- Email: [email protected]
The structure of two alluvial soils in Italy after 10 years of conventional and minimum tillage
soil
ELSEVIER
Soil & Tillage
Research
34 (1995)
209-223
8,
TUlase Research
The structure of two alluvial soils in Italy after 10 years of conventional and minimum tillage M. Pagliai a7*,M. Raglione b, T. Panini a, M. Maletta”, M. La Marcac a Istituto ’ Istituto
Sperimentale
per lo Studio e la Difesa de1 Suolo, 50121 Firenze, Speritnentale per lo Studio e la Difesa de1 Suolo, 02100 Rieti, ’ Istituto per la Chimica de1 Terreno, CNR, Accepted
Sezione di Fisica de1 Suolo, Piazza D’Azeglio 30, Italy Sezione di Conservazione de1 Suolo, Via Casette 1. Italy Via Corridoni 78. 56100 Piss, Italy
22 March
1995
Abstract
Micro and macroporosity, pore shapeand sizedistribution, aggregatestability, saturatedhydraulic conductivity and crop yield were analysed in alluvial silty loam (Fluventic Eutrochrept) and clay soils (Vertic Eutrochrept) following long-term minimum and conventional tillage. The soil structure attributes were evaluated by characterizingporosity by means of image analysisof soil thin sections prepared from undisturbed soil samples. The interaggregate microporosity, measured by mercury intrusion porosimetry, increasedin the minimally tilled soils, with a particular increasein the storage pores (0.550 pm). The amount of elongated transmissionpores (50-500 pm) also increasedin the minimally tilled soils.The resulting soil structure was more open and more homogeneous, thus allowing better water movement, as confirmed by the greater hydraulic conductivity of the minimally tilled soils.The aggregatestability was lessin the conventionally tilled soilsand this resultedin a greatertendency to form surfacecrusts and compacted structure, compared with the minimally tilled soils. The latter tillage practiceseemed to maintain, in the long-term, better soil structure conditions and, therefore, maintain favourable conditions for plant growth. In the silt loam, the crop yield did not differ significantly between the two tillage systems,while in the clay soil it decreasedin the minimum tilled soil becauseof problems of seedbed preparation at the higher surface layer water content. Keywords:
Soil porosity;
* Corresponding
Pore shape; Pore size distribution;
author:
0167-1987/95/$09.50 SSDIO167-1987(95)00471-8
Tel.
+ 39 55 249121 I; Fax. +39
0 1995 Elsevier
Science
Soil structure;
55 241485.
B.V. All rights
reserved
Soil tillage;
Image analysis
210
M. Pagliui
et al. /Soil
& Tilloge
Research
34 (1995)
209-223
1. Introduction
Intensive cultivation of some agricultural soils can lead to a deterioration in soil structure and other physical properties of the soil and, consequently, decreased crop yield (Catching et al., 1979; Marks and Soane, 1987). In addition, the need for tillage has been questioned, in the last few decades, partly because of the excessive erosion from farmland after tillage (Seta et al., 1993; Auerswald et al., 1994). Runoff from agricultural land may carry sediments, chemical fertilisers and pesticides, all of which can degrade water quality and upset the ecosystems of streams, lakes, reservoirs, and rivers. Research in this area frequently recommends the adoption of reduced tillage practices to prevent soil structural degradation, soil losses by erosion and to reduce consequent environmental impacts (Shipitalo and Protz, 1987; Pagliai et al., 1989). However, there are also some soil types that are unsuitable for minimum tillage, such as the clay soils developed on Pliocene marine clays (Typic Udorthent) which are so widespread in the hilly environment of central Italy (Pagliai and Pezzarossa, 1990). In order to evaluate the impact of management practices on the soil environment it is necessary to quantify the modifications to soil structure. Porosity is the best indicator of soil structure conditions and pore space measurements are increasingly being used to quantify soil structure because it is the size, shape and continuity of pores that affect many of the important processes in soils ( Ringrose-Voase and Bullock, 1984). A detailed insight into the complexity of the pore system in soil can be obtained by using mercury intrusion porosimetry to quantify pores less than 100 pm (equivalent pore diameter) inside the soil aggregates (Fies, 1992), and image analysis on thin sections prepared from undisturbed soil samples to quantify pores larger than 50 pm, i.e. macropores which determine the type of soil structure (Pagliai et al., 1983, 1984). Technological and theoretical advances, regarding both sample preparation and image analysis, have improved the methods for direct quantification of soil pores. Such methods allow the quantification of the effects of tillage practices on soil porosity and structure and in turn on the optimum tillage needs for sustainable agriculture (McBratney et al., 1992; Mermut et al., 1992; Moran and McBratney, 1992a,b). The aim of this research was to determine the effects of both long-term minimum tillage and conventional tillage on soil structural characteristics through the quantification of the differences in micro and macroporosity, aggregate stability, hydraulic conductivity and crop yield in two different soil types. The differences in porosity were determined not only in terms of total value but also in terms of size and shape distribution of pores and of their influence on soil structure morphology. The two soils investigated were a silt loam and a clay soil that are widespread and representative of the alluvial plains of central and south Italy. These soils are characterised by high vulnerability to structure degradation when intensively cultivated. It is well-known that in temperate areas, the silty and loamy soils are prone to crusting (Mucher and De Ploey, 1977) especially when cultivated (Pagliai et al., 1983, 1989). Similarly, the clay soil is characterised by the formation of large cracks and massive structure when dry, and by strong adhesiveness when wet, causing problems of workability.
M. Pagliai
et al. /Soil
& Tillage
Research
34 (199s)
209-223
211
2. Materials and methods 2.1. Soils and treatments A long-term field experiment was established in 1984 on two soil types. The first, named “Piedifiume”, was a silt loam soil classified as Calcaric Cambisol according to the Food and Agriculture Organization (FAO, 1988) and as Fluventic Eutrochrept following United States Department of Agriculture (USDA, 1985). The second, named “Casa Bianca” was a clay soil classified as Vertic Cambisol according to the FAO (1988) and as Vertic Eutrochrept following USDA ( 1985). Both soils are located on the experimental farm “San Pastore” near Rieti, Lazio, of the Institute for Soil Study and Conservation of Florence. Some characteristics of the soils are reported in Table 1. Both soils had the same crop rotations of maize-wheat. In the season 1992-1993 the crop was autumn-sown wheat (Triticum aestivum variety “Centauro”. Three replications of each of two management practices were tested with 330 m X 30 m plots on both soil types. The tillage treatments were minimum tillage (harrowing with a disc harrow to a depth of 10 cm) and conventional tillage (moldboard ploughing to a depth of 40 cm). Both the minimum and conventional tillage were applied at the end of October 1992, just after the maize harvest. In both treatments the same sowing machine was used for seeding. 2.2. Porosity measurements Six undisturbed samples were collected from the 0- 10 cm layer and from the 20-30 cm layer of each plot in July 1993 just before wheat harvest. After drying by acetone replacement of water (Miedema et al., 1974; Murphy, 1986) and impregnation with polyester resin (Crystic SR 17449, Scott Bader Co. Ltd., Wellingborough, UK), 6 cm X 7 cm vertically oriented thin sections were cut from each sample (Murphy, 1986). Photographs of the sections (Pagliai et al., 1984) were analysed by an image-analysing computer (Quantimet 570) to measure total porosity and to characterise pores according to their shape and size. These photographs covered 4.5 cm X 5.5 cm of the thin section, avoiding the edges where disruption could occur. The instrument was adjusted to measure pores larger than 50 pm. Pores were measured by their shape, which is expressed by the Table 1 Some characteristics
of the Ap horizon
of the soil types
Properties
Silt loam soil
Clay soil
Sand, 50 wrn-2 mm (g kg-‘) Silt, 2-50 wrn (g kg-‘) Clay, <2pm (g kg-‘) PH (W) Organic carbon (g kg-‘) Corbonate content (g kg - ’ ) CEC (meq kg-‘) Clay minerals
291 553 156 8.1 14.2 39 178 Smectite
84 416 500 7.7 17.2 123 356 Smectite
a Dominant
a
a
211
M. Pagliai
et al. /Soil
C? Tillage
Research
34 (1995)
209-223
shape factor (perimete?/4r( area) ) and divided into regular (more or less rounded) pores (shape factor l-2)) irregular pores (shape factor 2-5) and elongated pores (shape factor over 5). These classes correspond approximately to those used by Bouma et al. ( 1977). Pores in each shape class were further subdivided into size classes according to the equivalent pore diameter for regular and irregular pores and the width for elongated pores (Pagliai et al., 1983, 1984). Microstructure was also examined in the thin sections using a Zeiss R POL microscope at X 25 magnification. Six undisturbed samples were collected from the O-10 cm layer of each plot in the areas adjacent to those sampled for thin section preparation. Clods up to the size of 4 cm3 were air-dried and degassed prior to analysis using a Carlo Erba WS mercury intrusion porosimeter equipped with a Carlo Erba 120 macropore unit. The porosity and the pore size distribution were determined in the range 0.007-100 pm. 2.3. Aggregate stability A modification of the wet-sieving method of Malquori and Cecconi ( 1962) was used to determine the stability of the soil aggregates in water. Air-dried aggregates (l-2 mm) of soil were placed in 0.25 mm mesh sieves and moistened by capillary rise from a layer of wet sand, then immersed in deionised water and shaken with an alternative rotating movement (60 times min-‘) at room temperature. The water stability index (WSI) was calculated as lOO( l-A/B), where A and B are the weights of aggregates passing through the sieve after 5 and 60 min, respectively. Each determination was made at least in triplicate. 2.4. Saturated hydraulic conductivity and soil water content Six undisturbed cores (5.68 cm diameter, 9.5 cm high) were collected from the O-10 cm layer of each plot in areas adjacent to those sampled for thin section preparation. The samples were slowly saturated and the saturated hydraulic conductivity was measured using the falling-head technique (Klute and Dirksen, 1986). The gravimetric soil water content in the O-10 cm layer was measured at the time of sowing (November 1992). 2.5. Statistical analysis All data were analysed by analysis of variance (ANOVA) using CoStat (Cohort Software, 1990). The means were compared employing Duncan’s multiple range test.
3. Results 3.1. Porosity In the silt loam soil, total macroporosity (pores larger than 50 pm) did not show significant differences in the 0- 10 cm layer between the two treatments (Table 2). However,
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
213
Table2 Effectsof two tillagesystemson soil macroporosityin thin sectionsexpressed astotal areaoccupiedby pores largerthan50~m(m2m-*) (mean *SE) Tillagesystem
Soildepth(cm) O-10
20-30
0.16~0.0la 0.2 1 f 0.05a
0.19fO.Ola 0.09 + 0.02b
0.07fO.Ola
0.09fO.Ola 0.08 fO.Ola
Sill loam soil
Minimumtillage Conventionaltillage Clay soil
Minimumtillage ConventionalWage
0.07*0.01a
In each column and for each soil, the values of porosity differ significantly when followed by different letters at P=O.O5.
in the plots under minimum tillage the macroporosity was more homogeneously distributed in this layer than in the conventionally tilled plots. In the latter the porosity showed a greater variability as indicated by the standard deviation. In the minimally tilled soil the macroporosity was homogeneously distributed also in the 20-30 cm layer, while in the conventionally tilled soil the macroporosity significantly decreased in this layer, indicating a compact soil structure. In the clay soil the macroporosity did not show significant differences between the two types of tillage either in the 0- 10 cm layer or in the 20-30 cm layer (Table 2). In this soil the macroporosity was lower compared with the silt loam soil, thus indicating the presence of massive structure or compaction. Pore shape and size distributions in the 0- 10 cm layer of the minimally tilled plots of the silt loam soil showed large differences compared with conventionally tilled soil (Fig. 1). The higher porosity found in the latter soil was due to the proportion of irregular pores larger than 500 pm. These pores were “vughs”, i.e. “relatively large voids other than packing voids, spherical to elongated, not normally interconnected to voids of comparable size” according to the micromorphological terminology and definition of Bullock et al. ( 1985). These pores were not continuous in the soil matrix like the elongated pores. The proportion of elongated pores in the range of transmission pores (50-500 pm) was 0.10 m’ me2in the minimally tilled soil and 0.07 m* mm2in the conventionally tilled soil. In the 2030 cm layer of the minimally tilled soil the elongated transmission pores increased (0.12 m* m-‘), with respect to the surface layer, while in the conventionally tilled soil the significant reduction of macroporosity (Table 2) was mainly due to a reduction in the proportion of elongated transmission pores to 0.03 m* m-*. In the clay soil the main differences in pore shape and size distribution between soil under minimum and conventional tillage were represented by the reduction of the proportion of elongated transmission pores in both layers of the conventionally tilled treatment (Fig. 2). In this soil the proportion of irregular pores larger than 500 pm was higher in the conventionally tilled than in the minimally tilled treatment. In the O-IO cm layer of the silt loam soil, the total pore volume measured by the mercury intrusion porosimetry, inside the aggregates was greater in the minimally tilled treatment
M. Pagliai
er al. /Soil
& Tillage
Research
34 (1995)
209-223
hllYlhlL-hl
.oti
100.2002ow3?x!3w-100 ,w+ca m-ID00
Depth
TILL.\GE
2D-30
cm
PORE>
?Km
SIZE CLASSES (pm)
CONVENTIONAL TILLAGE Depth O-10 cm
COiSVEKTIONAL TlLLbCE .08-.
PORES 1 REGULAR a IRREGULAR 7
i .osj
Depth
20-30
cm
PORES 2 RECUL.AR TJ IRREGULAR g; ELOYGATED
3
Fig. I. Pore size distribution, expressed as equivalent pore diameter, for regular and irregular pores and width for elongated pores of the silt loam soil. Bars represent the standard error.
than in the conventionally tilled treatment (Fig. 3). Such a decrease in conventionally tilled soil was mainly due to the reduction in volume of storage pores (OS-50 pm). The proportion of pores 50- 100 pm was low in aggregates from both tillage treatments but their proportion was lowest in the conventionally tilled soil. Pores smaller than 0.5 pm, called “residual pores” according to the Greenland classification ( 1977), showed no differences between the two types of tillage. Such residual pores retain water at low potentials and this water is unavailable to roots or for drainage. These pores generally become dominant in dense soils (Ajmone Marsan et al., 1994). As with macroporosity (Table 1) , the total pore volume of the aggregates was smaller in the clay soil than in the silt loam soil (Fig. 3). A similar trend of significantly larger pore volume in the minimally tilled soil compared with the conventionally tilled was evident. The pore size distribution in the clay soil was different from that in the silt loam soil. In the clay soil the highest proportion of pores were residual pores; the volume of storage pores was much smaller than in the silt soil, and the conventionally tilled soil showed the lowest value of storage pores. 3.2. Soil structure The differences in pore shape and size distribution between the two types of tillage were reflected in the type of soil structure. Microscopic examination of thin sections revealed
M. Pagliai
et al. /Soil
& Tillage
UINIklUkI TILLAGE Depth
Research
34 (1995)
Depth
O-10 cm
215
209-223
‘2-30
c 111 POKE\ =: REGC LAK ; IRRECI’L.AK ELOUC.\TED
PORES
100-m
300~lol
,w-SW
500-1000
SIZE CLASSES (pm)
CONVENTIONAL TILLAGE Depth O-10 cm
cm-300
3uo-IW
mo-500
5cwIO””
SIZE CLASSES (km’)
(‘ON\ ENTIOYAL TILLAGE: Drpth ?O-30 cm
PORES D REGULAK 0 IRREGULAR Y ELONGATED
SIZE CLASSES ([id
Fig. 2. Pore size distribution, expressed as equivalent pore diameter, for regular and irregular pores and width for elongated pores of the clay soil. Bars represent the standard error.
that, after minimum tillage, a subangular blocky structure was homogeneously present down the profile of the silt loam soil (Fig. 4). In the conventionally tilled soil the structure was more complex: at the soil surface a compact platy crust was present; below this thin layer, different types of soil structure could be present, such as a vughy structure (Fig. 5), in which numerous irregular pores break up the continuity of fine materials (Bullock et al., 1985) or a crumbly to granular structure (Fig. 6). In the 20-30cm layer of the conventionally tilled soil a massive structure was visible (Fig. 7). The continuity of pores was reduced in the soil matrix. Also at this depth the conventional tillage treatment seemed to exhibit compaction, whereas in the minimum tillage treatment, as already mentioned, the porosity increased slightly. In the clay soil the structure was rather compact. However some differences between the two tillage systems were apparent. In the minimally tilled soil, where the elongated transmission pores were of higher proportions than in the conventionally tilled soil, the latter were sufficiently continuous to surround or separate large aggregates which were rather compact inside, thus producing an angular to subangular blocky structure or, in some cases, a prismatic structure (Fig. 8). In conventionally tilled clay soil (Fig. 9) the structure was almost of the massive type because there were no fully separated aggregates and the material appeared to be dense and its continuity broken up by very few irregular pores or elongated pores which were less continuous than in the minimally tilled soil (Fig. 8).
216
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
.l
a
i:, .y ,: 1 i._‘.._ :. /
.I j
;.::..I. :.:;. :, ..:;::.::..:I1.,t .:.:.: :.: : ...; :.:: \
SL-(“I
Fig. 3. Interaggregate porosity from the O-10 cm layer of the two soils measured by mercury intrusion (SL, silt loam soil; C, clay soil; MT, minimum tillage; CT, conventional tillage). Total porosity values differ significantly when followed by different letters at P = 0.05. Bars represent the standard error.
Fig. 4. Macrophotograph of vertically oriented thin section from samples of layer O-6 cm of minimally tilled silt loam soil. Plain light (in this case pores appear white).
3.3. Aggregate stability The effect of tillage systems on the stability of aggregates to water by wet sieving is reported in Table 3. Minimum tillage significantly increased the WSI compared with conventional tillage in both soils.
M. Pagliai
Fig. 5. Macrophotograph silt loam soil. A surface
Fig. 6. Macrophotograph silt loam soil. A surface appear black).
et al. /Soil
& Tillage
Research
34 (1995)
209-223
of vertically oriented thin section from samples of layer O-6 cm of conventionally crust and the vughy structure below are visible. Plain light.
217
tilled
of vertically oriented thin section from samples of layer O-6 cm of conventionally tilled crust and the crumbly structure below are visible. Crossed Yicols (in this case pores
218
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
3.4. Saturated hydraulic conductivity and soil water content
The saturated hydraulic conductivity of the O-10 cm layer was significantly lower in samples from conventionally tilled compared with minimally tilled plots (Table 3). The soil water content, measured at the time of sowing, did not differ in the two treatments of the silt loam soil, while the water content was significantly higher in minimally tilled compared with conventionally tilled clay soil (Table 3).
4. Discussion
According to the micromorphometric method, a soil is considered dense (compact) when the total macroporosity is less than 0.10 m* mm*,moderately porous when the porosity ranges from 0.10 to 0.25 m* m-*, porous when it ranges from 0.25 to 0.40 m* m-* and extremely porous over 0.40 m* m-* (Pagliai, 1988). For a thorough characterisation of soil macropores, the main aspects to be considered are not only the pore shape but also the pore size distribution, especially of elongated continuous pores, because many of these pores directly affect plant growth by easing root penetration and storage and transmission of water and gases. For example, according to Russell ( 1978) and Tippkiitter ( 1983), feeding roots need pores ranging from 100 to 200 pm to grow into. According to Greenland ( 1977) pores of equivalent pore diameter ranging from 0.5 to 50
Fig. 7. Macrophotograph of vertically oriented thin section from samples tilled silt loam soil. A rather massive structure is very evident. Plain light.
of 20-26
cm layer
of conventionally
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
219
Fig. 8. Macrophotograph of vertically oriented thin section from samples of O-6 cm layer of minimally tilled clay soil. A rather compact subangular stwture is visible. Plain light.
pm are the storage pores, which provide the water reservoir for plants and microorganisms, while transmission pores ranging from 50 to 500 pm (elongated and continuous pores) are important both in soil-water-plant relationships and in maintaining good soil structure conditions. Damage to soil structure can be recognised by decreases in the proportion of transmission pores. The increase of elongated transmission pores (50-500 pm) in minimally tilled soils indicates an improvement of soil structure (Pagliai et al., 1983, 1984, 1989). Pagliai and De Nobili ( 1993) have also shown that important biological activity such as soil enzyme activity, is positively correlated with pores ranging from 30 to 200 pm, and the root density is positively correlated with elongated transmission pores. The increase in the proportion of irregular pores larger than 500 pm in conventionally tilled soils could be ascribed to the effect of the type of tillage: during ploughing it is possible that there was the formation of packing pores between peds or clods, the face of which did not accommodate each other. The use of mercury intrusion porosimetry to determine the pore volume inside the aggregates can be very useful in assessing the suitability of soil to reduced tillage, since it allows the quantification of storage pores (0.5-50 pm). These pores are very important because they determine the amount of water available to plants (Pagliai, 1988). The results of the total pore volume of the aggregates, measured by mercury intrusion, were in agreement with the macroporosity results that the minimum tillage seemed to produce a more favourable porosity in soils. The increase of storage and transmission pores
220
M. Pagfiai
et al. /Soif
& Tiflcge
Research
34 (1995)
259-223
Fig. 9. Macrophotograph of vertically oriented thin section from samples of O-6 cm layer of conventionally tilled clay soil. A massive structure is evident. Plain light.
in the minimum tilled plots was important for improvement of the available water storage capacity and the water movement, respectively. The improvement of the soil pore system in many soil types under minimum tillage has been reported. For example, Hermawan and Cameron ( 1993) showed that in a silt loam soil the volume of pores in the size range 0.260 pm was higher in minimally tilled than in conventionally tilled soils. Table 3 Effects of tillage system on soil aggregate stability (O-10 cm), saturated conductivity (O-10 cm), crop yield, and gravimetric soil water content (O-10 cm) Property
Water stability index Saturated conductivity (mm h- ’ ) Crop yield (t ha-‘) Water content (kg kg-‘)
Tillage system a MT CT MT CT MT CT MT CT
Soil Silt loam
Clay
41.8a 29.61, 112.3a 12.9b 5.2a 5.Oa 0.22a 0.22a
75.2a 50.lb 26.Oa 6.8b 4.4a 5.lb 0.32a 0.25b
aMT, minimum tilfage; CT, conventional tillage. Values in each column differ significantly between treatments (P = 0.05) when followed by different letters.
h4. Pagliai
ef al. /Soil
& Tihge
Research
34 (1995)
259-223
221
The increase of water stability of aggregates following long-term minimum or reduced tillage is well known (Baldock and Kay, 1987; Hermawan and Cameron, 1993). The silt loam soil tends to form crusts and the surface crusts were much more developed in conventionally tilled soil (Fig. 5). This could be ascribed to the lower rain-stability of the surface soil aggregates compared with those from minimally tilled soil. The differences in shape and size distribution of macroporosity between tillage systems and the resulting soil structure influenced the hydrological properties of the two soils, The proportion of elongated transmission pores in conventionally tilled soil was not only lower than in the minimally tilled soil (Figs. 1 and 2), but their continuity was also reduced, as shown by the different types of soil structure observed in thin section (Figs. 7 and 9). The strong reduction of saturated hydraulic conductivity in the conventionally tilled samples of the silt loam soil could also be ascribed to the presence of surface crusts, which interrupted the vertical continuity of elongated pores. Total dry grain yield harvested by a combine was determined (Table 3). The wheat yield showed different trends in the two soils. In the silt loam soil, the crop yield showed no significant differences between the two tillage systems. However, the high value of crop yield was found in the minimally tilled soil, and this trend was also observed in the previous years ( 1984-1993). In contrast, in the clay soil the crop yield was significantly lower in the minimum tilled soil. This result appears to be in contrast to the results from physical properties that suggested physical properties improve in the minimum tilled soil, even though the differences were less pronounced with respect to the silt loam soil. Such a decrease of crop yield could be ascribed to the difficulty in preparing a good seedbed in this clay soil. This difficulty was mainly due to higher water content at the time of sowing in the minimally tilled soil. In this type of maize-wheat rotation (very common on the plain of central Italy), seedbed preparation for wheat occurs in November when the probability of rainfall is relatively high (153 mm). The higher water content in the minimally tilled clay soil did not allow good sowing operations such as covering the seeds with soil. In previous years in this clay soil the crop yield showed higher year to year variability than in silt loam soil; however, the trend reported here was similar in the years when the rainfall at the time of seedbed preparation was relatively high. Conventional tillage produced a higher macroporosity than minimum tillage, which allowed better drainage at seeding time and, thus, allowed better seedbed preparation, leading to higher germination and plant density. Natural soil settling during the wheat cycle, in this type of soil, nullified the effects of tillage and at the end of the crop cycle the porosity did not show differences with respect to the minimum tillage. Minimum tillage also improved the physical properties of the clay soil, though in a less pronounced manner than in the silty loam soil. The adoption of minimum tillage in clay soil may create difficulties for seedbed preparation when the soil water content is relatively high at the time of sowing. The difficulty of carrying out a good sowing (e.g. covering seeds) may induce failure in germination, thus leading to a decrease of crop yield. In the silty loam soil the higher porosity in the 20-30 cm layer of the minimally tilled soil allowed better drainage. In this case, seedbed preparation did not present particular difticulties.
222
M. Pagliai
et al.
/Soil & Tilfage Research
34 (1995)
209-223
5. Conclusions
The following conclusions were drawn from the results found in this study. ( 1) The long-term conventional tillage modified the physical properties of the soils, resulting in damage to soil structure compared with long-term minimum tillage. (2) The better quality of the pore system, with a higher proportion of elongated transmission pores, in the minimally tilled soil produced a more open and homogeneous structure that facilitated water movement, as shown by the higher values of hydraulic conductivity. (3) The increase of storage pores in the minimally tilled soil was important because it leads to an increase of available water to plants. (4) Soil aggregates from the conventionally tilled plots were less resistant to the destructive effect of water action, compared with those from the minimally tilled plots. Lower aggregate stability in conventionally tilled soil leads to a pronounced formation of surface crusts as in silt loam soil, or to a formation of compact (massive) structure as in clay soil. (5) The combination of mercury intrusion porosimetry-image analysis-micromorphological observations is very useful to assesssoil suitability for minimum tillage and can also help to explain differences in water movement and in aggregate stability observed between the two tillage practices.
Acknowledgements
The authors wish to thank M. Morandi Stiattesi for technical assistance. The authors also wish to thank Prof. A.R. Dexter, Silsoe Research Institute, Bedford, UK, for kind revision of the English. This work was supported by the Finalized Project PANDA, Subproject 1, Series 1, Paper No. 2.
References Ajmone Maraan, F., Pagliai, M. and Pini, R., 1994. Identification and properties of fragipan soils in the Piemonte Region (Italy). Soil Sci. Sot. Am. J., 58: 891-900. Auerswald, K., Mutchler, C.K. and McGregor, KC., 1994. The influence of tillage-induced differences in surface moisture content on soil erosion. Soil Tillage Res., 32: 41-50. Baldock, J.A. and Kay, B.D., 1987. Influence of cropping history and chemical treatments on the water-stable aggregation of a silt loam soil. Can. J. Soil Sci., 67: 501-511. Bouma, J., Jongerius, A., Boersma, O.H., Jager, A. and Schoonderbeek, D., 1977. The function of different types of macropores during saturated flow through four swelling soil horizons. Soil Sci. Sot. Am. J., 41: 945-950. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G. and Tursina, T., 1985. Handbook for Soil Thin Section Description. Waine, Wolverhampton, UK. Cohort Software, 1990. CoStat Statistical Software. Berkeley, CA, USA. Catching, W.E., Allbrook, RF. and Gibbs, H.S., 1979. Influence of maize cropping on the structure of two soils in the Waikato district, New Zealand. N.Z. J. Agric. Res., 22: 431-438. F&s, J.C., 1992. Analysis of soil textural porosity relative to skeleton particle size, using mercury porosimetry. Soil Sci. Sot. Am. J., 56: 1062-1067. Food and Agriculture Organization, 1988. FAOfUnesco soil map of the world, revised legend. World Resources Report 60. FAO, Rome, 138 pp.
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
223
Greenland, D.J., 1977. Soil damage by intensive arable cultivation: temporary or permanent? Ph. Trans. R. Sot. London, 281: 193-208. Hermawan, B. and Cameron, K.C., 1993. Structural changes in a silt loam under long-term conventional or minimum tillage. Soil Tillage Res., 26: 139-150. Klute, A. and Dirksen, C., 1986. Hydraulic conductivityanddiffusivity: laboratory methods. In: A. Klute (Editor), Methods of Soil Analysis, Part 1,2nd edn. American Society of Agronomy, Madison, WI, pp. 687-734. Malquori, A. and Cecconi, S., 1962. Determinazione seriale dell’indice di struttura de1 terreno. (Routine determination of soil aggregate stability.) Agrochimica, 6: 199-205. Marks, M.J. and Soane, G.C., 1987. Crop and soil response to subsoil loosening, deep incorporation of phosphorus and potassium fertilizer and subsequent soil management on a range of soil types, Part 1. Response of arable crops. Soil Use Manage., 3: 115-122. McBratney, A.B., Moran, C.J., Stewart, J.B., Cattle, S.R. and Koppi, A.J., 1992. Modifications to a method of rapid assessmentof soil macropore structure by image analysis. Geoderma, 53: 255-274. Mermut, A.R., Grevers, M.C.J. and de Jong, E., 1992. Evaluation of pores under different management systems by image analysis of clay soils in Saskatchewan, Canada. Geoderma, 53: 357-372. Miedema, R., Pape, Th. and van de Wall, G.J., 1974. A method to impregnate wet soil samples, producing high quality thin sections. Neth. J. Agric. Sci.. 22: 37-39. Moran, C.J. and McBratney, A.B., 1992a. Acquisition and analysis of three-component digital images of soil pore structure. 1.Method. J. Soil Sci., 43: 541- 550. Moran, C.J. and McBratney, A.B., 1992b. Acquisition and analysis of three-component digital images of soil pore structure. II. Application to seed beds in a fallow management trial. J. Soil Sci., 43: 551-566. Mucher, H.J. and de Ploey, J., 1977. Experimental and micromorphological investigation of erosion and redeposition of loess by water. Earth Surface Process., 2: 117-124. Murphy, C.P., 1986. Thin section preparation of soils and sediments. A B Academic Publishers, Berkhamsted, Herts., UK. Pagliai, M., 1988. Soil porosity aspects. hit. Agrophys., 4: 215-232. Pagliai, M. and de Nobili, M., 1993. Relationships between soil porosity, root development and soil enzyme activity in cultivated soils. Geoderma, 56: 243-256. Pagliai, M. and Pezzarossa, B., 1990. Structure and porosity of silty clay and clay soils in relation to different management practices. Agric. Medit., 120: 110-l 16. Pagliai, M., La Mama, M. and Lucamante, G., 1983. Micromorphometric and micromorphological investigations of a clay loam soil in viticulture under zero and conventional tillage. J. Soil Sci., 34: 391-403. Pagliai, M., La Mama, M.. Lucamante, G. and Genovese, L., 1984. Effects of zero and conventional tillage on the length and irregularity of elongated pores in a clay loam soil under viticulture. Soil Tillage Res., 4: 433444. Pagliai, M., Pezzarossa,B., Mazzoncini, M. and Bonari, E., 1989. Effect of tillage on porosity and microstructure of a loam soil. Soil Techuol., 2: 345-358. Ringrose-Voase, A.J. and Bullock, P., 1984. The automatic recognition and measurement of soil pore types by image analysis and computer programs. J. Soil Sci., 35: 673-684. Russell, E.W., 1978. Arable agriculture and soil deterioration. In: Trans. of the 1 lth hit. Congress of Soil Science, 19-27 June 1978, University of Alberta, Edmonton, Canadian Society of Soil Science, Alberta, Vol. 3, pp. 216-227. Seta, A.K., Blevins, R.L., Frye, W.W. and Barfield, B.J., 1993. Reducing soil erosion and agricultural chemical losses with conservation tillage. J. Environ. Qual.. 22: 661-665. Shipitalo, M.J. and Protz, R., 1987. Comparison of morphology and porosity of a soil under conventional and zero tillage. Can. J. Soil Sci., 67: 445-456. Tippkotter, R., 1983. Morphology, spatial arrangement and origin of macropores in some hapludalfs, West Germany. Geoderma, 29: 355-37 1. United States Department of Agriculture, 1985. Keys to Soil Taxonomy. Management Support Service. Tech. Monogr. No. 6, USDA, Cornell University, Ithaca, NY.
ELSEVIER
Soil & Tillage
Research
34 (1995)
209-223
8,
TUlase Research
The structure of two alluvial soils in Italy after 10 years of conventional and minimum tillage M. Pagliai a7*,M. Raglione b, T. Panini a, M. Maletta”, M. La Marcac a Istituto ’ Istituto
Sperimentale
per lo Studio e la Difesa de1 Suolo, 50121 Firenze, Speritnentale per lo Studio e la Difesa de1 Suolo, 02100 Rieti, ’ Istituto per la Chimica de1 Terreno, CNR, Accepted
Sezione di Fisica de1 Suolo, Piazza D’Azeglio 30, Italy Sezione di Conservazione de1 Suolo, Via Casette 1. Italy Via Corridoni 78. 56100 Piss, Italy
22 March
1995
Abstract
Micro and macroporosity, pore shapeand sizedistribution, aggregatestability, saturatedhydraulic conductivity and crop yield were analysed in alluvial silty loam (Fluventic Eutrochrept) and clay soils (Vertic Eutrochrept) following long-term minimum and conventional tillage. The soil structure attributes were evaluated by characterizingporosity by means of image analysisof soil thin sections prepared from undisturbed soil samples. The interaggregate microporosity, measured by mercury intrusion porosimetry, increasedin the minimally tilled soils, with a particular increasein the storage pores (0.550 pm). The amount of elongated transmissionpores (50-500 pm) also increasedin the minimally tilled soils.The resulting soil structure was more open and more homogeneous, thus allowing better water movement, as confirmed by the greater hydraulic conductivity of the minimally tilled soils.The aggregatestability was lessin the conventionally tilled soilsand this resultedin a greatertendency to form surfacecrusts and compacted structure, compared with the minimally tilled soils. The latter tillage practiceseemed to maintain, in the long-term, better soil structure conditions and, therefore, maintain favourable conditions for plant growth. In the silt loam, the crop yield did not differ significantly between the two tillage systems,while in the clay soil it decreasedin the minimum tilled soil becauseof problems of seedbed preparation at the higher surface layer water content. Keywords:
Soil porosity;
* Corresponding
Pore shape; Pore size distribution;
author:
0167-1987/95/$09.50 SSDIO167-1987(95)00471-8
Tel.
+ 39 55 249121 I; Fax. +39
0 1995 Elsevier
Science
Soil structure;
55 241485.
B.V. All rights
reserved
Soil tillage;
Image analysis
210
M. Pagliui
et al. /Soil
& Tilloge
Research
34 (1995)
209-223
1. Introduction
Intensive cultivation of some agricultural soils can lead to a deterioration in soil structure and other physical properties of the soil and, consequently, decreased crop yield (Catching et al., 1979; Marks and Soane, 1987). In addition, the need for tillage has been questioned, in the last few decades, partly because of the excessive erosion from farmland after tillage (Seta et al., 1993; Auerswald et al., 1994). Runoff from agricultural land may carry sediments, chemical fertilisers and pesticides, all of which can degrade water quality and upset the ecosystems of streams, lakes, reservoirs, and rivers. Research in this area frequently recommends the adoption of reduced tillage practices to prevent soil structural degradation, soil losses by erosion and to reduce consequent environmental impacts (Shipitalo and Protz, 1987; Pagliai et al., 1989). However, there are also some soil types that are unsuitable for minimum tillage, such as the clay soils developed on Pliocene marine clays (Typic Udorthent) which are so widespread in the hilly environment of central Italy (Pagliai and Pezzarossa, 1990). In order to evaluate the impact of management practices on the soil environment it is necessary to quantify the modifications to soil structure. Porosity is the best indicator of soil structure conditions and pore space measurements are increasingly being used to quantify soil structure because it is the size, shape and continuity of pores that affect many of the important processes in soils ( Ringrose-Voase and Bullock, 1984). A detailed insight into the complexity of the pore system in soil can be obtained by using mercury intrusion porosimetry to quantify pores less than 100 pm (equivalent pore diameter) inside the soil aggregates (Fies, 1992), and image analysis on thin sections prepared from undisturbed soil samples to quantify pores larger than 50 pm, i.e. macropores which determine the type of soil structure (Pagliai et al., 1983, 1984). Technological and theoretical advances, regarding both sample preparation and image analysis, have improved the methods for direct quantification of soil pores. Such methods allow the quantification of the effects of tillage practices on soil porosity and structure and in turn on the optimum tillage needs for sustainable agriculture (McBratney et al., 1992; Mermut et al., 1992; Moran and McBratney, 1992a,b). The aim of this research was to determine the effects of both long-term minimum tillage and conventional tillage on soil structural characteristics through the quantification of the differences in micro and macroporosity, aggregate stability, hydraulic conductivity and crop yield in two different soil types. The differences in porosity were determined not only in terms of total value but also in terms of size and shape distribution of pores and of their influence on soil structure morphology. The two soils investigated were a silt loam and a clay soil that are widespread and representative of the alluvial plains of central and south Italy. These soils are characterised by high vulnerability to structure degradation when intensively cultivated. It is well-known that in temperate areas, the silty and loamy soils are prone to crusting (Mucher and De Ploey, 1977) especially when cultivated (Pagliai et al., 1983, 1989). Similarly, the clay soil is characterised by the formation of large cracks and massive structure when dry, and by strong adhesiveness when wet, causing problems of workability.
M. Pagliai
et al. /Soil
& Tillage
Research
34 (199s)
209-223
211
2. Materials and methods 2.1. Soils and treatments A long-term field experiment was established in 1984 on two soil types. The first, named “Piedifiume”, was a silt loam soil classified as Calcaric Cambisol according to the Food and Agriculture Organization (FAO, 1988) and as Fluventic Eutrochrept following United States Department of Agriculture (USDA, 1985). The second, named “Casa Bianca” was a clay soil classified as Vertic Cambisol according to the FAO (1988) and as Vertic Eutrochrept following USDA ( 1985). Both soils are located on the experimental farm “San Pastore” near Rieti, Lazio, of the Institute for Soil Study and Conservation of Florence. Some characteristics of the soils are reported in Table 1. Both soils had the same crop rotations of maize-wheat. In the season 1992-1993 the crop was autumn-sown wheat (Triticum aestivum variety “Centauro”. Three replications of each of two management practices were tested with 330 m X 30 m plots on both soil types. The tillage treatments were minimum tillage (harrowing with a disc harrow to a depth of 10 cm) and conventional tillage (moldboard ploughing to a depth of 40 cm). Both the minimum and conventional tillage were applied at the end of October 1992, just after the maize harvest. In both treatments the same sowing machine was used for seeding. 2.2. Porosity measurements Six undisturbed samples were collected from the 0- 10 cm layer and from the 20-30 cm layer of each plot in July 1993 just before wheat harvest. After drying by acetone replacement of water (Miedema et al., 1974; Murphy, 1986) and impregnation with polyester resin (Crystic SR 17449, Scott Bader Co. Ltd., Wellingborough, UK), 6 cm X 7 cm vertically oriented thin sections were cut from each sample (Murphy, 1986). Photographs of the sections (Pagliai et al., 1984) were analysed by an image-analysing computer (Quantimet 570) to measure total porosity and to characterise pores according to their shape and size. These photographs covered 4.5 cm X 5.5 cm of the thin section, avoiding the edges where disruption could occur. The instrument was adjusted to measure pores larger than 50 pm. Pores were measured by their shape, which is expressed by the Table 1 Some characteristics
of the Ap horizon
of the soil types
Properties
Silt loam soil
Clay soil
Sand, 50 wrn-2 mm (g kg-‘) Silt, 2-50 wrn (g kg-‘) Clay, <2pm (g kg-‘) PH (W) Organic carbon (g kg-‘) Corbonate content (g kg - ’ ) CEC (meq kg-‘) Clay minerals
291 553 156 8.1 14.2 39 178 Smectite
84 416 500 7.7 17.2 123 356 Smectite
a Dominant
a
a
211
M. Pagliai
et al. /Soil
C? Tillage
Research
34 (1995)
209-223
shape factor (perimete?/4r( area) ) and divided into regular (more or less rounded) pores (shape factor l-2)) irregular pores (shape factor 2-5) and elongated pores (shape factor over 5). These classes correspond approximately to those used by Bouma et al. ( 1977). Pores in each shape class were further subdivided into size classes according to the equivalent pore diameter for regular and irregular pores and the width for elongated pores (Pagliai et al., 1983, 1984). Microstructure was also examined in the thin sections using a Zeiss R POL microscope at X 25 magnification. Six undisturbed samples were collected from the O-10 cm layer of each plot in the areas adjacent to those sampled for thin section preparation. Clods up to the size of 4 cm3 were air-dried and degassed prior to analysis using a Carlo Erba WS mercury intrusion porosimeter equipped with a Carlo Erba 120 macropore unit. The porosity and the pore size distribution were determined in the range 0.007-100 pm. 2.3. Aggregate stability A modification of the wet-sieving method of Malquori and Cecconi ( 1962) was used to determine the stability of the soil aggregates in water. Air-dried aggregates (l-2 mm) of soil were placed in 0.25 mm mesh sieves and moistened by capillary rise from a layer of wet sand, then immersed in deionised water and shaken with an alternative rotating movement (60 times min-‘) at room temperature. The water stability index (WSI) was calculated as lOO( l-A/B), where A and B are the weights of aggregates passing through the sieve after 5 and 60 min, respectively. Each determination was made at least in triplicate. 2.4. Saturated hydraulic conductivity and soil water content Six undisturbed cores (5.68 cm diameter, 9.5 cm high) were collected from the O-10 cm layer of each plot in areas adjacent to those sampled for thin section preparation. The samples were slowly saturated and the saturated hydraulic conductivity was measured using the falling-head technique (Klute and Dirksen, 1986). The gravimetric soil water content in the O-10 cm layer was measured at the time of sowing (November 1992). 2.5. Statistical analysis All data were analysed by analysis of variance (ANOVA) using CoStat (Cohort Software, 1990). The means were compared employing Duncan’s multiple range test.
3. Results 3.1. Porosity In the silt loam soil, total macroporosity (pores larger than 50 pm) did not show significant differences in the 0- 10 cm layer between the two treatments (Table 2). However,
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
213
Table2 Effectsof two tillagesystemson soil macroporosityin thin sectionsexpressed astotal areaoccupiedby pores largerthan50~m(m2m-*) (mean *SE) Tillagesystem
Soildepth(cm) O-10
20-30
0.16~0.0la 0.2 1 f 0.05a
0.19fO.Ola 0.09 + 0.02b
0.07fO.Ola
0.09fO.Ola 0.08 fO.Ola
Sill loam soil
Minimumtillage Conventionaltillage Clay soil
Minimumtillage ConventionalWage
0.07*0.01a
In each column and for each soil, the values of porosity differ significantly when followed by different letters at P=O.O5.
in the plots under minimum tillage the macroporosity was more homogeneously distributed in this layer than in the conventionally tilled plots. In the latter the porosity showed a greater variability as indicated by the standard deviation. In the minimally tilled soil the macroporosity was homogeneously distributed also in the 20-30 cm layer, while in the conventionally tilled soil the macroporosity significantly decreased in this layer, indicating a compact soil structure. In the clay soil the macroporosity did not show significant differences between the two types of tillage either in the 0- 10 cm layer or in the 20-30 cm layer (Table 2). In this soil the macroporosity was lower compared with the silt loam soil, thus indicating the presence of massive structure or compaction. Pore shape and size distributions in the 0- 10 cm layer of the minimally tilled plots of the silt loam soil showed large differences compared with conventionally tilled soil (Fig. 1). The higher porosity found in the latter soil was due to the proportion of irregular pores larger than 500 pm. These pores were “vughs”, i.e. “relatively large voids other than packing voids, spherical to elongated, not normally interconnected to voids of comparable size” according to the micromorphological terminology and definition of Bullock et al. ( 1985). These pores were not continuous in the soil matrix like the elongated pores. The proportion of elongated pores in the range of transmission pores (50-500 pm) was 0.10 m’ me2in the minimally tilled soil and 0.07 m* mm2in the conventionally tilled soil. In the 2030 cm layer of the minimally tilled soil the elongated transmission pores increased (0.12 m* m-‘), with respect to the surface layer, while in the conventionally tilled soil the significant reduction of macroporosity (Table 2) was mainly due to a reduction in the proportion of elongated transmission pores to 0.03 m* m-*. In the clay soil the main differences in pore shape and size distribution between soil under minimum and conventional tillage were represented by the reduction of the proportion of elongated transmission pores in both layers of the conventionally tilled treatment (Fig. 2). In this soil the proportion of irregular pores larger than 500 pm was higher in the conventionally tilled than in the minimally tilled treatment. In the O-IO cm layer of the silt loam soil, the total pore volume measured by the mercury intrusion porosimetry, inside the aggregates was greater in the minimally tilled treatment
M. Pagliai
er al. /Soil
& Tillage
Research
34 (1995)
209-223
hllYlhlL-hl
.oti
100.2002ow3?x!3w-100 ,w+ca m-ID00
Depth
TILL.\GE
2D-30
cm
PORE>
?Km
SIZE CLASSES (pm)
CONVENTIONAL TILLAGE Depth O-10 cm
COiSVEKTIONAL TlLLbCE .08-.
PORES 1 REGULAR a IRREGULAR 7
i .osj
Depth
20-30
cm
PORES 2 RECUL.AR TJ IRREGULAR g; ELOYGATED
3
Fig. I. Pore size distribution, expressed as equivalent pore diameter, for regular and irregular pores and width for elongated pores of the silt loam soil. Bars represent the standard error.
than in the conventionally tilled treatment (Fig. 3). Such a decrease in conventionally tilled soil was mainly due to the reduction in volume of storage pores (OS-50 pm). The proportion of pores 50- 100 pm was low in aggregates from both tillage treatments but their proportion was lowest in the conventionally tilled soil. Pores smaller than 0.5 pm, called “residual pores” according to the Greenland classification ( 1977), showed no differences between the two types of tillage. Such residual pores retain water at low potentials and this water is unavailable to roots or for drainage. These pores generally become dominant in dense soils (Ajmone Marsan et al., 1994). As with macroporosity (Table 1) , the total pore volume of the aggregates was smaller in the clay soil than in the silt loam soil (Fig. 3). A similar trend of significantly larger pore volume in the minimally tilled soil compared with the conventionally tilled was evident. The pore size distribution in the clay soil was different from that in the silt loam soil. In the clay soil the highest proportion of pores were residual pores; the volume of storage pores was much smaller than in the silt soil, and the conventionally tilled soil showed the lowest value of storage pores. 3.2. Soil structure The differences in pore shape and size distribution between the two types of tillage were reflected in the type of soil structure. Microscopic examination of thin sections revealed
M. Pagliai
et al. /Soil
& Tillage
UINIklUkI TILLAGE Depth
Research
34 (1995)
Depth
O-10 cm
215
209-223
‘2-30
c 111 POKE\ =: REGC LAK ; IRRECI’L.AK ELOUC.\TED
PORES
100-m
300~lol
,w-SW
500-1000
SIZE CLASSES (pm)
CONVENTIONAL TILLAGE Depth O-10 cm
cm-300
3uo-IW
mo-500
5cwIO””
SIZE CLASSES (km’)
(‘ON\ ENTIOYAL TILLAGE: Drpth ?O-30 cm
PORES D REGULAK 0 IRREGULAR Y ELONGATED
SIZE CLASSES ([id
Fig. 2. Pore size distribution, expressed as equivalent pore diameter, for regular and irregular pores and width for elongated pores of the clay soil. Bars represent the standard error.
that, after minimum tillage, a subangular blocky structure was homogeneously present down the profile of the silt loam soil (Fig. 4). In the conventionally tilled soil the structure was more complex: at the soil surface a compact platy crust was present; below this thin layer, different types of soil structure could be present, such as a vughy structure (Fig. 5), in which numerous irregular pores break up the continuity of fine materials (Bullock et al., 1985) or a crumbly to granular structure (Fig. 6). In the 20-30cm layer of the conventionally tilled soil a massive structure was visible (Fig. 7). The continuity of pores was reduced in the soil matrix. Also at this depth the conventional tillage treatment seemed to exhibit compaction, whereas in the minimum tillage treatment, as already mentioned, the porosity increased slightly. In the clay soil the structure was rather compact. However some differences between the two tillage systems were apparent. In the minimally tilled soil, where the elongated transmission pores were of higher proportions than in the conventionally tilled soil, the latter were sufficiently continuous to surround or separate large aggregates which were rather compact inside, thus producing an angular to subangular blocky structure or, in some cases, a prismatic structure (Fig. 8). In conventionally tilled clay soil (Fig. 9) the structure was almost of the massive type because there were no fully separated aggregates and the material appeared to be dense and its continuity broken up by very few irregular pores or elongated pores which were less continuous than in the minimally tilled soil (Fig. 8).
216
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
.l
a
i:, .y ,: 1 i._‘.._ :. /
.I j
;.::..I. :.:;. :, ..:;::.::..:I1.,t .:.:.: :.: : ...; :.:: \
SL-(“I
Fig. 3. Interaggregate porosity from the O-10 cm layer of the two soils measured by mercury intrusion (SL, silt loam soil; C, clay soil; MT, minimum tillage; CT, conventional tillage). Total porosity values differ significantly when followed by different letters at P = 0.05. Bars represent the standard error.
Fig. 4. Macrophotograph of vertically oriented thin section from samples of layer O-6 cm of minimally tilled silt loam soil. Plain light (in this case pores appear white).
3.3. Aggregate stability The effect of tillage systems on the stability of aggregates to water by wet sieving is reported in Table 3. Minimum tillage significantly increased the WSI compared with conventional tillage in both soils.
M. Pagliai
Fig. 5. Macrophotograph silt loam soil. A surface
Fig. 6. Macrophotograph silt loam soil. A surface appear black).
et al. /Soil
& Tillage
Research
34 (1995)
209-223
of vertically oriented thin section from samples of layer O-6 cm of conventionally crust and the vughy structure below are visible. Plain light.
217
tilled
of vertically oriented thin section from samples of layer O-6 cm of conventionally tilled crust and the crumbly structure below are visible. Crossed Yicols (in this case pores
218
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
3.4. Saturated hydraulic conductivity and soil water content
The saturated hydraulic conductivity of the O-10 cm layer was significantly lower in samples from conventionally tilled compared with minimally tilled plots (Table 3). The soil water content, measured at the time of sowing, did not differ in the two treatments of the silt loam soil, while the water content was significantly higher in minimally tilled compared with conventionally tilled clay soil (Table 3).
4. Discussion
According to the micromorphometric method, a soil is considered dense (compact) when the total macroporosity is less than 0.10 m* mm*,moderately porous when the porosity ranges from 0.10 to 0.25 m* m-*, porous when it ranges from 0.25 to 0.40 m* m-* and extremely porous over 0.40 m* m-* (Pagliai, 1988). For a thorough characterisation of soil macropores, the main aspects to be considered are not only the pore shape but also the pore size distribution, especially of elongated continuous pores, because many of these pores directly affect plant growth by easing root penetration and storage and transmission of water and gases. For example, according to Russell ( 1978) and Tippkiitter ( 1983), feeding roots need pores ranging from 100 to 200 pm to grow into. According to Greenland ( 1977) pores of equivalent pore diameter ranging from 0.5 to 50
Fig. 7. Macrophotograph of vertically oriented thin section from samples tilled silt loam soil. A rather massive structure is very evident. Plain light.
of 20-26
cm layer
of conventionally
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
219
Fig. 8. Macrophotograph of vertically oriented thin section from samples of O-6 cm layer of minimally tilled clay soil. A rather compact subangular stwture is visible. Plain light.
pm are the storage pores, which provide the water reservoir for plants and microorganisms, while transmission pores ranging from 50 to 500 pm (elongated and continuous pores) are important both in soil-water-plant relationships and in maintaining good soil structure conditions. Damage to soil structure can be recognised by decreases in the proportion of transmission pores. The increase of elongated transmission pores (50-500 pm) in minimally tilled soils indicates an improvement of soil structure (Pagliai et al., 1983, 1984, 1989). Pagliai and De Nobili ( 1993) have also shown that important biological activity such as soil enzyme activity, is positively correlated with pores ranging from 30 to 200 pm, and the root density is positively correlated with elongated transmission pores. The increase in the proportion of irregular pores larger than 500 pm in conventionally tilled soils could be ascribed to the effect of the type of tillage: during ploughing it is possible that there was the formation of packing pores between peds or clods, the face of which did not accommodate each other. The use of mercury intrusion porosimetry to determine the pore volume inside the aggregates can be very useful in assessing the suitability of soil to reduced tillage, since it allows the quantification of storage pores (0.5-50 pm). These pores are very important because they determine the amount of water available to plants (Pagliai, 1988). The results of the total pore volume of the aggregates, measured by mercury intrusion, were in agreement with the macroporosity results that the minimum tillage seemed to produce a more favourable porosity in soils. The increase of storage and transmission pores
220
M. Pagfiai
et al. /Soif
& Tiflcge
Research
34 (1995)
259-223
Fig. 9. Macrophotograph of vertically oriented thin section from samples of O-6 cm layer of conventionally tilled clay soil. A massive structure is evident. Plain light.
in the minimum tilled plots was important for improvement of the available water storage capacity and the water movement, respectively. The improvement of the soil pore system in many soil types under minimum tillage has been reported. For example, Hermawan and Cameron ( 1993) showed that in a silt loam soil the volume of pores in the size range 0.260 pm was higher in minimally tilled than in conventionally tilled soils. Table 3 Effects of tillage system on soil aggregate stability (O-10 cm), saturated conductivity (O-10 cm), crop yield, and gravimetric soil water content (O-10 cm) Property
Water stability index Saturated conductivity (mm h- ’ ) Crop yield (t ha-‘) Water content (kg kg-‘)
Tillage system a MT CT MT CT MT CT MT CT
Soil Silt loam
Clay
41.8a 29.61, 112.3a 12.9b 5.2a 5.Oa 0.22a 0.22a
75.2a 50.lb 26.Oa 6.8b 4.4a 5.lb 0.32a 0.25b
aMT, minimum tilfage; CT, conventional tillage. Values in each column differ significantly between treatments (P = 0.05) when followed by different letters.
h4. Pagliai
ef al. /Soil
& Tihge
Research
34 (1995)
259-223
221
The increase of water stability of aggregates following long-term minimum or reduced tillage is well known (Baldock and Kay, 1987; Hermawan and Cameron, 1993). The silt loam soil tends to form crusts and the surface crusts were much more developed in conventionally tilled soil (Fig. 5). This could be ascribed to the lower rain-stability of the surface soil aggregates compared with those from minimally tilled soil. The differences in shape and size distribution of macroporosity between tillage systems and the resulting soil structure influenced the hydrological properties of the two soils, The proportion of elongated transmission pores in conventionally tilled soil was not only lower than in the minimally tilled soil (Figs. 1 and 2), but their continuity was also reduced, as shown by the different types of soil structure observed in thin section (Figs. 7 and 9). The strong reduction of saturated hydraulic conductivity in the conventionally tilled samples of the silt loam soil could also be ascribed to the presence of surface crusts, which interrupted the vertical continuity of elongated pores. Total dry grain yield harvested by a combine was determined (Table 3). The wheat yield showed different trends in the two soils. In the silt loam soil, the crop yield showed no significant differences between the two tillage systems. However, the high value of crop yield was found in the minimally tilled soil, and this trend was also observed in the previous years ( 1984-1993). In contrast, in the clay soil the crop yield was significantly lower in the minimum tilled soil. This result appears to be in contrast to the results from physical properties that suggested physical properties improve in the minimum tilled soil, even though the differences were less pronounced with respect to the silt loam soil. Such a decrease of crop yield could be ascribed to the difficulty in preparing a good seedbed in this clay soil. This difficulty was mainly due to higher water content at the time of sowing in the minimally tilled soil. In this type of maize-wheat rotation (very common on the plain of central Italy), seedbed preparation for wheat occurs in November when the probability of rainfall is relatively high (153 mm). The higher water content in the minimally tilled clay soil did not allow good sowing operations such as covering the seeds with soil. In previous years in this clay soil the crop yield showed higher year to year variability than in silt loam soil; however, the trend reported here was similar in the years when the rainfall at the time of seedbed preparation was relatively high. Conventional tillage produced a higher macroporosity than minimum tillage, which allowed better drainage at seeding time and, thus, allowed better seedbed preparation, leading to higher germination and plant density. Natural soil settling during the wheat cycle, in this type of soil, nullified the effects of tillage and at the end of the crop cycle the porosity did not show differences with respect to the minimum tillage. Minimum tillage also improved the physical properties of the clay soil, though in a less pronounced manner than in the silty loam soil. The adoption of minimum tillage in clay soil may create difficulties for seedbed preparation when the soil water content is relatively high at the time of sowing. The difficulty of carrying out a good sowing (e.g. covering seeds) may induce failure in germination, thus leading to a decrease of crop yield. In the silty loam soil the higher porosity in the 20-30 cm layer of the minimally tilled soil allowed better drainage. In this case, seedbed preparation did not present particular difticulties.
222
M. Pagliai
et al.
/Soil & Tilfage Research
34 (1995)
209-223
5. Conclusions
The following conclusions were drawn from the results found in this study. ( 1) The long-term conventional tillage modified the physical properties of the soils, resulting in damage to soil structure compared with long-term minimum tillage. (2) The better quality of the pore system, with a higher proportion of elongated transmission pores, in the minimally tilled soil produced a more open and homogeneous structure that facilitated water movement, as shown by the higher values of hydraulic conductivity. (3) The increase of storage pores in the minimally tilled soil was important because it leads to an increase of available water to plants. (4) Soil aggregates from the conventionally tilled plots were less resistant to the destructive effect of water action, compared with those from the minimally tilled plots. Lower aggregate stability in conventionally tilled soil leads to a pronounced formation of surface crusts as in silt loam soil, or to a formation of compact (massive) structure as in clay soil. (5) The combination of mercury intrusion porosimetry-image analysis-micromorphological observations is very useful to assesssoil suitability for minimum tillage and can also help to explain differences in water movement and in aggregate stability observed between the two tillage practices.
Acknowledgements
The authors wish to thank M. Morandi Stiattesi for technical assistance. The authors also wish to thank Prof. A.R. Dexter, Silsoe Research Institute, Bedford, UK, for kind revision of the English. This work was supported by the Finalized Project PANDA, Subproject 1, Series 1, Paper No. 2.
References Ajmone Maraan, F., Pagliai, M. and Pini, R., 1994. Identification and properties of fragipan soils in the Piemonte Region (Italy). Soil Sci. Sot. Am. J., 58: 891-900. Auerswald, K., Mutchler, C.K. and McGregor, KC., 1994. The influence of tillage-induced differences in surface moisture content on soil erosion. Soil Tillage Res., 32: 41-50. Baldock, J.A. and Kay, B.D., 1987. Influence of cropping history and chemical treatments on the water-stable aggregation of a silt loam soil. Can. J. Soil Sci., 67: 501-511. Bouma, J., Jongerius, A., Boersma, O.H., Jager, A. and Schoonderbeek, D., 1977. The function of different types of macropores during saturated flow through four swelling soil horizons. Soil Sci. Sot. Am. J., 41: 945-950. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G. and Tursina, T., 1985. Handbook for Soil Thin Section Description. Waine, Wolverhampton, UK. Cohort Software, 1990. CoStat Statistical Software. Berkeley, CA, USA. Catching, W.E., Allbrook, RF. and Gibbs, H.S., 1979. Influence of maize cropping on the structure of two soils in the Waikato district, New Zealand. N.Z. J. Agric. Res., 22: 431-438. F&s, J.C., 1992. Analysis of soil textural porosity relative to skeleton particle size, using mercury porosimetry. Soil Sci. Sot. Am. J., 56: 1062-1067. Food and Agriculture Organization, 1988. FAOfUnesco soil map of the world, revised legend. World Resources Report 60. FAO, Rome, 138 pp.
M. Pagliai
et al. /Soil
& Tillage
Research
34 (1995)
209-223
223
Greenland, D.J., 1977. Soil damage by intensive arable cultivation: temporary or permanent? Ph. Trans. R. Sot. London, 281: 193-208. Hermawan, B. and Cameron, K.C., 1993. Structural changes in a silt loam under long-term conventional or minimum tillage. Soil Tillage Res., 26: 139-150. Klute, A. and Dirksen, C., 1986. Hydraulic conductivityanddiffusivity: laboratory methods. In: A. Klute (Editor), Methods of Soil Analysis, Part 1,2nd edn. American Society of Agronomy, Madison, WI, pp. 687-734. Malquori, A. and Cecconi, S., 1962. Determinazione seriale dell’indice di struttura de1 terreno. (Routine determination of soil aggregate stability.) Agrochimica, 6: 199-205. Marks, M.J. and Soane, G.C., 1987. Crop and soil response to subsoil loosening, deep incorporation of phosphorus and potassium fertilizer and subsequent soil management on a range of soil types, Part 1. Response of arable crops. Soil Use Manage., 3: 115-122. McBratney, A.B., Moran, C.J., Stewart, J.B., Cattle, S.R. and Koppi, A.J., 1992. Modifications to a method of rapid assessmentof soil macropore structure by image analysis. Geoderma, 53: 255-274. Mermut, A.R., Grevers, M.C.J. and de Jong, E., 1992. Evaluation of pores under different management systems by image analysis of clay soils in Saskatchewan, Canada. Geoderma, 53: 357-372. Miedema, R., Pape, Th. and van de Wall, G.J., 1974. A method to impregnate wet soil samples, producing high quality thin sections. Neth. J. Agric. Sci.. 22: 37-39. Moran, C.J. and McBratney, A.B., 1992a. Acquisition and analysis of three-component digital images of soil pore structure. 1.Method. J. Soil Sci., 43: 541- 550. Moran, C.J. and McBratney, A.B., 1992b. Acquisition and analysis of three-component digital images of soil pore structure. II. Application to seed beds in a fallow management trial. J. Soil Sci., 43: 551-566. Mucher, H.J. and de Ploey, J., 1977. Experimental and micromorphological investigation of erosion and redeposition of loess by water. Earth Surface Process., 2: 117-124. Murphy, C.P., 1986. Thin section preparation of soils and sediments. A B Academic Publishers, Berkhamsted, Herts., UK. Pagliai, M., 1988. Soil porosity aspects. hit. Agrophys., 4: 215-232. Pagliai, M. and de Nobili, M., 1993. Relationships between soil porosity, root development and soil enzyme activity in cultivated soils. Geoderma, 56: 243-256. Pagliai, M. and Pezzarossa, B., 1990. Structure and porosity of silty clay and clay soils in relation to different management practices. Agric. Medit., 120: 110-l 16. Pagliai, M., La Mama, M. and Lucamante, G., 1983. Micromorphometric and micromorphological investigations of a clay loam soil in viticulture under zero and conventional tillage. J. Soil Sci., 34: 391-403. Pagliai, M., La Mama, M.. Lucamante, G. and Genovese, L., 1984. Effects of zero and conventional tillage on the length and irregularity of elongated pores in a clay loam soil under viticulture. Soil Tillage Res., 4: 433444. Pagliai, M., Pezzarossa,B., Mazzoncini, M. and Bonari, E., 1989. Effect of tillage on porosity and microstructure of a loam soil. Soil Techuol., 2: 345-358. Ringrose-Voase, A.J. and Bullock, P., 1984. The automatic recognition and measurement of soil pore types by image analysis and computer programs. J. Soil Sci., 35: 673-684. Russell, E.W., 1978. Arable agriculture and soil deterioration. In: Trans. of the 1 lth hit. Congress of Soil Science, 19-27 June 1978, University of Alberta, Edmonton, Canadian Society of Soil Science, Alberta, Vol. 3, pp. 216-227. Seta, A.K., Blevins, R.L., Frye, W.W. and Barfield, B.J., 1993. Reducing soil erosion and agricultural chemical losses with conservation tillage. J. Environ. Qual.. 22: 661-665. Shipitalo, M.J. and Protz, R., 1987. Comparison of morphology and porosity of a soil under conventional and zero tillage. Can. J. Soil Sci., 67: 445-456. Tippkotter, R., 1983. Morphology, spatial arrangement and origin of macropores in some hapludalfs, West Germany. Geoderma, 29: 355-37 1. United States Department of Agriculture, 1985. Keys to Soil Taxonomy. Management Support Service. Tech. Monogr. No. 6, USDA, Cornell University, Ithaca, NY.