Interrelationships of soil resources and cropping systems
Soil pores and water characteristics
Soil fabric, density and strength
Soil structure and crop growth
How management directly affects soil structure
Field indicators of physical problems
Management for maintenance of soil physical properties
The key resources for sustainable dryland cropping are listed in Table 2. The soil resources in this table fall into three broad groups, reflecting their underlying interrelationships: those related to soil structure (water use, structure, erosion) are dealt with in this Chapter, those related to nutrition or biological factors are discussed later.
As this Bulletin deals with dryland crops, water availability is the major constraint on crop performance. Table 3 suggests that water use efficiency (WUE) is a key measurement of performance. WUE reflects water availability (seasonably of rainfall) but also, importantly for management, it integrates the influences of factors such as the volume of soil exploitable by roots. Long-term trends with time in WUE and trends between areas provide estimates of the relative sustainability of specific cropping systems. This is illustrated in Figure 12: crops below the theoretical WUE line are less efficient or effective than they should be. One reason for such ineffectiveness is poor management during the growing season, for example weediness and nutrient deficiency. This is correctable, as shown by the vertical lines on Figure 12, where WUE was increased during experimental treatments. Other reasons for poor WUE are related more to fundamental issues associated with soils.
A traditional view of the influence of soil is that it provides an opportunity, or a constraint on the type of cropping system that can be implemented and its productivity. A more responsible view is that 'the soil' combines various properties which interrelate and are directly influenced by the procedures of cropping. Figure 13 attempts to illustrate this for a non-sustainable cropping system, in which soil resources are declining through changes in surface properties, subsoil compaction, loss of organic matter and reduced biological activity. It shows that each of these properties may affect the others and that all can directly and indirectly reduce plant performance, as well as affecting other aspects including erosion, salinization and acidification. Some of the mechanisms underlying these interconnections are described below.
Soils are made up of three parts: mineral materials, organic matter and space (called pore-or void-volume). The relative importance of these varies with soil type but pore space can occupy about half the volume of a medium-textured soil. At optimum water content for plant growth, approximately half the pore space is filled with water and half with air. The proportions of water and air can change rapidly depending on weather, evapotranspiration and other factors.
The dimensions (size, shape and arrangement) and number of pore spaces are most important in determining soil water and soil structure.
Porosity is the volume of soil voids (pore space). It is expressed in relation to the bulk volume of the soil. The water holding capacity of a soil depends on its porosity, and the size distribution of its pores. Small pores retain water at greater suctions than larger pores (Table 7). The moisture (or water) potential is the amount of energy required to remove water from a soil; field capacity is the water-holding capacity after a free-draining soil has been allowed to drain. The suction corresponding to this state has variously been defined as 0.33, 0.1 and 0.05 bar and so the convention used should always be checked. Wilting point, beyond which plants cannot exert sufficient suction to remove water from a soil, is generally considered to correspond to a suction of 15 bar.
FIGURE 13 - Interrelationships between management and soil as they impact on crop productivity
TABLE 7
Generalized view of pore size groups and functions
|
Pore diameter (mm) |
Function |
Equivalent particle or aggregate size1 |
Biological cause (if not due to natural particle arrangement) |
Equivalent soil water2 tension (kPa) |
|
>0.5 |
Aeration and water transmission |
>1.6 (mostly gravel size, some coarse sand size) |
ants, worms |
<0.6 |
|
0.50-0.05 |
Water transmission (infiltration, permeability) |
0.16-1.6 (mostly coarse sand size, some fine sand size) |
roots |
0.6-6.0 |
|
0.0005-0.05 |
Water storage |
0.0016-0.16 (mostly silt and fine sand size, some clay size) |
lateral roots, root hairs |
6.0-600 |
|
< 0.0005 |
Residual (bound) water, unavailable to plants |
<0.0016 (mostly clay size) |
fungal hyphae and bacteria |
>600 |
1 Equivalent particle size = 3.2 x pore size (assuming spherical, uniform size particles).
2 Based on equation: Pore diameter (mm) = 0.30/soil water tension (kPa).
TABLE 8
Typical values of water-holding capacity of soils (adapted from Salter and Williams 1967)
|
International textural class |
Field capacity (10 kPa) (gravimetric) % |
Wilting point (gravimetric) % |
Available mm water/m soil |
|
Coarse sand |
8 |
4 |
80 |
|
Sand |
14 |
4 |
150 |
|
Loamy sand |
18 |
7 |
160 |
|
Sandy loam |
26 |
9 |
180 |
|
Loam |
30 |
13 |
180 |
|
Silty loam |
34 |
16 |
200 |
|
Sandy clay loam |
26 |
15 |
150 |
|
Clay loam |
34 |
18 |
180 |
|
Silty clay loam |
43 |
20 |
190 |
|
Sandy clay |
29 |
19 |
140 |
|
Clay |
42 |
25 |
180 |
TABLE 9
Typical values of saturated hydraulic conductivity based on texture and degree of structure
|
Texture |
Structure |
Infiltration |
Permeability (mm/h) |
|
Sand |
Apedal |
Very rapid |
>120 can be measured >250 |
|
Sandy loam |
Weakly pedal |
Very rapid |
>120 |
|
Apedal |
Rapid |
60-120 |
|
|
Loam |
Peds evident |
Rapid |
60-120 |
|
Weakly pedal |
Mod. rapid |
20-60 |
|
|
Apedal |
Mod. rapid |
20-60 |
|
|
Clay loam |
Peds evident |
Mod. rapid |
20-60 |
|
Weakly pedal |
Moderate |
5-20 |
|
|
Apedal |
Slow |
2.5-5 |
|
|
Light clay |
Highly pedal |
Moderate |
5-20 |
|
Peds evident |
Slow |
2.5-5 |
|
|
Weakly pedal |
Very slow |
<2.5 |
|
|
Medium to heavy clay |
Highly pedal |
Slow |
2.5-201 |
|
Peds evident |
Very slow |
<2.5 |
|
|
Weakly pedal |
Very slow |
<2.5 |
|
|
Clay |
Sodic and saline |
Moderate |
8.0 |
|
Sodic |
Very slow |
<2.5 |
|
|
Highly sodic |
Extreme |
<1.0 |
1 Strongly structured polyhedral subsoils, e.g. Krasnozem.
Available soil water (ASW) is the amount of water which is available for uptake by plants, namely that held at suctions between wilting point and field capacity. It varies with soil type and can be correlated with the clay content and structural arrangement of the soil. It varies also with soil treatment because the size and distribution of pores in the topsoil reflects surface exposure, normal seasonal wetting and drying, and management. Williams et al. (1983), studying the water content of 244 soil samples, found that the ASW of well-structured soils was one-third to twice as large as that in comparable (similarly-textured) poorly structured or degraded soils. Bearing in mind that ASW varies with natural weathering and management, Table 8 gives typical values of ASW for various soil texture classes.
Hydraulic conductivity (K) of a soil is its conductivity to movement of water down a pressure gradient. High values of K are associated with well-structured soil and contiguous pores; they allow high infiltration rates and rapid drainage. Earthworm channels, which can have populations of 500 m-2 in Mediterranean climates (Barley 1959), and continuous deep voids left by dead roots (5-10 000 m-2) contribute greatly to hydraulic conductivity. Hydraulic conductivity varies with soil type and management (Table 9). K values below 10 mm/h are low and likely to cause runoff following rainfall or problems with irrigation, given that steady rain falls at about 10 mm/h. K values of 10 to 20 mm/h can give intermittent runoff (a downpour falls at about 50 mm/h) while values up to 120 mm/h are associated with occasional, increasingly rare runoff. Values above 120 mm/h may facilitate regular drainage to the groundwater, causing potential problems for heavily-fertilized soils, and those treated with effluent, herbicides or pesticides.
Both soil water content and saturated hydraulic conductivity generally relate to the number and continuity of pores, particularly the larger macro-pores. It is, however, difficult to measure these soil attributes and they are highly location-specific, so that variability is great and they sometimes have little interpretive value. Moran et al. (1988), however, in a study of a soil in a wet-and-dry environment, show that a soil treated with minimum tillage had more pores, identified directly by image analysis, and higher hydraulic conductivity, measured in the field, than did a similar soil traditionally cultivated (Figure 14). Figure 14 also illustrates the impact of management on soil pores and soil water characteristics; this is discussed further in the section below.
Surface sealing and crusting are common in wet-and-dry climates. Sealing increases runoff and seriously reduces the amount of water infiltrating into the soil thus reducing the water held in the soil. Sealing can also increase ponding at the surface and thereby evaporation.
Infiltration rates are often reduced 1000-fold by crusting. The crust can have a skin with a conductivity of only about 0.1 mm/h, able only to accommodate the lightest rate of precipitation (fine mist) and commonly overlies a layer of poorly-aggregated material which also has a conductivity substantially lower than that of the underlying soil.
Chase and Boudouresque (1989) and Chase et al. (1989) illustrate the impact of crusting in the Sahel on increased runoff from soils. They show how runoff may be reduced, and the depth of wetting increased, by covering the soil surface with mulch.
The solid soil material is arbitrarily divided into fractions, the proportions of which determine the texture of the soil. The fractions in the International Method of particle-size analysis are:
FIGURE 14 - Photographic images of vertical slices of soils: the vertical faces of soil were taken from a direct drill (DD) treatment (a) and a conventionally tilled soil (b) at the same location. Hydraulic conductivities were 42.5 and 5.0 mm/h respectively (Moran et al. 1988)
FIGURE 14 - (a)
FIGURE 14 - (b)
Sand: grains, which feel gritty and are large enough to be seen and felt individually; coarse sand has particle sizes between 2 and 0.2 mm and fine sand 0.2 and 0.05 mm.
Silt: imparts a smooth, soapy or silky and only slightly sticky feeling, silt grains cannot be individually detected; their particle sizes range from 0.05 to 0.002 mm.
Clay: gives a sticky feel to the soil. Clay particles are less than 0.002 mm diameter.
These solid fractions contribute to the consistence and strength of the soil, and their packing determines bulk density.
Bulk density is a measure of the packing or compression of the three constituents of soil. Just as the inherent bulk density of a soil will vary by 30% according to its constituents, so the limiting values of bulk density for root penetration will range from about 1.4 g cm3 in a soil of clay texture to 1.8 g cm3 in a sandy one (see next section).
Soil strength is the resistance of soil to shearing or structural failure. This reflects the friction which is built up between the soil and an implement, and depends on the density, and the roughness and shape of the soil particles. The shear strength of an individual clod decreases with wetting but, more importantly, the strength of the bulk soil increases with increasing moisture to about the lower plastic limit (known to field operators as the 'sticky point'), at which each particle is surrounded by a film of water which acts as a lubricant. Soil strength drops sharply from that point to the upper plastic limit, where the soil becomes viscous. The difference between the moisture content at the upper and lower plastic limits, termed the plasticity index, is an index of the workability of the soil. A large range or high plasticity index implies a need for large amounts of energy to work the soil to a desired tilth.
Soil physical properties affect root and shoot growth directly and indirectly, the latter for example through poor drainage causing pores to fill with water and plants to suffer from anaerobiosis. Root growth has been described under various soil physical conditions, but relationships have only rarely been established between features such as crop yield, root growth and soil pore size distribution or conductivity, a more aggregate measure.
The difficulty in establishing simple relationships does not mean that soil has little influence upon root growth; rather it points to the complexity of the interactions (Figure 13) and the internal homeostasis which plants maintain. Roots both elongate and proliferate and spread laterally as they grow and age. Concomitantly some roots die and others become suberized and function as conduits, but not as absorbers, of water and nutrients. Roots can elongate downwards as fast as 8 cm/d, as for example, soybean growing in a silt loam in a rhizotron (Kaspar et al. 1978). Deep-rootedness and maximum rooting depth reflect soil properties (for example, roots will not grow through pores that they cannot deform to a larger diameter than the root). However, relationships are not often reported. Maximum rooting depth varies with species and soil type. For example, wheat roots penetrated to 0.8 m in heavy-textured soils and to 1.2 m in a loamy sand (Rickert et al. 1987) but it is often found that a variety will have a consistent rooting depth across similar soil types in a particular year (Hamblin and Hamblin 1985) or in one soil across several years (Pearson et al. 1991). Angus et al. (1983) found that rice and six dryland crops (mung bean, cowpea, soybean, groundnut, maize and sorghum) extracted different amounts of stored soil water (ranging from 100 mm for rice to 250 mm for groundnut) and that extraction was, in part, related to rooting depth.
The spread of roots with age (Figure 15a) can be related to the growth (increase in weight) of the whole plant, and to accumulated temperature or growing day-degrees (GDD) (Figure 15b); indeed, there is some evidence that temperature influences the direction of newly-appeared roots as well as the rate of appearance and extent of growth (Tardieu and Pellerin 1991). Clearly, however, there are factors other than plant size, temperature and soil which influence root proliferation. Otherwise the plants sown at three different times of year in the same soil in Figure 15b would align their root growth along a single growth-GDD relationship. These other factors, of which day length is probably particularly important, tend to mask underlying relationships between growth and soil structure.
Figure 15c shows that tillage can affect root length, though in this case its effects took three years to develop. Measurable differences in soil porosity developed under two tillage treatments: in the first year both root growth and water infiltration (K approximately 5 mm/h) were the same under minimum tillage and conventional tillage. By the third year, when differences were measured between roots, infiltration rates were 84 mm/h in minimum tillage and 0.2 mm/h under conventional tillage. Despite the differences in root growth there were no substantial differences in grain yield, reflecting the overall constraint of climate in the semi-arid environment.
One of the clearer associations between soil porosity and plant growth is described by Tisdall (1978) and Cockroft and Tisdall (1978). Soil was ameliorated by deep ploughing, inserting gypsum at depth, and incorporation of straw and green manure in the topsoil. This treatment gave 10-fold increases in the number of earthworms and a 4-fold increase in soil pores. Infiltration rates also increased 10-fold relative to untreated soil. Root growth was not measured, but fruit yields from peach trees increased from 18 to 75 t/ha.
FIGURE 15 - Spread of wheat roots with age, and effects on root growth of minimum or direct drilling compared with conventional tillage (Pearson et al. 1991)
TABLE 10
Values of air-filled porosity (%) and bulk density (g cm3) which are critical and which limit root growth for various soils (Source: Pierce et al. 1983)
|
Texture class |
Non-limiting |
Critical1 |
Limiting |
|
|
Air-filled porosity |
||||
|
Fine loamy |
20 |
10 |
5 |
|
|
Coarse silty |
20 |
10 |
5 |
|
|
Fine silty |
20 |
10 |
5 |
|
|
Clay: |
|
|
|
|
|
|
35-45 |
15 |
10 |
5 |
|
>45 |
15 |
10 |
5 |
|
|
Bulk density |
||||
|
Sandy |
1.60 |
1.69 |
1.85 |
|
|
Coarse loamy |
1.50 |
1.63 |
1.80 |
|
|
Fine loamy |
1.46 |
1.67 |
1.78 |
|
|
Coarse silty |
1.43 |
1.67 |
1.79 |
|
|
Fine silty |
1.34 |
1.54 |
1.65 |
|
|
Clayey: |
|
|
|
|
|
|
35-45% |
1.40 |
1.49 |
1.58 |
|
45% |
1.30 |
1.39 |
1.47 |
|
1 "Critical" is defined as causing <20% reduction in root growth; "limiting" is about the value at which root growth ceases.
Increases in soil density or strength retard root penetration and thus limit the volume of soil exploited by the crop and the water available. It is difficult to quantify the relationships between these soil parameters and plant growth. In the cases of bulk density and strength, particularly, a gross measure of either for an undisturbed mass of soil can give only a remote indication of what a root encounters. A determination of gross bulk density does not assess whether a root is growing within a pore (in which case it may deform surrounding soil before its radial environment reaches the density or strength of the gross soil) or if it is growing within the soil material, in which case it has already exerted a radial force equivalent to that measured for the gross soil.
This problem of scale - what is measured in a gross estimate cannot assess the micro-environment of the root, to which it (and through hormonal signals, the plant top) responds -does not invalidate some general hypotheses about plant behaviour in response to soil compaction and structural arrangement. Roots stop growing when they are unable to deform their micro-environment. This probably occurs at suctions about 60 kPa when they cannot generate adequate internal turgor. It is thought that root elongation declines curvilinearly with increasing either bulk density or shear strength. In the case of bulk density, little effect is noted until a 'critical value' and root elongation ceases within a further 10% increase in bulk density (Table 10). As explained earlier, these values vary with soil type.
Root elongation declines asymptotically with increasing soil strength (Figure 16) though the actual critical values would be expected to vary with soil type, water content and the method of measurement (those in Figure 16 were measured with a penetrometer).
Studies of the effect of bulk density or strength on other plant processes, particularly germination and shoot elongation, also suffer from the same technical problem of scale. Shoots are able to explore macropores without being subject to the gross values of the soil they are in. The actual local values which inhibit shoot elongation appear to be quite small, for example, 0.76 kPa (Addae and Pearson 1992). These values from controlled situations contrast sharply with gross field values and with the large number of inconsistent correlations which arise from attempting to correlate crop performance with grossly-measured soil structure. Perhaps, most encouragingly, studies of genetic variation in the sensitivity of crops to soil strength suggest that there is appreciable range in plant sensitivity. The relative ranking of genotypes is, however, the same when under near-critical stress as when growing with virtually no mechanical stress. Genotypes suited to stressful situations may be selected, therefore, by screening at a single soil strength (Addae and Pearson 1992).
TABLE 11
Grain and stover yield (t/ha) of maize and seasonal water runoff and soil loss under maize grown with and without alley cropping, with two tree legumes, and tillage in Nigeria (Source: Kang and Ghuman 1991)
|
Treatment |
Maize |
Runoff1 |
Soil loss (t/ha) |
|
|
grain |
stover |
|||
|
Without alley cropping |
||||
|
Tilled control |
2.3 |
3.1 |
66.0 (9.4) |
6.18 |
|
No-tillage |
2.4 |
3.2 |
5.6 (0.8) |
0.43 |
|
Alley-cropped |
||||
|
2 m Gliricidia |
3.2 |
4.6 |
4.8 (0.7) |
0.57 |
|
4 m Gliricidia |
2.8 |
4.2 |
23.1 (3.3) |
1.44 |
|
2 m Leucaena |
3.4 |
4.9 |
2.6 (0.4) |
9.17 |
|
4 m Leucaena |
3.1 |
3.9 |
10.7 (1.5) |
0.82 |
1 Seasonal rainfall (March-July 1988) = 704.2 mm.
The above comments relate to the direct effects of soil physical properties on crop growth. Two further points may be made. First, changing soil properties may not cause, or at least not immediately cause, measurable differences in plant performance (Figure 15). This neither invalidates an association between the soil and the plant, nor the need to remain concerned about changes in soil properties. It is common but not often reported, that changes in a cropping system and soil characteristics first affect aspects other than the crop. It is only when a system has become significantly degraded, and there are associated environmental impacts, that reduced plant performance is noticed. This is illustrated (Table 11) in a study at Ibadan, Nigeria, where no-till maize in rotation with cowpea gave similar yields to conventionally cultivated crops. The no-till fields had only one-tenth the runoff and erosion of those conventionally cultivated. Likewise, alley-cropping of the annuals between six year-old hedgerows of tree legumes (Leucaena leucocephala, Gliricidia sepium) caused increased crop yields, but within the alley-cropping systems the yields of maize were very similar despite substantial variations in runoff and soil loss.
TABLE 12
Desirable crop attributes that sustain soil productivity
|
Attribute |
Contributing characteristic |
|
Rapid establishment |
(Relatively) large seed for establishment and seedling vigour; abundant seed for propagation |
|
Groundcover to reduce soil exposure, suppress weeds |
Early branching; perhaps rhizomes or stolons; horizontal leaves (high canopy-extraction coefficient) |
|
Low requirements for nutrients |
· Colonization by associative bacteria (e.g., Brachyrhizobiun), micorrhizae (e.g., Glomus) and free-living organisms (e.g., Azospirillum) |
|
Efficient water use |
Short growth duration (to utilize residual moisture after crop); high water use efficiency |
|
Deep rooting to reduce water table (salinity) and recover nutrients at depth and increase macropores |
Vertical root distribution; roots penetrate high impedence soils |
|
Useful products |
High leaf/stem ratio; edible seeds; easily digestible; no nutritional compounds in material for livestock; leaf retention on stems for cut and carrying to livestock |
|
Non-host for diseases and pests of main crop (to break disease cycle) or decoy (to attract diseases from concurrent crops) |
Botanically unrelated to main food crop(s) |
|
Suppress other species |
Allelopathy (leachates, exudates which suppress or kill other plant species); also physical attributes (as above) |
The second point about soil-crop relations is that the crop also affects the soil through ground-cover, depth of rooting and other properties. The crop attributes that most influence soil physical properties are speed of establishment and development of foliage cover. Rapid establishment and growth minimizes topsoil structural decline and soil erosion by wind and water. Thereafter, deep-rooting directly affects soil structure, particularly if deep-rooted crops, such as safflower, are grown in rotation as a 'biological plough' to create macropores and these are minimally disturbed before the next crop is sown. A general list of desirable crop characteristics is given in Table 12. Not all these are universally applicable (nor indeed, accepted by all scientists) because some attributes such as stolons and rhizomes, on the one hand, can provide advantageous ground cover and bind the soil, but they are undesirable if the species presents weeding or other problems. The list includes both herbaceous and tree species. Ezenwa (1991) considers the attributes important for the choice of trees in Sahelian areas as the ability to: (a) enrich a microsite by depositing litter; (b) fix nitrogen and not be allelopathic; (c) avoid competition; (d) withstand stress, either water or wind; and (e) provide alternative produce such as fodder, fuel or fruit.
The three management practices which most affect soil physical properties (and consequent in-situ degradation and erosion) are groundcover, tillage and traffic load. Additionally, water harvesting techniques (FAO 1991) can be used to increase the amount of water stored within the soil and in surface catchments, though this does not necessarily affect soil structure.
Ground cover is crucial for the maintenance of soil structure in wet-and-dry climates. In marginal cropping areas in particular, plant populations (of weeds or pasture in non-crop periods) range widely and affect crop productivity. Their effects in semi-arid Syria are shown in Figure 17.
On resource-poor farms near the dry margins of the wet-and-dry croplands, populations of crop and weedy species tend to be small and variable during the cropping period. The effects of such sparse cover are clear, erosivity is directly related to the area exposed. In practice, during the crop phase there is a five-fold range in erosive risk associated with the area covered by living plants, consistent with the crop factor (C) values which are used in the Universal Soil Loss Equation (Table 13).
TABLE 13
Effect of crop cover on crop management factor C in the Universal Soil Loss Equation
|
Crop cover |
C1 |
|
No cover |
1.0 |
|
Maize, sorghum |
0.3-0.9 |
|
Groundnut |
0.4-0.8 |
|
Cassava |
0.2-0.8 |
|
Cotton, tobacco |
0.5 |
|
Oil palm, coffee, cacao with cover crops |
0.1-0.3 |
|
Rice |
0.1-0.2 |
|
Rapidly growing cover crop |
0.1 |
|
Savannah or pasture (without grazing) |
0.01 |
|
Forest or crop with thick layer of mulch |
0.001 |
1 C values range from almost zero, when the vegetation stops loss of soil directly related to rain, to 1 when the soil is fully exposed to rain.
The very low value for forests in Table 13 has encouraged advisers to advocate planting of trees as part of mixed crop systems (Table 11). Young (1989) points out, however, that trees are usually part of spatially-heterogeneous systems, such as hedgerows. Thus the tree canopy is usually not extensive and does not cover and protect the cropped land. He believes that such tree canopies are not likely to reduce erosion and may indeed increase it. Under agroforestry systems, however, the use of cut leaves and litter as groundcover may directly or indirectly improve protection by providing barriers to lateral water flow and wind.
Mineral ions are lost during lateral surface erosion. Eroded sediment commonly contains a higher proportion of organic matter and nutrients than the topsoil from which it is derived. This enrichment, frequently 1.5-4-fold, but sometimes as high as 10-fold, is because the eroded soil comes from the uppermost layer with the more organic matter than the soil below and because water differentially removes light particles and soluble nutrients.
Loss of plant cover also increases downward movement of ions and fine soil particles. The loss of mineral ions to groundwater is illustrated in Table 14. Such losses contribute to the negative nutrient balance of cropping systems (Chapter 3) and may also pollute groundwater. Under bare soil, downward translocation of silt and clay particles may fill subsoil pores and thus reduce infiltration. Vertical loss of organic material and fine particles may create a less-well-structured surface layer with less water-holding capacity, poorer infiltration and a tendency to crust (Figure 14).
The best ground covers are living crops and pastures. Soil stability declines in the order: living mulch; dead mulch on surface and incorporated dead mulch; and lastly, no mulch. The benefits of mulch incorporation depend on slope, soil erodibility, and on the type of organic matter. These and the type of tillage used determine whether the dead mulch will remain on the surface. Costs and availability of both mulch and labour (human, animal or tractor) are also important. Though farmers may appreciate the benefits of surface mulching, it is unlikely that it will become practice in those tropical and temperate wet-and-dry climates where there is a shortage of feed for livestock in the dry period(s).
TABLE 14
Influence of plant cover of catchments on ion leaching (Source: Ryszkowski 1989)
|
Type of catchment |
Catchment |
Percent of area |
Outflow of elements during 2 years (g m2) |
||||||
|
Arable |
Forests |
Grass-lands |
N-N03 |
P-PO43 |
K+ |
Ca2+ |
Mg2+ |
||
|
Larger contribution of cultivated fields |
1 |
53 |
12 |
35 |
1.63 |
0.03 |
3.32 |
37.66 |
2.82 |
|
2 |
62 |
8 |
28 |
1.43 |
0.04 |
2.99 |
35.42 |
3.30 |
|
|
3 |
53 |
32 |
13 |
1.13 |
0.04 |
2.57 |
30.95 |
2.28 |
|
|
4 |
51 |
21 |
27 |
0.71 |
0.02 |
2.91 |
32.40 |
2.70 |
|
|
Mean |
|
55 |
18 |
26 |
1.22 |
0.03 |
2.36 |
34.13 |
2.78 |
|
Smaller contribution of cultivated fields |
1 |
38 |
44 |
17 |
0.60 |
0.02 |
1.58 |
23.33 |
1.80 |
|
2 |
29 |
45 |
26 |
0.48 |
0.01 |
1.00 |
25.63 |
1.44 |
|
|
3 |
21 |
65 |
14 |
0.39 |
0.01 |
0.91 |
22.54 |
1.56 |
|
|
4 |
32 |
47 |
21 |
0.44 |
0.01 |
0.91 |
15.26 |
1.20 |
|
|
Mean |
|
30 |
50 |
19 |
0.48 |
0.01 |
1.10 |
21.69 |
1.50 |
TABLE 15
Objectives, advantages and disadvantages of soil tillage in crop production
|
Objective |
Type of tillage |
Advantage |
Disadvantage |
|
Soil conditioning |
Cutting, loosening, granulating |
Weed control, water conservation, structure improvement, seedbed preparation, better drying of wet soils |
Greater erosion potential, high energy input1, increased evaporation |
|
Eradication or control of plants or plant materials |
Cutting, inverting, mixing |
Weed control, volunteer plant control, water conservation, establish desirable plant populations, pest control2, better drying of wet soils, mineralization of soil nutrients |
Greater erosion potential, may cause compaction, high energy input, decreased soil organic matter, increased evaporation |
|
Establishing soil boundaries and surface configurations |
Cutting as with coulters to improve ploughing, operation, land forming |
Weed control, soil conservation, water conservation, residue incorporation, seedbed preparation, better drying of wet soils, warmer soil temperatures |
Greater erosion potential, may cause compaction, high energy input, increased evaporation |
|
Incorporating, covering, or handling foreign materials |
Cutting, inverting, mixing |
Weed control, residue incorporation, mineralization of soil nutrients, fertilizer and pesticide incorporation, pest control, better drying of wet soils, warmer soil temperatures |
Greater erosion potential, may cause compaction, high energy input, decreased soil organic matter, increased evaporation |
|
Segregation |
Move soil materials from one layer to another |
Wind erosion control, better drying of wet soils |
High energy input, increased evaporation |
|
Mixing |
Mixing |
Better drying of wet soils, improved soil amendment distribution, fertilizer and pesticide incorporation, soil texture improvement (mixing of two or more layers), soil structure improvement, mineralization of soil nutrients |
Great erosion potential, high energy input, decreased soil organic matter, increased evaporation |
|
Compaction or firming |
Rolling or pressing |
Improved seed-soil contact |
May cause compaction |
1 High energy input: may include fuel for tractors, feed for animals, labour and equipment inventories or usage.
2 Pests controlled may be insects, diseases, rodents, etc.
The objectives, advantages and disadvantages of tillage are summarized in Table 15. Had this been written 8-10 years ago, tillage would have been emphasized (for example, Unger 1984). Currently, tillage is seen as appropriate for some objectives, for example to break impervious layers of subsoil and to incorporate organic matter topsoil, but generalizations about the value of tillage or the 'best' type of tillage are not regarded as transportable from one system to another. Tillage may be defined as the mechanical manipulation of soil to enhance the outcomes from a cropping system. So, given the broad range of system outcomes (Chapter 1), it is likely that tillage will continue for reasons such as tradition and satisfaction long after its beneficial benefits on soil structure and weed control have been discounted.
FIGURE 18 Variation in crop yield for different soil depths of a Vertisol in the Central Highlands of Queensland, Australia (Source: Littleboy et al. 1989)
When advocating particular tillage methods, or interpreting experimental results and their transferability to other environments, it should be noted that tillage is usually affected by the type of ground cover. The variables of tillage are the degree of contact with the soil, the type of manipulation, and the number of implement passes through the soil. Thus, minimum tillage (one-pass) and conventional tillage (perhaps 3-5 passes, in the Australian farming system), might have the same effects on the soil, differing only in degree. Conservation tillage, which uses different implements and ground covers, is defined as 'any tillage or cultivation system that maintains at least 30% of the soil surface covered by residue after planting to reduce soil erosion by water; or where soil erosion by wind is the primary concern, maintains at least 450 kg/ha of flattened small grain residue equivalent on the surface during critical erosion periods' (Mannering et al. 1987).
In seasonally wet-and-dry climates, the most apparent benefits from tillage are weed control and reduced evapotranspiration from weeds preceding the crop. This increases the water available at the beginning of the crop period and the volume of soil into which roots may penetrate (and thus, the available soil water). The impact of tillage on soil volume is modelled in Figure 18. This shows that rooting depth (or soil depth in this example) has a marked effect on the probability of achieving certain crop yields. Thus, exploitation of larger soil volumes may both increase yield and increase the likelihood of attaining a particular yield, that is, reduce the year-to-year risk.
The variable effects of type of tillage on crop growth (Figure 15) are interpreted by recognizing that some effects of tillage, such as increasing the potential root volume, occur within the season of tillage, while other effects are longer term. Tillage will, over several years, reduce the outer permeability of clods, restricting water-holding capacity and increasing the power required for further effective tillage. In some soils it dis-aggregates clods, causes downward movement of silt and clay which fills soil pores and reduces infiltration. It can also accelerate the breakdown of organic matter. Some of these effects on plant growth are discussed in the previous section and Unger (1984) presents further data on the impact of different types of tillage implement on soil and erosion.
Compaction by machinery or by treading by livestock is common and is reviewed elsewhere. The ultimate example of soil degradation through tillage and compaction is the formation of impermeable beds for rice production. Figure 19 illustrates the effects of compaction and plant activity on bulk density. Three levels of compaction were created (0, 40 and 100 kPa) under grassland. At depths below 15 cm the porosity and pore length reflected these pressures. In some places the volume of pores was halved by the heaviest compaction treatment. The subsoil bulk densities also reflect the deleterious effect of pressure. From 1987 to 1990, however, all bulk densities near the surface decreased. Grass roots were apparently able to offset the effects of machinery pressure.
General field indicators of poor soil physical conditions include:
1. Patchiness or absence of vegetation. This can be an obvious sign of degraded structure or other factors. When structural, it may reflect surface structure degradation (see previous sections) or non-wetting characteristics which give rise to poor infiltration, or subsoil impermeability.2. Weedy vegetation. Cyperaceae or Juncaceae may indicate soil structural decline because they flourish where water has been ponded on the surface, suggesting poor infiltration or an impermeable subsurface horizon.
3. Rill and sheet erosion. Erosive runoff may be symptomatic of poor surface structure. The turbidity of water in ponds and lakes after rain may be a good indicator of erosion.
4. Surface crusts.
5. Hard-setting surfaces.
6. Poor infiltration and ponding. This may be indicated by puddles following rain in an area where one would expect rapid infiltration, or by wetting to only a shallow depth (as seen when dug with a spade).
7. Pale surface soil colour and absence of organic matter. The surface of degraded soils may be brittle and pale, lacking organic matter and having lost clay either through eluviation (differential movement downwards) or by water or wind erosion.
8. Cloddiness. This may be apparent if after a single cultivation, large, tough clods are formed requiring further cultivation to form a reasonable seedbed.
9. Restricted root growth. This can be seen by digging with a narrow-faced spade and washing the roots free of soil. The root mass can be restricted to the upper soil or be constricted in particular places such as a less pervious layer, above and below which the roots may proliferate.
When assessing soil degradation on a large scale as for a catchment, Hamblin (1991a) argues that most precise (scientific) measurements are time-consuming and complex, and that robust surrogate assessments amenable to remote sensing are appropriate. Vegetation cover is one useful surrogate. Ranges in groundcover (often easier to estimate than plant densities) provide a direct index of soil productivity and they usually, although not necessarily, correlate closely with underlying problems of soil structure or fertility. Sparse groundcover might also, of course, reflect inappropriate management of non-degraded soil. This could suggest inefficient management which will lead to loss of sustainability though there are no current problems with soil structure. The surrogate features Hamblin appears to favour are turbidity of water, transient ponding and waterlogging. Transient ponding is particularly attractive as it can be sensed remotely, relates directly to the field being measured, and reflects several important physical aspects. These latter include surface degradation, continuity of vertical macropores, and subsurface compaction, none of which can currently be satisfactorily measured directly to give cheap meaningful data.
At farm level, attuned observers and local farmers will probably identify, often subconsciously, more indicators than those in the above list. Farmers are often good observers of their fields and can recognize soil degradation. For example, interviews with 55 farmers in Mindanao, Philippines, found that 90% reported yield declines associated with soil erosion: 'Many farmers said their soils initially had been dark on top and reddish underneath but over time the top layer had eroded away. The infertile, red subsoils were exposed. Farmers estimated that erosion reduced the depth of the darker soil from 50 cm in 1976 to 10 cm in 1986' (Fujisaka and Garrity 1991). Table 16 illustrates their insights to declining soil productivity. Many farmers recognize changes in soil structure (e.g., compaction) through the increased power needed during tillage, or by the formation of brick-like clods rather than a fine crumby tilth. These observations, however, are made at the beginning of the growing season, when there is little opportunity for soil amelioration. Farmers should be encouraged to dig small inspection holes to examine roots during the life of the crop. This supplements observations described above. Live roots are easily exposed and reflect many soil structural factors. Such examinations allow the farmer to consider soil amelioration, using stubble and crop residues, prior to the next crop. Figure 20 illustrates clear differences in root patterns revealed by digging in three tillage systems in India.
TABLE 16
Examples of farmer concepts/statements concerning aspects of sustainable crop production (Source: Fujisaka and Garrity 1991)
|
Crops and soil nutrients Nutrient depletion Fallows Weeds Soil erosion Erosion control |
It is important too that farmers, whether resource-poor or advanced, understand the conditions under which soil degradation is most likely to be severe. Broadly, human-induced structural damage occurs during tilling, when heavily trampled by livestock, and by poor management of soil cover.
Weather-induced damage is episodic rather than continuous. Virmani (1990) explains wind and water erosion as follows:
Wind erosion is a serious problem in the arid and semi-arid tropics on sandy, loamy sand or sandy loam surface soils low in organic matter with weak topsoil structure in areas with high winds (>20 km/h). In tropical areas where wind erosion occurs the annual rainfall is generally less than 600 mm and is seasonal. The rest of the year is dry. In the tropics, 80-90% of the annual wind erosion losses occurs in the hot and dry period preceding the short rainy season. During this period the daily maximum temperature usually exceeds 35-40°C, the wind speeds exceed 20 km/h, and open pan evaporation exceeds 8 mm/d. As a rule of thumb, on uncultivated bare sandy soils, the soil displaced by wind erosion is around 1 t/ha/WED, where WED is the sum of the number of days during which the wind speed exceeds 20 km/h, the day temperature exceeds 35 °C and the wind fetch is more than 60 km. For freshly cultivated soils the amount of soil eroded may be as large as 50 t/ha/WED.
Water erosion is serious in areas with an average annual rainfall of 600-1100 mm. Using the concept of a rain erosion month (REM), that is a month in which rainfall exceeds pan evaporation, Virmani (1990) states that water-induced erosion at rates of approximately 1 t/ha/REM occurs on cropped Vertisols in areas where the rain/pan evaporation ratio ranges between 1 and 2 and where improved soil and water conservation practices have been applied. He records rates of 4 to 8 t/ha/REM where practices are not improved.
'In the struggle against aridization and desertification, dry-farming is no less important or urgent than revegetation and irrigation' (Kovda, 1982). Maintenance or improvement of soil structure requires: (a) recognition of the interrelated nature of soil attributes and their interaction with cropping; (b) the ability to recognize the symptoms indicating physical problems or their likely existence; and (c) action.
The interrelated nature of soil and plant attributes, and measures of degradation, are discussed above. The size of the problem, regionally and internationally, is also well recognized. Likewise, the magnitude of local soil losses is established: for example, values as high as 200 t/ha/y in Niamey (Virmani 1990) and measured losses of 282 t/ha/y in the highlands of Ethiopia (Gander 1990). Lastly, and significantly, it is becoming recognized that soil physical characteristics may be less robust than chemical attributes. In Australia, the rates of degradation of structure and compaction are faster, on the same soil types, than the degradation of chemical attributes (Table 17).
TABLE 17
Rate of change in soil physical and chemical characteristics from various locations in Australia (Source: Hamblin 1991b)
|
Characteristic |
Measure of change |
Time for change to occur (years) |
|
Surface structure |
loss |
2-10 |
|
Subsoil compaction: |
formation |
1-5 |
|
pH |
decrease by |
7-12 sand |
|
Salinity |
not quantified |
10-100 |
The issue now is to reflect in a general way, as is possible only in a bulletin like this, on appropriate action.
There are many regionally-based assessments of land degradation and various studies on how management may improve, or reduce the rate of deterioration in, soil physical structure (e.g. Thomas et al. 1989). Regional identification of the size of the problems, and general prescriptive strategies to ameliorate them, are appropriate for planning and allocation of resources. An example from Africa (FAO 1986) is outlined in the extensive quotation that follows:
TABLE 18
Changes in cropping patterns and calculated erosion hazard in three regions in Italy between 1954 and 1976 (Source: Chisci 1986)
|
Location |
Catchment and area (ha) |
Change in cropping systems |
Change in calculated erosion hazard (t/ha/year) | |
|
Tuscany |
(1) |
98 |
Mixed crops replaced with monoculture, particularly vines, using mechanical, non-contour tillage; minimal pastures in new catchments |
6.4 to 7.7 |
|
Emilia |
(1) |
38 |
Traditionally forested; catchment logged and abandoned for regrowth and not cultivated |
4.2 to 16.2 |
|
Piedmont |
(1) |
92 |
Mixed cropping; some reduction in cropping while retaining contour tillage |
40.0 to 33.5 |
The African environment is being seriously degraded as a result of the over-exploitation of cropland. The degradation of cropland can best be halted by improving soil fertility, limiting soil erosion, and introducing water harvesting and dryland farming techniques. Options for the first two include:
· replacing shifting cultivation with perennial tree crops such as oil palm, coconut, coffee and cacao;· growing crops between alleys of leguminous trees and shrubs, and supplementing the nitrogen and organic matter they supply with farmyard manure;
· using zero or minimum tillage systems to minimize soil erosion;
· adopting integrated plant nutrition to supply nitrogen by using animal manure alone or mixed with mineral fertilizers, or by growing leguminous crops; and
· constructing physical barriers to soil erosion, such as earth bunds, bench terraces and tied ridges.
Where population pressure has led people to cultivate hillsides with marginal soils, or to settle in semi-arid areas better suited to pastoralism, three simple measures can reduce degradation: the construction of small dams to conserve water, and tree planting on upper slopes; the use of dryland farming techniques; and, finally, the use of short-season, drought-resistant cultivars.
To tackle the problem at a local level work needs to be done in cooperation with the farmers themselves: 'start with what they know, build on what they have' (Altieri 1988). Table 18 illustrates a locally-targeted analysis of erosion risk in the hills of Italy. It combines calculations of erosion risk with on-farm assessment of changes in land use. There is a predictable increase in erosion hazard associated with deforestation and increased cropping of monocultures, especially under vines, where there is no additional ground cover and where the land is not ploughed around the contours. Erosion risk fell where cropping declined over the 20-year study period.
It seems most likely that lasting changes to cropping practice and control of erosion will be based on farmers' recognition of trends such as those in Tables 16 and 19, coupled with strategies which farmers find appropriate for their goals. These strategies are listed in Table 19, which broadly sets out the options. Soil physical sustainability depends on activities which are explicitly aimed at its maintenance, and if necessary, any activities needed to ameliorate or control damage already done. Water harvesting can be used as a complementary strategy, the options for which are summarized by Critchley and Siegert (FAO 1991). All the strategies in Table 19 are described above and some have been illustrated. It is appropriate to repeat, however, that few such strategies are universally applicable in detail. For example, minimum soil disturbance may, after several years, enhance sustainability in temperate semi-arid Australia and USA (e.g. Figure 15) whereas tillage experiments in Africa and Asia support conventional, even deep, ploughing (e.g. Figure 20). While the philosophy of minimal disturbance may be biologically 'right', differences in soil texture may require different approaches to sustainability. For example, Sahelian soils are commonly coarse-textured with high bulk densities and form crusts. They do not readily develop macropores. Though it is desirable to mulch such soils with straw, there is none available as it is used to feed livestock.
TABLE 19
Aspects to consider for maintenance or amelioration of soil physical properties
|
Maintenance: prevention of physical degradation - Crop choice.
- Crop cultural practices
- Inter-crop ley and fallow
- Mulches
Amelioration to control damage - Management of water erosion
- Management of wind erosion
- Soil surface management
- Compaction
|
