiments have lasted longer than 25 years (Greenland, 1993 ). The difficulty of financing long-term experiments and measurement series has probably increased with today's project-based thinking. To overcome this problem is a great challenge for the research community within soil productivity research.
11. Concluding remarks There are three paradoxes in agriculture: - surplus production versus malnutrition; - technological advances versus low income; - agricultural subsidies versus benefits of free trade (Hjortshoj-Nielsen, 1988). It is very difficicult to reconcile these paradoxes and, by adding a fourth problem, productivty versus environmental effects, agriculture becomes difficult to plan and manage. The last problem might be approached by combining the sustainability with a suitable management unit. The boundaries of the management unit should be widened from the farm to the catchment of a waterway, such as a stream, of an area of, say, 0.5-10.0 km ~. This unit would be suitable for management of long-term productivity and maintenance of the quality of the environment. Soil, water and nutrient balances within the catchment would give us the necessary inputs for a sustainability calculation based on aggregated indexes for inputs and outputs. The catchment should be subdivided into production zones according to land
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suitability for different intensities, and zones for conservation, recreation and self-purification for catching and controlling losses of soil and nutrients, and for converting these losses to recirculation. Intensities would include drainage, irrigation, cultivation and fertilization, according to crop requirement and land potential. Long-term experiments and long-term monitoring are necessary methods for achieving and maintaining a productive and sustainable agriculture. References Allmaras, R.R., Langdale, G.W., Unger, P.W., Dowdy, R.H. and van Doren, D., 1991. Adoption of conservation tillage and associated planting system. In: R. Lal and F.J. Pierce (Editors), Soil Management for Sustainability. Soil and Water Conservation Society, Iowa, pp. 53-83. Blaxter, K.L., 1972. The limits to agricultural improvement. Univ. Newcastle Agric. Soc., 1972-74: 3. Borresen, T. and Njos, A., 1994. The effect on soil properties and crop yields of ploughing depth and seedbed preparation from 1940 to 1990 on a loam soil in South Eastern Norway, Soil Tillage Res., submitted. Carter, V.G. and Dale, T., 1974. Topsoil and civilization. University of Oklahoma Press, 292 pp. Chambers, J.D. and Mingay, G.E., 1970. The agricultural revolution 1750-1880. (First publ. 1966) B.T. Batsford Ltd., London, 222 pp. Christensen, L.R., 1975. Concepts and measurements of agricultural productivity. Am. J. Agric. Econ., 57: 910-915. Clark, C. and Haswell, M., 1970. The economics of subsistence agriculture. (First publ. 1964) Macmillan & Co. Ltd., 267 pp. Consultative Group on InternationalAgricultural Research (CGIAR), 1988. Sustainable agricultural production: Implications for internationalagricultural research. TAC Report (Adr/FAC: IAR18722 Rev.2), The World Bank, Washington D.C. Conway, G.R., 1985. Agroecosystem analysis. Agric. Admin., 20: 31-55. Dahlberg, K.A., 1993. Regenerative food systems: broadening the scope and agenda of sustainability. In: P. Allen (Editor), Food for the Future: Conditions and Contradictions of Sustainability, John Wiley & Sons Inc., pp. 75-102. Ehui, S.K. and Spencer, D.S.C., 1990. Indices for measuring the sustainabilityand economic viability of farming. RCMP Monograph 3, IITA, Ibadan, Nigeria 1990, 28 pp. Feller, C. Bernhardt-Reversat, F., Garcia, J.L., Pantier, J.J, Roussos, S. and van Vliet-Lanoe, B., 1983. l~tude de la matirre organique de diffbrentes fractions granulomrtriques d'un sol sableux tropical. Effet d'un amendement organique (compost). Cah. ORSTOM, 20(3): 223-238. Food and Agriculture Organisation (FAO), United Nations, 1986. The cost of soil erosion in Zimbabwe in terms of the loss of three major nutrients. Consultants working paper No. 3, AGLS, FAO, Rome. Greenland, D., 1993. Long-term experiments in developing countries: History and needs. Rothamsted 150th Anniversary Conference. Abstract. Greenland, D.J. and Nye, P.H., 1959. Increases in carbon and nitrogen contents of tropical soils under natural fallows. J. Soil Sci., 9: 284-289. Hammer, D.A., (Editor), 1989. Constructed Wetlands for Wastewater Treatment - Municipal, Industrial and Agricultural. Proceedings from the First International Conference on Constructed Wetlands for Wastewater Treatment, Chattanooga, Tennessee, 1988. Hammer, D.A., 1992. Designing constructed wetlands systems to treat agricultural nonpoint source pollution. Eco. Eng., 1:49-82 Hannerberg, D., 1971. Svenskt agrarsamh~ille under 1200 hr. G~rd och hker. Skrrd och boskap. L~iromedelsforlagen, Stockholm, 131 pp.
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Hjortshoj-Nielsen, A., 1988. Demand, production, restriction and policies. An economic view of the role and possibilities for agriculture in Europe. K. Skogs - och Lantbr. akad. Tidskr. Suppl., 21: 73-83. Hornick, S.B. and Parr, J.F., 1987. Restoring the productivity of marginal soils with organic amendments. Am. J. Alternative Agric., 2: 64-68. IITA (International Institute of Tropical Agriculture), 1992. Annual report. IITA, Ibadan, Nigeria. llTA (International Institute of Tropical Agriculture), 1993. Unlocking Africa's potential, IITA, lbadan, Nigeria. IITA Medium Term Plan, 1994-1998. lsraelsen, O.W., 1950. Irrigation principles and practices. 2nd edn., John Wiley & Sons Inc., New York. Jenssen, P.D., M~ehlum,T., Roseth, R., Braskerud, B., Syversen, N., Njos, A. and Krogstad, T., 1994. The potential of self purifying measures. ENS-93. Marine Poll. Bull., in press. Johnston, A.E., 1993. The Rothamsted "Classical" Experiments. Rothamsted 150th Anniversary Conference. Abstract. Lal, R., 1982. No-till farming. Monograph 2. International Institute of Tropical Agriculture, Ibadan, Nigeria, 68 pp. Lal, R., Eekert, D.J., Fausey, N.R. and Edwards, W.M., 1990. Conservation tillage in sustainable agriculture. In: C.A. Edwards, R. Lal, P. Madden, R.H. Miller and G. House (Editors), Sustainable Agricultural Systems, Soil and Water Conservation Society, Iowa. Larson, W.E. and Robert, P.C., 1991. Farming by soil. In: R. Lal and F.J. Pierce (Editors), Soil Management for Sustainability, Soil and Water Conservation Society, Iowa, pp. 103-112. Leach, G., 1976. Energy and food production. IPC Science and Technology Press Ltd., UK. Lynam, J.K. and Herdt, R.W., 1989. Sense and sustainability: Sustainabilityas an objective in international agricultural research. Agric. Econ., 3:381-398. Mulungoy, K. and Akobundu, I.O., 1992. Agronomic benefits of N contributed by legumes in live mulch and alley cropping systems. IITA Research Bulletin, March, 1992. Njos, A. and Borresen, T., 1991. Long-term experiment with straw management, stubble cultivation, autumn and spring ploughing on a clay soil in S.E. Norway. Soil Tillage Res., 21: 53-66. Oram, P.A., 1988. Building the agroecological framework. Environment (Special Topic: Sustainable Agriculture in Developing Countries). 30 ( 9 ): 14-17, 30- 36. Pieri, C., 1989. Fertilit6 des terres de savane. Bilan de trente ans de recherche et de d6veloppement agricole au Sud du Sahara. Minist6re de la Coop6ration et CIRAD-IRAT, Paris, 444 pp. Ruttan, V.W., 1988. Sustainability is not enough. Am. J. Alternative Agric., 3 (2/3 ): 128-130. Sanchez, P.A., 1976. Properties and Management of Soils in the Tropics. J. Wiley & Sons Inc_New York, 618 pp. Sandor, J.A. and Eash, N.S., 1991. Significance of ancient agricultural soils for long-term agronomic studies and sustainable agriculture research. Agron. J., 83: 29-37. Southwood, R., 1993. The importance of long-term experiments. Rothamsted 150th Anniversary Conference. Abstract. Stewart, B.A., Lal, R. and E1-Swaify,S.A., 1991. Sustaining the resource base of an expanding World agriculture. In: R. Lal and F.J. Pierce (Editors), Soil Management for Sustainability, Soil and Water Conservation Society, Iowa, pp. 125-144. Stout, B.A., Myers, C.R., Hurand, A. and Faidley, L.W., 1979. Energy for World Agriculture. FAO Agric. Series No. 7, Rome. Thomas, H., 1989. An Unfinished History of the World. Pan Books, London, 794 pp. USDA, 1988. National Food Security Act. Title 180, 2nd edn., Part 510-518, USDA, SCS Washington D.C. World Commission on Environment and Development, Gen6ve, 1987. Our Common Future. Oxford University Press.
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Soil & Tillage . Researcn
Future land utilization and management for sustainable crop production A r n o r Njos JORDFORSK, Centre for Soil and Environmental Research, N-1432 .~s, Norway
(Accepted 21 December 1993)
Abstract Soil cultivation caused a great change in the ecological state, from a situation with the soil surface protected by vegetation to a sitation with bare soil exposed to climatic forces during part of the year. Annual crops require annual soil cultivation, and the increased mineralization of organic matter and the loss of soil productivity by erosion, leaching and other degradation processes remain a problem for sustainable food production. The challenge for the future is to manage the agricultural landscape in units which are catchments. These units would contain intensively drained and cultivated areas, permanent vegetation zones, as well as natural and constructed wetlands working as condensors and self-purifying units. The management would be differentiated according to a land suitability approach with intensity of use based on productivity and requirements for maintenance of environmental quality. Key words: Catchment;Management;Productivity;Sustainability;Unit
1. Introduction In agricultural history there have been several technological breakthroughs, sometimes called agricultural revolutions (Blaxter, 1972 ). According to Thomas (1989) the first agricultural revolution occurred between about 12 BC and 7000 BC and involved several steps towards settled agriculture, such as the introduction of tillage to create a seedbed for cultivable plants. Thomas (1989) used the wording "robust historians" about authors who spoke of revolution, when the change might have taken such a long time. The tillage was very simple, scratching the surface for placing the seeds. Early ploughs were little more than hoes. In: Egypt a two-man plough, with one man pulling a rope in front and a second m a n 0167-1987/94/$07.00 © 1994 ElsevierScienceB.V. All fightsreserved SSDIO167-1987(94)O8029-Y
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pressing the plough's point into the ground, was used around 7000 BC. It became evident to the early farmers that the competition from weeds was serious for the survival of the cultivated plants, and tillage was necessary to create a seedbed and to eliminate or decrease the competition from weeds. The preparation of a seedbed for crops is probably the major break with a natural or ecological balance, if it ever existed. If the cultivation started on a steppe or river plain, clearing was simpler and less laborious than in a forest. Energy was now spent on a small area to increase its productivity. Previously, energy had been used for hunting. According to Stout et al. (1979), hunters and gatherers would need an area of 1.5 km 2 per person for survival under good conditions, while under poor conditions 60-80 km 2 would be needed. Clark and Haswell (1970), after collecting material from several authors, state that in cold climate the Eskimos and Indians of north western Canada would need about 140 km 2 per head, while on the American prairies 25 km 2 would suffice. On the shores of rivers rich in salmon the figure might fall to 1.5 km 2. Pastoral people in East Africa would need 0.5 to 1.5 km 2 per person, and settled agricultarists with shifting cultivation around 0.15 km 2 per person. Generally, from the introduction of soil cultivation and seeding, agriculture might be defined as a planned and implemented ecological imbalance to increase the productivity per unit area by introducing more energy into the system. In millenia after the first agricultural revolution, cultivated fields were generally permanent in the better agroecological regions, and grain yields varied around two to four fold (two to four times the quantity of seeds sown). This would correspond to a yield of about 0.4-0.8 t h a - 1 (Hannerberg 1971; Blaxter, 1972 ). In many parts of Europe, with the three field system (2 years of grain, 1 year of rest) the yields on the average soils were not above this. On the large river plains of Asia and Europe and in the Nile valley of Northern Africa yields were higher, for example, 1.0-1.5 t ha-1. The same applied to volcanic soils. For the river valley soils in ancient Egypt a figure of 2 t h a - 1 has been estimated (Clark and Haswell, 1970). The supply of plant nutrients was the limiting factor in the subhumid and humid areas; water in the semiarid and arid areas.
2. The crop rotation system in Europe The setting for the second agricultural revolution was the industrial revolution and the growth in Europe's population. With increasing demand for food, a larger area had to be cultivated or the productivity per unit area had to be increased. In this case the result was increased productivity per unit area. Legumes, such as clover, were grown for forage and to supply nitrogen for next year's crop. A new agricultural technology developed, based on the turning effect of the steel moldboard plough, and crops grown in a time sequence in the same field. Crops, such as cereals and root crops were grown in a sequence with a legume-grassland ley. In Europe, England is considered the home of crop rotation agriculture, because of the Norfolk crop rotation. A good description of this agricultural revolution is
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given by Chambers and Mingay (1970). The steel moldboard plough has become more or less the symbol of the crop rotation agricultural system. Grain yields increased to 1.5-2.0 t h a - ~ (Blaxter, 1972). Looking at this period in retrospect, many people feel that this was the era of a sustainable agriculture and of an ecologically balanced system. However, the system was not entirely balanced. Soil cultivation, and the turning of the sod, contributed to mineralization of the organic matter conserved during the sod years. This showed up as a gradual decrease in soil organic matter with time. The human labour was partly replaced with animal horse power. Fertilizers and lime were gradually added as inputs. Fields were drained, which further increased the mineralization of organic matter.
3. The development of agricultural technology since 1940 The third agricultural revolution, from about 1940 onwards (also called the Chemistry Revolution), was characterized by the tremendous rise in yields by chemical and other inputs, a strong decrease in human labour and a replacement of animal power with full mechanization. The input of energy by fertilizers, pesticides, mechanical power, drying, irrigation and drainage increased many times. The crop rotation of grain-rootcrops-grain-ley yielded to grain-grain or grassgrass. An important change to the ecological balance was the drainage of wetlands. The wetlands, open ditches and small ponds and lakes are, in a broad sense, the natural elements of self-purifying mechanisms in the agricultural landscape. The mineralization process was speeded up simultaneously by increased cultivation and drainage. The infiltration rate and the structural stability of the annually cultivated soils decreased and was followed by erosion. In the drier areas of the World, improper irrigation often resulted in rising water tables in depressions, giving rise to salinization and alkalinization (see, for instance, Israelsen, 1950 or Carter and Dale, 1974). The Chemistry Revolution has raised serious questions about environment, ecological balance and sustainability. This issue was brought forward through the Brundtland report "Our Common Future" (World Commission on Environment and Development, 1987). Before discussing the question and philosophy of sustainabillity, it should be mentioned that the present developments in agriculture might be expressed as the fourth agricultural revolution or the 'Biology Revolution'. The developments within biotechnology, genetic engineering and information technology are forceful tools for a new type of agriculture, where inputs of agrochemicals might decrease and thereby energy input per unit produced.
4. Sustainability----definitions The expression sustainability in many ways is similar to bearing capacity or to common expressions about good husbandry. In German, "Haltbarkeit" would
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provide a reasonable cover of the meaning. In French "durabititr" would probably be close to sustainability. There are several definitions of sustainability. (a) Conway (1985 ): The ability of a system to maintain productivity in spite of a major disturbance, such as caused by intensive stress or a large perturbation. (b) Oram (1988 ): A system's ability to maintain productivity when exposed to external stress or shock. (c) Consultative Group on International Agricultural Research, (1988). The successful management of resources for agriculture to satisfy the changing human needs, while maintaining or enhancing the quality of the environment and conserving natural resources. Some authors have stressed the ability to survive 'a shock' while others stress the environmental aspect. Should the system be strong enough to survive natural disasters (such as pests and large floods), volcanic eruptions (such as Lakagigar in Iceland in 1783 ) or human induced shocks (such as the 30-Years War in Europe)? It is easy to agree that a system cannot be expected to endure a major climatic change, such as an ice age. Thus, a reasonable requirement might be that the system should be able to sustain normal variations over, say, a generation or a century. Lynam and Herdt (1989) included the system's ability to maintain production at an average historical level within historical variability. Ruttan ( 1988 ) proposed to include the requirement that a technology is developed which maintains and/or enhances the quality of soil and water resources, improves plant and animal materials and paves the way for a transition from a chemical to a biological technology. The inclusion of the environmental requirements might be seen as a new addition to the old perception of good husbandry, which was summarized in the proverb "leave the farm to the next generation in a better shape than it was received". Consider what has happened in the Nile catchment through some thousand years. The erosion in Ethiopia was always considerable and the loss of soil was the base for the fertility of the river plain and the delta of the Nile. Similar examples are found in many river plains in Asia, such as Mesopotamia, the Indus and Ganges floodplains on the Indian Sub-continent and the large river plains in China. Taking the World as a whole, most agricultural civilizations have not survived more than 30 generations (Carter and Dale, 1974). However, the river plains together with some volcanic areas and the irrigated rice cultures in the East Asia have persisted for longer. The causes for persistance are generally a good and regular water supply and some mechanism of refreshing the nutrient supply, for instance through flooding or volcanic ashes. There are interesting long-term effects of ancient agriculture on soils. Sandor and Eash ( 1991 ) report lower organic C values for the upper layers of previously cultivated terrace soils than for uncultivated soils in New Mexico, although cultivation ceased more than 800 years ago. The authors also describe work on ancient terrace soils in Peru. In the Colca valley in Peru, agricultural terrace soils have properties indicating favourable tilth and fertility after 1500 years of cultivation. In this case the cultivated soils had a higher organic C content than their uncultivated counterparts. Traditional soil and crop management practices might
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have been responsible for this condition. The soils were mollisols in the study area in New Mexico as well as in the study area in Peru. In New Mexico the mollisols had a very strong argiUic horizon with a shallow upper boundary. In Peru the farming system actually was a crop rotation system with tillage being done with a chisel-like plough working shallowly and not inverting the soil (Sandot and Eash, 1991 ). Although Carter and Dale (1974) stated that most agricultural civilizations did not last more than 30 generations because of soil degradation, well-known exceptions took place on river plains and volcanic soils.
5. Sustainability and productivity Productivity is the base for sustainability. Productivity is the ratio of output to input in a given system, where inputs and outputs must be measured in the same units. Economic viability is the criterion used to compare the productivity of several systems at a given time. The time derivative of the productivity is a measure of sustainability. The basic idea behind the concept of a sustainable system is that of productivity which does not decrease with time. There are ways of solving the differential equation that results from the time derivative of the productivity. The solution is likely to contain the natural logarithms of outputs and inputs. Very often the difficulty is the handling of an aggregated index for products, as well as an aggregated index for inputs. Christensen (1975 ) used actual prices in different years for weighting. Ehui and Spencer (1990) calculated sustainabilities for tropical farming systems, including also the loss of soil fertility estimated by the decrease in soil nutrients. The latter would be a change in resource base. Similarly, if erosion were a problem, soil losses might be recalculated as losses of nutrients and available water storage. Dahlberg (1993) also includes ecology, ethics and equity in a concept which he describes as regenerative systems. If ethics and equity are included, the sustainability concept would not be described sufficiently by productivity.
6. The boundaries of the system Setting the boundaries at the farm fence, the system is the farm. If there were to be no pressure from the outside world, this inner world would continue to produce with regard to its own resources, such as the upper metre of the soil, the slopes and depressions and the small streams. The environment would be the local landscape, the air, the unpredictable weather with too little or too much o f everything. The ground water would be outside the range of interest except for water supply, and the volumes of gases leaving as carbon dioxide and ammonia would also be outside the system. The system might also be the catchment. Here, it is practical to think of a catchment of a lake, stream, canal or a tile drainage system of a few square kilometres, mainly with agricultural land and activities, with a mixture of forests, arable fields
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and pastures. The quality of the water, measured at the outlet of the catchment, would be a measure of the health of the system and its potential influence on the environment, which might be a larger watershed. The main difference from the farm as a system would be that the measures against soil and nutrient losses could be based on a clear hydrology or agrohydrology concept. Transport of water within the catchment would include surface runoff, drainage discharge through canals or tile systems, or discharge from ground water to a stream, the total discharge being accountable at the outlet of the system. The inputs would be accountable both for the farm and the catchment, but the total effect on the water leaving the area would not be easy to associate with the farm. Self-purifying processes, such as trapping sediments and nutrients in lakes, streams, ponds, natural and planted vegetation zones and constructed wetlands as well as denitrification in shallow waterways could operate both at the farm and catchment level, but the planning, implementing and monitoring would have to be based on a catchment approach. Constructed wetlands might be used for treating agricultural non-point sources of pollution (Hammer, 1992; Jenssen et al., 1994), as well as wastewater (Hammer, 1989). Generally the catchment would be a proper way for mastering nonpoint, as well as point sources in the agricultural landscape. For gases, the catchment offers about the same problem as for the farm, but the total of liquid phase and gaseous phase balances would be more easily accountable in a catchment.
7. Soil and water management Clearing the natural vegetation, land levelling, drainage and soil cultivation are important changes in the natural landscape. Drainage and tillage both increase the aeration and thereby the mineralization of organic matter. Drainage and the closing of natural waterways have removed many small lakes, ponds, streams, swamps and other natural wetlands. Their self-purifying processes, such as trapping suspended soil material with phosphorus and removing excess nitrogen through denitrification, as well as trapping nutrients through plant and algae growth and fish production, have been important for keeping surface waters relatively unpolluted. An actual measure would be to re-open closed waterways, construct ponds, dams (for irrigation water and sediment traps), construct artificial wetlands and establish border zones with permanent vegetation close to streams. The required area for such water conservation measures is not excessive. It is easier to limit the necessary area for a self-cleaning waterscape in a region with more relief than in a flat landscape. It seems rational to choose, as a management unit, the catchment of the nearest natural waterway for planning measures to conserve soil and water resources and prevent pollution of the larger waterways and rivers. A future agricultural landscape would consist of production units, based on natural production capacity, with a regulated intensity of hydrotechnical and agricultural inputs, based on the risk of soil and nutrient loss, and conservation units for trapping moving resources and for self-purification of surface and drainage waters. This calls for cooperation and organization of the
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farmers, such that they take responsibility for their catchment and waterway, and a government policy of incentives to maintain sustainability management. At farm level (and at catchment level) the 'farming by soil' concept outlined by Larson and Robert ( 1991 ) is a strong possibility for the future. This would involve the application of intensity according to digitized soil and land characteristics. The spatial variability of soil parameters is enormous, and the only way to handle it is by using the methods for information technology. Farming by soil can increase the profitability, reduce the potential for chemical leaching and reduce the potential for erosion and sedimentation on parts of the landscape. According to Larson and Robert (1991 ) the tools for farming by soil are digitized soil maps, a ground position system, fertilizer application implements with automatic adjustment to soil requirements and a microcomputer.
8. Soil management for sustainability on a World scale
Stewart et al. ( 1991 ) adapted a model by Hornick and Parr ( 1987 ), based on soil degradation processes, soil productivity and soil conservation processes to develop a "Difficulty of achieving sustainability" index for selected sites around the World. The calculated indices showed the greatest difficulty for hot and dry climates and the least for humid and cool regions. This seems to be an interesting approach. In arid and semi-add regions, lack of water is a serious limitation not only because of the importance of water in the production, but also because of the low half-life of organic matter and the potential problems of salinity and alkalinity with time, especially on the more fine-textured soils. In the Sudano-Sahel region up to 50% of the organic matter may be composed of plant residues at various stages of humification (Feller et al., 1983). The lack of fine particles tends to increase the rate of organic matter decomposition. In dry savannas of Senegal and Burkina Faso, Pied (1989) found an annual decomposition rate of 4% in sandy soils and 2% in loamy sands. Over-application of irrigation water is the main reason for salinity problems (and eventually alkalinity problems) developing in depressions where the ground water levels tend to be high. In the humid tropics the forested areas are often characterized by an accumulation of soil organic matter in the topsoil as a result of litter fall and the superficial nature of forest tree roots, and when soils are brought under cultivation the organic matter decreases rapidly (Sanchez, 1976). Greenland and Nye (1959) give an example from a maize-cassava rotation in Ghana, where the decomposition rate of organic matter was 4.7% of the total organic matter per year, and from a dean-weeded bare fallow in Zaire, where the decomposition rate was 12.8% per year. Weeds are a problem, and minimum tillage or no-till methods of soil cultivation, therefore, are correspondingly difficult to use successfully. As Lal ( 1982 ) has pointed out, these techniques may be feasible only under a high level of management. Trees, and in many cases legume trees, seem to be well adapted to the tropical environment. Scientists of IITA (Mulungoy and Akobundu, 1992 ) have
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calculated that leguminous woody species may fix between 130 and 270 kg N ha-~ and that in alley cropping, hedgerow prunings may contain even more. As B.T. Kang (personal communication, 1992) of IITA pointed out, farmers' willingness to adopt agroforestry may be a question of alternatives. In regions with subsistence agriculture, with no nearby markets and poor accessibility, farmers have shown greater interest in switching from shifting cultivation to agroforestry than in areas with good infrastructure and some markets. However, IITA ( 1993 ) maintains its push for alley farming as the most promising technology yet developed for stabilizing agriculture in the humid lowland tropics, and possibly in other zones. Bushy crops, such as cassava, are also interesting in the humid tropics. Cassava is especially important to resource-poor farmers in less well endowed areas, is tolerant to poor soils and drought and its low and flexible labour requirement make it an ideal component in mixed systems, mainly devoted to other more demanding crops (IITA, 1993 ). In the moist savannas with precipitation around 800-1500 mm year -~, conditions are often suitable for a maize-legume rotation. Under annual crops in savanna areas of Ghana, Senegal and Sudan, Greenland and Nye (1959) found a decomposition rate for soil organic matter of 2.5-6.6% of the total organic matter per year. In Nigeria the rotation maize-soybean in the Guinea savanna region has recently been rather successful according to IITA (1992). This region is the African equivalent of the American Mid-West. It is certainly well worth following up the potential use of this agroecological zone. However, it is important to underline that there are many caveats. If prices of inputs were to rise, it may well be that the system would not prove sustainable. It is to be expected that there would be an increasing need for further additions of inputs, such as micronutrients, lime, and other macronutrients in addition to ordinary NPK fertilizers. In semi-arid landscapes, the length of the growing season is generally limited by the amount and reliability of rainfall. Risk of losses are due to water and wind erosion, and even leaching after heavy, long-lasting rains. For Zimbabwe, FAO (1986) presented figures for the losses of nitrogen, organic C and phosphorus by different farming groups (Table 1 ). The annual soil losses varied from 3 to 75 t ha-~. The total annual cost for N and P losses of all soils was calculated at 2.5 billion US dollars. Over-grazing may very often be the main reason for soil erosion in semi-arid regions. The structure Table 1 Total losses of nitrogen, organic carbon and phosphorus in Zimbabwean soils as related to land use Group
Commercial grazing Commercial arable Communal arable Communal grazing
Annual loss in 1000 t Nitrogen
Organic carbon
Phosphorus
57 19 134 1425
454 1548 1324 13679
5 2 20 209
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of property rights (individual ownership of the animals and c o m m o n property rights to limited areas of land) is a likelyexplanation of over-grazing. In the US, Allmaxas et al. ( 1991 ) investigated the use of conservation tillage and planting. They observed that the surface cover of crop residue, shallow burial of residue and surface roughness wcrc all important factors for controlling water and wind erosion and improving soilwater relations.Furthcrmorc, it will bc important to control ground water contamination resulting from the use of agrochemicals. The types of plant disease and the interaction of disease treatment with residues arc important for the total effectof the tillagesystem on yield and the environment. It is expected that there will be an increased area of combined chemical and mechanical wccd control. Lal ct al. (1990) asserted that conservation tillageis a package of cultural practices that arc specificallydeveloped and adopted to conserve soil and water resources, sustain high and satisfactory returns, minimize degradation of soiland environment and maintain the resource base. For the World as a whole, the greatest threats to sustainability arc probably intensive cultivation of arabic fieldsand over-grazing of communal land. In the future it may bc necessary to return to a crop rotation system where perennial crops are interchanged with annuals on many areas used for cereals,cotton and soybeans. The measures for increased sustainabilitygiven by the U S Food SecurityAct (USDA, 1988) e.g.conservation rcscrvc,compliance soiltillage,etc.may bc a model to build on. There is a general nccd to develop a differentiatedsystem for soilmanagement, and a government incentive system that makes conservation measures attractive to farmers. The landscape/catchment planning must bc based on good information about agroecological factors,such as climate, terrain and soilsand information on socioeconomic factors, such as property rights and structure, education level, credits, markets, information on the public's interest of acccssibility for recreation, the nccd for conservation areas for water, plants and animals and the long-term effects of different management measures on the productivity and on the environment. Considering the long-term effectsof different soil management systems, these effectsmay show up quickly under humid tropical conditions, but more slowly in cool or temperate climates. One experiment on a clay soil in the cool climate of Norway, at 59 ° North, with autumn and spring ploughing, stubble treatment and straw treatment, showed no negative effectof spring ploughing during the first6 years. However, for the next 14 years there was a steady long-term yield decrease of the spring-ploughed treatments, especially on plots with straw and no stubble treatment in autumn (Njos and Borrcscn, 1991 ). In a ploughing depth experiment which lasted from 1940 to 1990 (Borrcscn and Njos, 1994), thcrc wcrc slightdifferences in yields during the first7-8 years. Over the whole period of the experiment, however, the shallow ploughing (12 cm) reduced yields considerably as compared with the 18 c m depth (considered normal, at the start of the experiment), and cvcn more so when related to the 24-cm ploughing depth. An interesting result was that the organic matter content down to 40 c m was not
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influenced by ploughing depth. For the shallowest ploughing depth ( 12 cm), the organic matter was highest in the upper layer (0-12 cm ); for the deepest ploughing (24 cm), the organic matter was highest in the 18-24 cm layer.
9. Other possibilities One major breakthrough for soil conservation and hence sustainability might come from genetic engineering methods, if these methods could be used to produce perennial grain crops giving high yields without annual soil cultivation and sowing of seeds. Dry periods, during the early establishment of annual crops with subsequent heavy rains, may cause uneven grain quality because of late tillers, less yields than fertilized for, and heavy leaching losses (resulting in eutrophication of water courses). A good agronomic practice, both for productivity and conservation, would be to add water at the time of tillering for small grain crops and to split the nitrogen application in portions according to water supply and crop development. The author observed a similar type of irrigation for winter wheat on the Ganges Plain. Lal et al. (1990) maintained that the population growth would lead to intensification of agriculture to produce sufficient food. Leach (1976) pointed out that the energy productivity in low input crop production generally is higher than in intensive agriculture. The latter productivity is still higher than unity. Transferring biological nitrogen fixation properties to major cereal crops might be a reasonable expectation for future plant improvement work, as would be perennial grain crops. The latter would certainly be a major contribution towards reduced soil erosion.
10. Long-term experiments--the base for planning a sustainable management system Sustainable agricultural production is difficult to envisage without proper sets of data for inputs in the productivity calculations. The classical experiments started by Sir John Lawes at Rothamsted (in cooperation with Sir Henry Gilbert) celebrated its 150th anniversary in 1993. As pointed out by Southwood (1993) the value of long-term experiments in agriculture is the ability to study: (i) Variation in crop yield with time, and the effects of new agricultural methods, as well as environmental changes. (ii) The way in which patterns of diseases and pests change with time. (iii) Changes in soil parameters, including pH, organic matter content and pollutants derived from contaminants in fertilizers or manures, or changed inputs from e.g. the atmosphere.
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(iv) Nutrient cycles in soils to identify the factors affecting the efficiency of nutrient use as well as the processes that allow nutrients to be lost to the environment. He also mentioned that long-term data sets have been used for statistical ecology based on the changes in relationships between species over time, and the development of many statistical methods by e.g. Fisher and Yates, who worked at Rothamsted. The only practical way to test the sustainability of an agricultural system, and the external and internal factors affecting sustainability, are continuous, longterm experiments (Johnston, 1993 ). He maintained that such experiments must (a) test modifications to enhance productivity or maintain sustainability; (b) provide a resource of soil and plant material, including archived samples, for research into soil and plant processes; (c) assess non-agricultural activities on soil fertility and crop quality; (d) provide sets of data for mathematical modelling. There are some important long-term experiments outside Rothamsted, such as the Morrow Plots in Illinois, USA from 1876, the Sanborn Field Experiment in Missouri, USA from 1888, the Magruder plots in Oklahoma, USA from 1892, the Permanent Rotation Trial at the Waite Agricultural Research Institute, Adelaide, Australia from 1926, Halle, Germany 1878, Askov, Denmark from 1894, Moscow, Russia from 1912 and Skierniewice, Poland from 1923. In developing countries, very few experiments have lasted longer than 25 years (Greenland, 1993 ). The difficulty of financing long-term experiments and measurement series has probably increased with today's project-based thinking. To overcome this problem is a great challenge for the research community within soil productivity research.
11. Concluding remarks There are three paradoxes in agriculture: - surplus production versus malnutrition; - technological advances versus low income; - agricultural subsidies versus benefits of free trade (Hjortshoj-Nielsen, 1988). It is very difficicult to reconcile these paradoxes and, by adding a fourth problem, productivty versus environmental effects, agriculture becomes difficult to plan and manage. The last problem might be approached by combining the sustainability with a suitable management unit. The boundaries of the management unit should be widened from the farm to the catchment of a waterway, such as a stream, of an area of, say, 0.5-10.0 km ~. This unit would be suitable for management of long-term productivity and maintenance of the quality of the environment. Soil, water and nutrient balances within the catchment would give us the necessary inputs for a sustainability calculation based on aggregated indexes for inputs and outputs. The catchment should be subdivided into production zones according to land
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