Chapter 5
Soil crusting and sealing

J. Morin, The Institute of Earth Sciences, The Hebrew University of Jerusalem

As many as 500 million people live in the semi-arid regions of the world, and most of them depend on agriculture for their livelihood. The populations of many of the countries in such regions have doubled in the past three decades and are still growing rapidly. These increases have not been accompanied by similar rises in food production. The pressing need to assure an adequate and reliable food supply is obvious.

The dominant characteristic of the semi-arid zone is an insufficient water supply to support stable agriculture. Not only is there insufficient rainfall, but its occurrence is also highly erratic between years, during the year, and spatially, during any single rainfall event. In general, the rainfall pattern becomes more variable as the mean annual rainfall decreases.

The semi-arid zone includes climates of two main kinds: the Mediterranean climate, where rain falls during the cool season of the year, and the semi-arid tropics where most or all of the rainfall occurs during the warm summer months. The minimum annual rainfall necessary to support a crop in the Mediterranean zone is approximately 250-400 mm, while in the semi-arid tropics it is 400-600 mm or even higher.

It is in the sub-humid to semi-arid tropics, however, that the problem of sealing, hardsetting, or soil capping, appears to be the most serious. Such conditions are typically found in the Sudan-Sahelian region of West Africa as well as in large parts of eastern and southern Africa, India, Thailand and elsewhere. Where the bare soil surface is sealed rainfall cannot penetrate and runs off laterally, even on very gentle slopes.

In many such localities the overall climatic conditions could allow one annual crop in most years. A 3-month rainfall of 700-1000 mm is sufficient for economic yields of crops like millet, sorghum, cowpeas and groundnuts. The success of dryland agriculture depends, however, on whether this rainwater can penetrate and be stored in the soil.

Moisture storage is usually not a restriction, unless the soils happen to be very sandy or very shallow. Many of the soils in the semi-arid tropics have effective depths of approximately 100 cm, with a subsoil structure and a clay minerals assemblage which guarantee an effective soil moisture storage capacity of at least 100 mm. A critical factor, however, is the degree to which the surface allows the rainwater to penetrate, right from the start of the rainy season. A significant part of the water does not enter the soil even though the water storage capacity of the soil is far from taken up.

It is not generally realized that runoff losses from a field can amount to 30-35% of storm rainfall (Hoogmoed and Stroosnijder 1984). Much of the rain in semi-arid zones falls at high intensities, causing runoff and severe erosion. Since African rainstorms are usually heavy, such erosion is common even on moderate slopes.

Sealing also impedes seedling emergence, because of the strength needed to break through the crust, and the formation of an oxygen-deficient layer immediately below the crust.

When trees and bushes are cut, leaving the soil surface bare and unprotected, crust formations develop rapidly, preventing adequate moisture from entering the soil. This is the main cause of the progressive desertification in the Sudan-Sahelian region.

TERMINOLOGY

There are three main kinds of crusts and seals. These are described below.

Surface seal is defined as the orientation and packing of dispersed soil particles which have disintegrated from the soil aggregates due to the impact of rain drops. By definition surface seals are formed at the very surface of the soil, rendering it relatively impermeable to water.

A structural crust is defined as a surface layer of the soil, ranging in thickness from a few millimetres to a few centimetres, which is much more compact than the material beneath. The import of external materials is not involved in the formation of the crust.

Structural crusts are formed also by physical forces as a result of trampling by livestock or through traffic by agricultural machinery and other vehicles.

Depositional crusts form when soil particles, suspended in water, are deposited on the soil surface as the water infiltrates or evaporates. Externally-derived materials are always involved in the construction of depositional crusts.

FACTORS AND MECHANISM OF CRUST FORMATION

The processes and mechanisms of crust formation are discussed in this section. Special emphasis is given to surface seal, which is dominant in most cultivated and exposed soils.

Surface Seal

The rapid drop in infiltration rate of most bare soils during rainstorms is due mainly to the formation of surface seal. The permeability of the seal is lower by several orders of magnitude, than the subsurface permeability. Surface sealing, as well as most other crust formations, results from three processes (Agassi et al. 1981; Morin et al. 1981).

• Physical disintegration of soil aggregates and their compaction, caused by the impact of raindrops.
• Chemical dispersion of the clay particles. The low electrical conductivity of the rainwater as well as the organochemical bonds between the primary particles of the surface aggregates, dictate the rate and degree of dispersion.
• An interface suction force which arranges suspended clay particles into a continuous dense layer. Such almost impermeable layers form right on the surface of the soil or in the immediate subsurface washed-in layer, as discussed by McIntyre (1958).

The separation above is artificial. The marked reduction in infiltration rate depends on the combined action of the three processes.

Figure 16 shows the relative influence of raindrop impact and chemical processes (Agassi et al. 1985). It stresses the importance of the electrical conductivity of rainwater and the high levels of soil sodicity in reducing infiltration rates. Since high electrolyte rainwater has an impact force only, eliminating the dispersion effect of the distilled water, the relative influence of these two processes can be seen. In general the beating action of the drops enhances the chemical dispersion of the soil.

FIGURE 16
Effect of electrolyte concentration on the infiltration rate of a loess soil at two ESP levels (Agassi et al. 1985)

Rain characteristics and their effects on surface seal formation

Rain impact forces depend on size distribution of the raindrops, their velocities and intensities. Table 11 gives such information for major rain types. It shows that light rain has a very low impact force compared with heavy rain. The low impact results from the smaller drop size (and hence velocities) and the lower rain intensity.

The relationship between rain characteristics and infiltration rates can be studied using a rainfall simulator. The relationships between soil infiltration, rain depth and rain momentum are demonstrated by Figures 17 and 18. Figure 17 presents the decline in infiltration as a function of cumulative rain depth, generated by different drop sizes and velocities, while Figure 18 presents it as a function of accumulated momentum. Both figures demonstrate that the decline of infiltration rate is greater and the final values are lower, with increase in height of the drops' fall. Figure 19 presents the final infiltration rate as a function of momentum per kg of mass (1 mm m^-2). The final infiltration rate of the loess soil decreases asymptotically as the impact of the drops increases. There are very small differences for the momentum values higher than 5 N s^-1.

TABLE 11
Kinetic energy and number of drops for rainfall of various intensities (from Lull 1959)

The infiltration and raindrop momentum relationships can be summarized as follows: the impact momentum of raindrops is a key factor in the quantitative evaluation of soil infiltration. When considering seal formation, a differentiation should be made between the rate of seal formation and the final permeability of the seal. The rate of seal formation depends on the accumulated momentum of raindrops beating the soil surface. The soil surface consists of aggregates of various strengths and drops with low impact are not able to break the more stable aggregates so they are not as effective in forming continuous impermeable seals. On the other hand, there is a threshold of the impact momentum values, and no further destruction of the aggregate is achieved by applying higher momentum force. Therefore, surface seal relations can be evaluated quantitatively using the rainfall momentum characteristics. The marked impermeability of surface seals in most African soils is directly related to the high impact values of the rainstorms there.

Structural Crusts

Hardsetting Crusts

Hardsetting of cultivated soil is a process of compaction, with increase in bulk density, that occurs without the application of an external load. In practice, it is difficult to distinguish between the effects of an externally applied load and the internal effect caused by the wetting of weak unstable soil. In previously loosened topsoil, during and after wetting, hardsetting involves the collapse of some or all of the aggregated structure (tilth). The hardsetting processes can be divided into two physically distinct processes: slumping and uniaxial shrinkage.

Slumping: Slumping is not limited to hardsetting soils. It occurs during and after the wetting of a soil horizon formed of water-unstable aggregates. The aggregates soften and swell simultaneously, and some or all of the silt and clay-sized material becomes suspended (although not necessarily as individual particles). Under appropriate ionic conditions, some of the clay fractions disperse. Aggregates disintegrate because they have insufficient strength to withstand the stresses set up by rapid water uptake, caused by rapid release of heat on wetting, trapped air, the mechanical action of rapidly moving water (Collis-George and Greene 1979), or by differential swelling (Emerson 1983). The matric potential of the soil, prior to wetting, also influences the incidence or severity of slaking. Moist aggregates slake less readily than air-dry ones because they have already completed some or all of their swelling and some pores are already water-filled.

Uniaxial shrinkage: Shrinkage is of importance because the closer proximity of particles that it entails makes a contribution to the increase in strength upon drying hardsetting soils. Laboratory experiments on the behaviour of aggregate beds of a hardsetting soil, wetted under tension or at zero potential, show that, at least during the early stages of drying, uniaxial shrinkage occurs. Since uniaxial shrinkage is, by definition, anisotropic, it follows that it must be accomplished by realignment of the disrupted aggregates and/or the internal fabric of the soil. Such a realignment can occur without cracking only if the forces holding the soil together are long-range and nonspecific. this point is important because this kind of force is likely to be provided by the matric potential, and therefore, when uniaxial shrinkage occurs, this may be an indication that effective stress contributes a dominant component of soil strength.

Mullins et al. (1987) have proposed the following explanation for the development and increase in strength observed in hardsetting soils, starting with a cultivated bed consisting of dry aggregates:

• Wetting of the system mobilizes some or all of the silt and clay. This may occur through slaking and/or dispersion.
• During the early stages of drying, the mobilized material is carried behind the retreating water meniscus to occupy concavities on the surface of sand grains and any remaining aggregates, forming annular bridges between them. This phenomenon has been observed under the microscope by Kemper et al. (1987).
• As drying proceeds, despite air entry into the soil, the mobilized material remains saturated until a very low potential is reached; consequently, the contribution of matric potential to the effective stress provides a major component of the soil strength.

Traffic Crust

Traffic crusts are discussed only briefly as they are outside the general scope of this article. Surface compaction by farm machinery and animals can cause a serious reduction in penetrating water and seedling emergence. Overgrazing can induce crust formation by two mechanisms:

1. surface compaction of wet or moist soils; and
2. mechanical destruction of the surface aggregates, which on dry soil can powder the soil surface.

This tends to speed up crust and surface seal formation at the oncoming of the rains.

Depositional Crusts

Depositional crusts, formed as suspended particles are deposited on the soil surface, are common in a limited area of cultivated and non-cultivated soils. The main sources of fine eroded soil particles are:

• Flood and furrow irrigation water
• Raindrop impact splash of loose soil particles from ridges and surrounding higher ground
• Overflow and floods from rivers
• Runoff and sheet erosion

The permeability of a depositional crust to water depends mainly on the clay mineralogy and the electrolyte concentration of the carrying water. The clay and silt particles in turbid suspension can either disperse or flocculate. They flocculate when the electrolyte concentration in the suspension exceeds the flocculation threshold of the clays (Oster et al. 1980). Depositional crusts formed from flocculated particles have an open structure and high permeability. Conversely, when the suspension electrolyte concentration is below the flocculation threshold, dispersed particles settle to form the depositional crust, the hydraulic conductivity of which is several orders of magnitude lower than that of the parent soil.

SOILS LIKELY TO SEAL AND CRUST

Effects of Clay Mineralogy

Crusting of all forms involves the initial destruction of soil aggregates. It is therefore important to review the materials and factors which affect the forces binding soil aggregates. Aggregates are mainly held together by electrochemical forces which bind the clay mineral particles; most of these clusters are plate-shaped. The possible arrangements of clay particles are illustrated in Figure 20.

Aggregate stability depends on the arrangement of the actual soil particles. Clay mineral type, their electro-chemical characteristics and the electrical concentration of the soil solution, determine the kinds of association.

Clay Mineral Types

Kaolinite

The diagrammatic sketch of the structure of kaolinite presented in Figure 21 helps us to understand the main factors involved in the binding of kaolinite plates.

Plate 2, a scanning electron micrograph, shows the shape of kaolinite particles, each of them composed of several hundred 1:1 sheets. Van der Waals' forces as well as O-OH connections between the different plates, hold these plates close together so that water and electrolytes cannot penetrate in between them. The CEC (cation ex-change capacity) is less than 10 me per 100 g of clay.

Stable kaolinite aggregates have edge-to-edge and edge-to-face connections (Figure 20) between the clusters, since some positive charges exist on the broken edges, while the broad particle faces expose a negative charge and thus repel each other.

Smectite

A diagrammatic sketch (Figure 22) of the smectite group structure shows that this 2:1 mineral type differs markedly from the 1:1 kaolinite type. The weak connection between the element-ary 2:1 units allows water as well as exchangeable cations to penetrate. The negative clay faces attract hydrated cations which cover them completely. The main binding connections between the elementary sheets and clusters are Van der Waals' forces. Connection types (edge-to-face relations) and strength are influenced to a great extent by the electrolyte concentration and the kinds of cations, much more than for the kaolinite group. Swelling on wetting and shrinking while drying are the characteristics of the smectite clay group, since some of the soil water solution forms part of their construction. The CEC of varies between 80 and 150 me per 100 g (including both internal and external surfaces).

The SEM micrograph in Plate 3a and b (Keller et al. 1986) presents the swelling type of Na-montmorillonite, where the plates are expanded by high hydration (Plate 3a); and Ca-montmorillonite, with low hydration, where the plates are held tightly (Plate 3b).

Illite

Illite is a 2:1 mineral type with a general structure similar to the smectite group as presented in Figure 22. The units are the same, except for some differences in isomorphic substitution.

The potassium cation, which occupies some of the holes of the hexagonal oxygen ring, is part of the mineral structure. Most of the 1:2 units are held together firmly by this non-exchangeable potassium. As for kaolinite, water and electrolytes cannot penetrate in between the sheets. The CEC varies between 20 and 40 me per 100 g. There are some non-potassium hydrated layers that would be responsible for some internal surface which explains the CEC variations. Electron micrographs reveal that illite particles have irregular surfaces and their planar surfaces are terraced (Greene et al. 1978). As the particles approach each other, the irregular surfaces permit only poor contact between the edges and planar surfaces. This weakens edge-to-face attraction, and flocculation then requires a high electrolyte concentration. Illite soils are susceptible to clay dispersion. A scanning electron micrograph of illite is shown in Plate 4, which emphasizes the cluster of tight units with their relatively large, flat-lying plates.

Effects of Soil Chemistry

Electrolyte composition and concentration of soils solution

The formation of all soil crusts involves aggregate disintegration and dispersion. The dispersion of soil colloids is controlled by the nature and the distribution of the exchangeable cations held by permanent and/or variable charge on the colloidal surfaces. The short-range attractive forces, mainly Van der Waals' forces, are responsible for the coagulation or flocculation of the separate clay particles and for holding them in bigger clusters. The two major factors controlling flocculation are cation type, and the concentration of the electrolyte solution. Figure 23 demonstrates the effect of these two factors, for various concentrations of sodium and calcium, on the flocculation of a montmorillonite suspension. It is seen from the figure that sodium (Na⁺) is a much more dispersive cation than calcium (Ca⁺⁺). As the percent of Na⁺ rises, a solution of much higher concentration is needed to cause flocculation. In other words, clay with a high exchangeable sodium percentage will disperse quickly under the dilution effect of rainwater at the soil surface. In general, the higher the absorbed cation charge, the higher the clay stability. In most soils, the cation-binding effect on stability is in the following order:

Al⁺⁺⁺ > Fe⁺⁺⁺ > Cu⁺⁺ > Ca⁺⁺ > Mg⁺⁺ > K⁺ > Na⁺

Organic Matter

Although the beneficial effects of organic matter are widely accepted, there is less agreement on how it improves aggregate stability. The role of organic substances in stabilizing soil structure can be divided into two: on the one hand, organic substance can reduce interaction of water with inorganic colloids; on the other hand, it can bind soil particles together physically or chemically. The damaging effect of re-wetting is diminished in the presence of organic substances. Soil structure is also rendered more stable by the binding action of organic matter. For example, the physical binding of soil particles in a network of microbial mucilage. When analysing the mechanisms involved in the formation of clay-organic complexes, the nature of the organic and clay mineral colloids must both be considered. It has been shown that organic matter consists of high-molecular-weight polymers of various compositions and shapes. The organic polymers expose along their long complex surfaces, negative and positive edges of amides, carboxyl, hydroxyl and other polarizable groups. The organopolymer-clay connections are strong and water-stable.

In summary, the mechanisms by which organic colloids stabilize soil structure can be attributed to the binding of organic polymers to clay surfaces by: cation bridges; hydrogen binding; Van der Waals' forces; and the formation of sesquioxide-humus complexes.

The organic colloids compete with water molecules for space and surface, reducing wetting and swelling and increasing the strength of the aggregates through cementing effects.

Iron, Aluminium Oxides and pH Relationships

The association of stable structures with large contents of iron oxides has led to the view that iron oxides bind soil particles together. Emerson (1983) pointed out that soils rich in iron oxides and with a stable structure are generally acid. It may well be that structural stability is due to the effect of low pH on the positive surface charge and aluminium oxides, rather than to the binding of iron oxides (Keren and Singer 1990). The pH effect is indicated by the fact that higher pH values will diminish the positive edge charges in the clusters. The strong edge-to-face connection will collapse in this situation. This is particularly pronounced in kaolinite soils.

Combined Effects

From consideration of the clay mineralogy and soil chemistry of soils, we can conclude that hardsetting crusts are most readily formed in kaolinitic soil types, as indicated in Figure 24.

The non-uniform swelling and shrinking of the smectite group helps to prevent layer collapse by flood or saturation, a feature typical of kaolinite. The slow rate of smectite swelling accounts for its stability, in spite of the fact that Ca-montmorillonite swells approximately 25 times more than Ca-kaolinite. On the other hand, the dispersivity of the top surface and surface sealing are more pronounced among soils dominantly formed from the smectite group. In all soil types sodium is the cation which most influences crust formation by its high dispersivity effect.

Sensitivity of the Main African Soils

Taxonomy

In the preceding sections, the hazards and qualities of crusts are related to clay minerals and soil chemistry. In this section, a description of the major soils in the sub-humid and semi-arid zone of Africa is presented. The various soil types are evaluated according to their crust and surface seal restrictions. It should be noted that there are inconsistencies, because even where classifiers use similar soil properties in classifications, they can be used at different levels within the various classifications.

Figure 25 shows the distribution of the main African soil types. Crust and surface seals can be formed on all these soil types, where the soil is bare and exposed to heavy rain. Runoff and water deficiency are common for all the bare soils, except Oxisols. In these soils surface aggregates are stabilized by the strong connections of their kaolinite coated particles, with the oxides and oxihydrated complex of iron and aluminium. A high infiltration level is maintained throughout rainstorms. Vertisols, Luvisols and Planosols, the main cultivated soils in the sub-tropics, are evaluated specifically for northern Cameroon, but it is believed this evaluation can be extrapolated to similar areas elsewhere in Africa.

Soil properties for Vertisols, Luvisols and Planosols, the three main soil orders in northern Cameroon, are presented in Table 12. This information is used later, in the soil descriptions and evaluation of crust restrictions.

TABLE 12
Some physical, mineralogical and chemical properties of the dominant soils in northern Cameroon (Shainberg et al. 1988)

Vertisols extend over 40% of northern Cameroon. Their clay content ranges between 30% and 45%, and the silt content between 10% and 20% (Table 12). The clay is predominantly montmorillonite and the CEC ranges between 23 and 35 me/100 g soil. Calcium is the dominant cation on the exchange complex, accounting for 60 to 80% of the total exchangeable cations. Exchangeable Mg ranges between 20% and 30% of the CEC, and the exchangeable sodium percentage is low (<1.0%) at the soil surface, increasing in depth, to a value of 10 at 2.0 metres. The Vertisols in Cameroon are non-calcareous. The presence of exchangeable Na, exchangeable Mg and the low electrical conductivity of the soil solution may explain the dispersivity of these soils, especially when exposed to the high intensity rains. Surface seals are formed rapidly and reduce the final infiltration rates to 3-4 mm h^-1. As these soils are deep and rich in swelling clays, wide cracks are formed, allowing better penetration through them (Plate 5).

Luvisols cover about 30% of north Cameroon. Characteristically, the clay content increases with depth (Table 12). The clay content in northern Cameroon Luvisols ranges between 8% and 13% in the A-horizon; the CEC ranges between 6 and 6.5 me/100 g soil. of organic matter is low in cultivated soils. The CEC values, together with X-ray data, indicate that the dominant clay is kaolinite, but impurities of 10-20% of 2:1 layer clays are present. The Luvisols have weakly developed structure, caused by the combined contribution of a low clay content (particularly in the surface horizon) and the inactivity of the main clay minerals (kaolin, with small proportions of 2:1 clays and sesquioxides), together with the low organic matter in cultivated soils. Instability of aggregation is the major cause of their tendency to display a rapid surface sealing following rainfall, and hardsetting crusting with subsequent drying. Other consequences of the unstable structure are slaking, packing and compaction. The instability of the surface structure and the formation of a seal or crust are followed by excess runoff, soil erosion and poor seedling emergence, even early in the season.

Planosols cover some 30% of the northern Cameroon area. Characteristically they possess a clayey compact sub-surface layer, impermeable to water and plant root development. The clay percentage in the surface layer is between 11% and 14%. CEC values for the upper layer are between 4 and 10 me/100 g soil. The low CEC indicates their mixed clay mineralogy. This is supported by X-ray measurements that show a mixture of smectite, illite and kaolinite. The soil surface develops a seal which causes low infiltration on bare cultivated fields. Runoff of 50% to 60% of a rainstorm has been recorded (Morin 1989). The subsurface impermeable layer in these soils causes considerable runoff even where there is mulch-protected surface. Hardsetting is common in Planosols, probably because of repeated cycles of wetting and drying, partly the result of the impermeable subsurface horizon. Plate 6 shows a condensed thick surface layer from a bare Planosol mottled with spots of iron compounds.

Surface Seal

Surface seal is the dominant factor in reducing infiltration in most African soils. The work of Stern et al. (1991), as presented in Figure 26 and Table 13, shows infiltration values for 19 soil types in South Africa. The detailed analyses in Table 13 help us to extrapolate the results to other African soils. The final infiltration rates of 19 South African soils and three soils from Israel are presented in Figure 26. It is evident that the South African soils can be divided into two main groups:

1. stable soils, with final infiltration rates for untreated soils greater than 8.0 mm h^-1;
2. dispersive soils, with final infiltration rates less than 4.5 mm h^-1.

The clay content of the 19 soils ranges from 13% to 39% (Table 13), the range where soils are most susceptible to sealing (Ben-Hur et al. 1985). The exchangeable sodium percentage of the soils is less than 1.9 and its distribution is similar in both the dispersive and stable groups. The organic carbon contents are quite similar in both groups, with a slight increase in the higher infiltration group. The pH of the soils range from acid to neutral, with a trend for the pH of the dispersive soils to be slightly higher than that of the stable soils. No correlation is found between the clay and silt contents on the one hand, and the final infiltration rates and soil loss rates on the other. The mineralogical data in Table 13 show that kaolinite is the dominant clay mineral in the 19 soils. Many of the soils contain illite, but only the dispersive soils contain some smectite, whereas the stable soils do not. It is possible that small amounts of smectite may disperse kaolinitic clays by the deposition of smectite platelets on the positively charged edges of kaolinite. In doing so, they prevent the edge-to-face flocculation that occurs in pure kaolinite. A similar phenomenon is reported by Frenkel et al. (1978). Thus, the presence of smectities in small quantities may affect dramatically the degree of dispersion. This mechanism may explain the rapid decrease in infiltration rates of soils which contain smectites compared with those which do not (Figure 26). The influence of clay type on soil infiltration, as determined by Stern et al. (1991) for South African soils, can be applied to soils elsewhere in Africa.

TABLE 13
Physical, chemical and mineralogical properties of 19 soils from South Africa (Stern et al. 1991)

K = Kaolinite, I = Illite, S = Smectite, V = Vermiculite, Cl = Chlorite, Is = Interstratified
¹ 1 = weak .... 5 = strong, relative peak intensities of the major peaks of each mineral on the diffractogram

MANAGEMENT TECHNOLOGY FOR CRUSTED SOIL

The monsoon rainfall, common in the sub-tropical regions of Africa, is commonly of very high intensity and short duration. Almost every year the monsoon gives one or more storms. Water losses through runoff occur whenever the rainfall intensities exceed the infiltration capacity of the soil, providing there is no physical obstruction to the surface flow. It is almost impossible to prevent runoff completely, even in areas where there is an economic need to use every drop of rainfall. Methods of water conservation are described below.

Infiltration Improvement

Soil infiltration is commonly limited by surface crusting rather than by deeper profile properties. Increased infiltration reduces runoff, thus increasing the water available for vegetation in situ. Three methods of augmenting infiltration are discussed here:

1. conservation farming;
2. the use of soil amendments; and
3. tillage management.

These methods may be used separately or combined.

Conservation Farming

Vegetation and mulch cover reduce water loss and erosion dramatically. Conservation farming concepts include some specific tillage systems, such as: no tillage, mulch farming, stale mulch and minimum tillage. These systems are widely used in the USA and Australia, to counteract water and wind erosion.

There are two main goals for conservation farming:

1. to improve the physical and chemical conditions of the soil; and
2. to shield the soil surface from the destructive force of the raindrops using mulches or protective cover crops.

Together these two approaches are highly successful.

For high levels of production, conservation farming demands skilled labour, expensive equipment and the costly use of herbicides to reduce weed competition for moisture and nutrients. So it can only be used to a limited extent in the African sub-tropics. Lal (1987) demonstrates the urgent need to adopt the conservation farming concepts in Africa.

Soil Amendments

One way to reduce the risk of crusting is to improve soil structure and aggregate stability at the soil surface. Increasing electrolyte concentration by spreading phospho-gypsum on the soil surface, results in a moderate decrease in infiltration rates (Agassi et al. 1981). Phosphogypsum dissolves quite readily during rainstorms and releases Ca⁺⁺ and SO₄^-- ions into the soil solution to support concentrations high enough to prevent clay dispersion. Mined gypsum is much less effective than phosphogypsum in preventing crust and surface seal formation, as it is less readily dissolved (Keren and Shainberg 1981). Adding phosphogypsum improves the physical properties of the soils, by replacing Na⁺ by Ca⁺⁺ on the soil colloids. Soil saturated with Ca⁺⁺ is more stable and more permeable than Na⁺-saturated soil. Improving soil structure and reducing crust formation by using organic polymers (for example, PAM-Poly Acryl Amide) has been under intensive investigation for many years. The main limits to their use, especially in low-income agriculture, is cost. Using such methods in experiments often helps to identify the main problem, however.

Figure 27 shows runoff percentages from experimental plots during the 1989 rainy season in north Cameroon. Straw mulch and amendments were tested on three major soils in the area (Vertisols, Luvisols and Planosols) for their relative effectiveness in improving infiltration. The runoff from the control area (bare soil) was very high, greater than 60% of the season's rainfall. Mulching the Luvisol to eliminate the surface seal reduced the runoff to 10% of the total rainfall. The Vertisol was affected quite dramatically by the mulch application, and runoff was reduced from 65% to 25 %. On the other hand, the Planosol (in Salak), with its hardsetting crust, was less affected. Runoff was reduced from 60% to 55% by the mulch treatment. The compact dense upper layer (Plate 6) was responsible for this. The gypsum and PAM treatment was better than the mulch, reducing the runoff to 45%. Figure 30 shows that even complete mulching is insufficient to eliminate runoff under African conditions.

Tillage management

Tillage management, to overcome surface seal and crusting, has to be evaluated mainly on its ability to improve aggregate stability and to increase surface storage. It is important to evaluate the relative importance of these two factors.

Aggregate stability: Previous sections have described the forces and mechanisms of aggregate stability. The evaluation here covers the relations between field aggregates and crust formations. Many experiments have proved that aggregate size plays an important role in controlling the crust phenomenon. The efficiency of surface seal in reducing the infiltration, depends greatly on the continuity of the impermeable crust layer. Large aggregates that break through such a layer, give an uncrusted or unsealed surface with greater infiltration rates. Figure 28 shows the infiltration of rain in cultivated wheat fields in Israel. Three tillage methods were used: mouldboard plough and disk harrow; disk harrow (two passes); and chisel plough.

Infiltration rates were highest for the chisel plough, even though the cultivation depth was only 10 cm. The lowest rates were on the disk harrow field which showed the fastest reduction in infiltration rate after the commencement of the rain. Infiltration was reduced to 8 mm h^-1 after the first 60 mm of rain. The chisel-ploughed field had rates of 43 mm h^-1 infiltration after the same amount of rain. These differences can be understood by the differences in aggregate distribution. Crusts are formed rapidly in the case of the small aggregates produced by the disk harrow. The moldboard plough and disk, despite achieving the deepest cultivation (40 cm), was less good than the chisel plough in preserving high infiltration rates as it produced smaller aggregates.

Silty soils are considered to have the highest crust hazard (Kemper and Miller 1974). This is possibly because cultivation leaves the silty soil surface with much smaller aggregates than similar cultivations on soil with higher clay contents. Figure 29 shows that for the same aggregate size, a surface seal develops faster in clay soils. This is true especially for soils with a low organic matter content.

In summary, any operation that will increase aggregate stability and/or size, helps to reduce crust formation.

Surface Storage: The balance among rain intensity, infiltration rate and runoff involves another, very important factor - surface storage. The runoff-balance equation can be written as follows:

Runoff = rainfall - infiltration - surface storage

Figure 30a and b demonstrates the surface storage effect on the runoff from a storm of 82 mm. Raising the storage from 1 mm (Figure 30a) to 15 mm (Figure 30b) results in reducing the runoff from 36 to 11 mm. The 25 mm reduction in the runoff is much greater than the storage itself. This is because surface storage is a dynamic parameter, which allows maximum infiltration for low-intensity rain segments, at the same time renewing the storage capacity free volume.

The general advantage of deep ploughing is the resulting rough soil surface, which enhances infiltration, primarily by the surface retention of water in the small holes and depressions. Stable, large aggregates, like those occurring in many clay soils, can preserve both high infiltration rates as well as storage throughout the rainy season. On the other hand, aggregates collapse quite quickly in weakly-structured soils, like silty loess, giving a smooth sealed surface. In such cases, special tillage methods can be used to preserve and control the surface storage.

Improvement of Seedling Emergence

The following management techniques, which are modified from those suggested by Taylor (1971) and by Goyal et al. (1982), can be used to reduce the damage to the plant caused by crusting.

• Before planting, the seeds can be soaked in order to hasten their emergence. This technique is especially useful in countries where manual labour is still available. The time that various kinds of seeds need to be soaked requires further investigation to avoid damage to the seeds by mechanical planters.
• Where crusting is a serious problem, seedling vigour is a characteristic that should be considered in the selection of varieties and breeding programmes. Varieties capable of exerting and rapidly achieving strong emergence forces should be selected (Parihar 1974).
• Soil moisture contents close to field capacity are most favourable for seed emergence in crusty soils. Timely wetting of the soil surface by irrigation, especially sprinkling, is a common practice to weaken the crust and improve seedling emergence.
• Planting seeds on ridges may also be advantageous, because any crusts formed on ridges are likely to be thinner than those formed on a flat surface. An additional advantage is that cracks tend to form on top of the ridge because of concentrated stress.
• As the soil crust is less strong on the slopes of a furrow, and here soil crusting is not severe, seeds can be planted on the sloping sides of furrows.
• Seeds can be planted in groups to generate greater emergence force (Hanegreefs and Nelson 1986).
• Surface soil crust can be broken by special equipment before or after seeding. Such equipment, recommended by Kemper and Miller (1974).

The timing of operations can be critical. Cultivation of the field too early, on moist soil, can encourage compaction and an even harder crust subsequently. Cutting the dry hard crust of the ridge tops by a dragged bar, and seeding into the moist soil, is recommended for hard-crusting soils. In Arizona, cotton is sown like this and the top of the ridge covered by dry soil from the furrow to prevent fast drying (Kemper and Miller 1974).

• The mechanical impedance by soil is reduced by the presence of organic material. Preserving organic material in and on the soil surface can be achieved by conservation farming.
• In arid regions, the restoration of dispersed soils by using gypsum should be considered only as a preliminary step to the establishment of better soil structure, and should be followed by farming methods that build up organic matter contents in the soil.

CONCLUSIONS

Crusting and surface sealing are widespread in the African sub-tropics. They result from the following factors:

• the generally low organic matter contents of the soils resulting from high temperature and cultivation;
• the heavy raindrop impact that commonly occurs during intensive rainstorms;
• the weak topsoil structure of most cultivated soil types;
• widespread destruction of the vegetation cover, which protects the soil surface from the impact of heavy rain.

Effective management to overcome crusting depends heavily on a local community's infrastructure culture and means. Western methods cannot simply be copied and applied under African conditions. Agricultural development to increase food production should be gradual and handled wisely while preserving the local community life.

Developing effective management systems for specific local conditions, demands a quantitative in-depth understanding of particular soil restrictions, of which crust formation and surface sealing are key aspects.

The problems caused by crusting and sealing can be readily ameliorated opening the possibility of significant increases in food production in the African sub-tropics.