4. Soil preparation

4.1 Soil fertility

All cropping measures must be directed to the conservation of soil fertility. Soil fertility is defined as the natural and sustainable potential of the soil with respect to the production of crops (Klapp, 1967). The fertility of the soil is decisively influenced by the soil-preparation methods undertaken. The following physical, chemical and biological components determine the soil fertility.

The texture, i.e. the relative proportion of fine and coarse particles present, provides information on and leads to conclusions regarding the pore distribution, structure stability and nutrient supply. Soils with a high silt content can store the most amount of moisture available to the plants. Sandy soils usually hold little moisture for the plants, as they cannot counteract the forces of gravity. They do not generally possess a stable structure, since the surface forces of the sand grains are minimal. Thus, the organic components of water-storage capacity and the structure of the soil are decisive. Loamy and clayey soils indicate generally stable structures, since the greater inner surface area leads to stronger attractive forces between the soil particles. As also clay minerals are the carriers of cation exchange capacity, the natural fertility of these soils is better than sandy soils. (Dehn, 1981)

The soil structure is the conglomeration of various soil particles in aggregates as well as their shape and arrangement. It determines the distribution of coarse, medium and fine pores that affect moisture availability and drainage. This is extremely important for precipitation conditions in the tropics. There, the soil structure, namely the building up of aggregates, is created by swelling and shrinking, root growth, activity of larger soil fauna and soil tillage. The soil structure depends, among other things, upon the proportion of iron and aluminium oxides and the types of clay minerals.

Severely weathered soils have a high percentage of iron oxides and kaolinite, which has the property that it does not swell with exposure to moisture and subsequent drying. Less weathered soils are characterized by a high share of minerals that are able to swell. The proportion of coarse pores is significant for infiltration and the exchange of gases. In tropical rainforest infiltration rates of several hundred mm/h are reached due to the high proportion of pore volume, so that even with intensive rainfall there is no surface runoff (Sanchez, 1976). The coarse pores also determine the space in which roots can potentially grow.

The bulk density is determined by the share of pore volume as well as the relationship of mineral to organic matter. For optimal root growth the soil should be loose. Bulk densities of between 1.0 and 1.2 g/cm³ are reasonable. The compaction of a loose soil by 0.15 g/cm³ can already reduce root growth to about half (Trouse, 1979: in Dehn, 1981). Thus, the achievable moisture potential is reduced for the plant, which is crucial in zones having high rainfall fluctuations. Particularly critical for plant growth are abrupt density changes (such as clay concentration horizons, plow sole), which also can lead to a perched water table.

Plants often react more sensitively towards soil than air temperature. When the soil is protected from direct sunlight the soil temperature corresponds essentially to the air temperature in the humid tropics. Without cover this can rise to more than 15°C higher. Soil temperatures of over 35°C approach the upper limitation for plant growth.

Building up a high stable content of humus by supplementing the soil with organic material and hindering a too rapid decomposition must be the priorities of a sustainable agriculture. Humus and organic matter can decisively improve the properties of the soil. Nevertheless, the practice of removing organic matter and residues, for example by burning or the pasturing animals, is common. Measures for supplying organic material, such as green manure and application of animal dung, are not being exploited in most of the regions investigated.

4.2 Soils

In most cases the regions in the survey do not possess similar types of topsoils, rather soil associations are encountered that are influenced by a host of factors; here the landscape relief plays a prominent role.

The properties of the components of the associations are important for soil preparation. Moisture and clay content of a soil determine the soil stability and thus the tillability. The optimal range for tillage is very narrow for clayey soils. The more clay a soil contains and the drier it is, the harder is its condition. The space for roots, one of the most important aspects for plant growth, is limited in shallow soils.

Furthermore, the moisture supply for the plants in the thin soil layer is not always assured due to low moisture storage capacity. This factor is especially important where short dry periods also occur during the vegetation period. The risk of erosion is critical, since the thin arable layer can rapidly be removed in contrast to soils having a greater depth. Shallow, stony soils are difficult to till. Oxisols (USST), the most severely weathered of all soil formations, are predominantly found on relatively flat, old land surfaces. They are very deep and usually have a stable structure, and are very suited for mechanized cropping. Despite the high clay content (upto 80 %) they often occur as loam or loamy sand because of the building up of stable micro-aggregates in the fields. The bulk density is very low, so that in part with compaction (e.g. in tractor tracks) higher yields are achieved due to a better moisture supply. Two days after a heavy rainfall the soil can be tilled.

Most possess few nutrients except for those originating from volcanic primary rock. On slopes, from which weathering products are constantly being removed by means of water erosion and soil flow, Ultisols and Alfisols (USST) occur as recent formations. They have a somewhat higher natural soil fertility, but are structurally less stable. In part they are characterized by greater texture differences between the A and B horizon, so that there is a severe risk of erosion.

Moreover, gravel deposits or stone layers can limit the tillability near the soil surface on the upper slopes. During dry seasons the Alfisols become very hard, rendering soil preparation impossible. These so-called "millet soils" in West Africa tend to form crusts and to possess a higher bulk density, which makes root growth difficult (Klaij and Serafini, 1988).

Inceptisols, a classification of newer soil formation, occur where the soil removal process has reached hardpan. Relatively fertile soils can be created on freshly weathered hardpan and in the sedimentation basin of rivers, if the sediment did not originate from Oxisols from the older highlands. Vertisols (USST) are nutrient-rich lower lying soils having a high clay content, which have originated from basalt or are created on a stowage level in depressions beside older soils (reformation of clay minerals), where however nutrient deficits can occur, e.g. phosphorous and potassium. A pre-condition for the creation of Vertisols is a changing wet climate, in which they are subject to high moisture fluctuations and regular drying out. Vertisols represent an extreme case here, due to their high clay content and the high proportion of swelling clay minerals. The most suitable range for tillability between too wet and too dry conditions is very narrow (minute soils).

They can therefore not be optimally tilled. Sandy and silty soils having a low structural stability tend to form a sealed surface, crust immediately and therefore undergo risk of erosion. The breaking up of crust formations can increase infiltration and thus reduce the surface water runoff, the trigger for water erosion; the impact of this measure for weakly structured soils is rapidly reduced, especially with rainfall. Sand achieves a high bulk density of 1.5 g/cm³, and under heaviest compaction upto 1.7 g/cm³. The compactions are solid, and thus no roots can penetrate them. This is most evident with fine sand, which has the densest compactions. Drier sand can be tilled; the measure may be useless however since it does not retain its structure produced by the tillage operation. (Roth, 1989)

Aside from stones in the narrow sense, laterite concretions can render the soil preparation difficult. In a semihumid/semiarid climate iron-rich amorphic mass (Plinthite layer) can occur deeper in the soil at the break-off point on edges of slopes, which can arrive on the surface by tillage and dry out irreversibly (Sol Ferralitique RemaniFS). In soils which have often been tilled pea-size concretions are found, which can take up to 50 % of the soil profile, e.g. in the humid tropics of West Africa.

4.3 Toposequence and soil types

The soils change along a slope with regard to depth and clay content. These changes can occur within a few hundred meters, depending upon the topography

In the humid tropics steep slopes are seldom found. Wavy, hilly landscape without rugged edges (half-oranges) occur or flat land, such as found in the Congo and Amazon basins. Stones are rare. On slopes there are soil sequences, e.g Oxisol, Ultisol, Inceptisol (USST). In the wet and dry climates of the humid tropics (e.g. South Brazil) their are more jagged slopes. In savanna climates the sequence can consist of Alfisol, Ultisol and Vertisol (USST) ("le rouge, le gris et le noir"). (Roth, 1989) On the upper part of the slope the soil can be flat and stony. The risk of erosion is high due to the inclination and the shallow soils allow no margin for soil loss. The clay content, the depth and the water storage capacity increase at lower levels. The soils on the upper slopes are correspondingly easier to till, also manually. Alluvial soils, heavy black soils having a substantial quantity of organic matter, are found in valley bottoms. Because of the soil moisture they can be used year round as pasture (e.g. Vertisols in the valley bottom with a changing wet climate, such as in Zambia, Malawi, Tanzania or Ethiopia). They require high investment of energy; in part, they can only be cultivated after considerable expenditures for water management and drainage. On the other hand, the risk of drought is greatest on the upper slopes. The farmer must weigh the lower power input requirement against the greater risk of drought. This risk decreases with increasing humidity.

(Pingali et al., 1987)

The zone preferred for cropping depends upon the climate and the population density. In arid areas the lower slopes or valley bottoms receive preference. In semiarid areas cropping begins on middle slopes and replaces the pastures on the lower slopes and valley bottoms with increasing population pressure. In humid regions the upper slopes are also cultivated. Labour-intensive water management measures are only worthwhile in lowlands when sufficient labour resources are available on the basis of the population development.

The intensification of soil preparation on the medium slopes leads to severe erosion problems for many tropical soils. Due to the heavy soils in the valleys the transition from the hand hoe to the plow takes place here first, according to Pingali et al. (1987). This does however not apply generally, as the example of Casamance (Senegal) shows, where animal traction is utilized more on the plateau. Also in south Paranhe plow becomes more widespread on the upper slopes.

4.4 Objectives

Various aims are pursued with soil-preparation measures:

- weed control, especially prior to sowing,

- creation of a certain surface structures (e.g. ridges); seedbed preparation for smooth operation of seeders; crumbling of soil for special crops; preparation for irrigation,

- loosening of poorly structured, tightly compacted soils; creation of coarse pores for better root penetration,

- working in of organic material or chemical fertilizers,-increasing the infiltration by means of loosening soil, especially breaking of crust,

- reduction of evaporation by destroying capillary structure or hindering growth (full fallow).

In general, loosening only serves a purpose when the soils have previously become compacted, e.g. by heavy tractors, implements or animals. Further aims such as bringing leached soil components to the surface are of lesser importance with the shallow working depth of draft-animal implements.

4.5 Various aspects of soil preparation

4.5.1 Impact of utilizing implements

The mechanization of soil preparation alone does not produce a quality gain in comparison to the hand hoe, and thus does not improve the area performance (yield per ha) (Pingali et al., 1987). Weed control with the hand hoe, also a soil-preparation measure, is considerably more effective. With draft-animal use beside the increase of labour productivity only the possibility of cultivating unused heavier soils is given.

The mouldboard plow has become widely distributed in the tropics and subtropics at the level of animal traction, in contrast to motor mechanization where disk implements dominate. Its decisive advantage is an effective weed control. It leaves a finer seedbed than the ard or chisel plow. Frequently, the implement, which is adapted to cropping in temperate climates, has been introduced by European settlers in new agro-ecological zones. The implements used for the subsequent work operations are designed for work on well prepared fields following plowing.

Less intensive preparation with the chisel plow or the ard are particularly widespread in the semihumid/semiarid regions. The soil is loosened without turning. In some soils, e.g. Vertisols in Ethiopia, the ard is the only implement used for soil preparation. Further work operations can hardly be carried out due to an unsuitable soil structure or clogging. Access to the wet, poorly drained fields is very difficult, for example for weed control.

Contradictory investigations have been apparently conducted on the advantages and disadvantages of soil preparation, especially with the plow. These deviating statements can be attributed to the considerably differing basic conditions of soil type and climate, however. Yield increases after plowing (Charreau, 1974: in Pingali et al., 1987) and a reduction of erosion have been determined (Charreau, 1972 in: Sanchez, 1976) in semihumid/semiarid regions having soils that tend to become compacted, while in humid regions less significant yield growth (Vincente-Chandler, 1966 in: Sanchez, 1976) and an increase of erosion has been measured (Marquez and Bertoni, 1961 in: Sanchez, 1976).

Plowing causes a temporary reduction of soil bulk density. The enlargement of pore volume however does not apply to all pore size classifications. Plowing creates essentially large pores favouring root growth, especially important on soils having a higher bulk density and non-swelling clay minerals (kaolinite). Thereby an increase of the infiltration rate is achieved, at least for a certain period of time. The medium and fine pores determining the moisture content capacity can only be created biologically or physically (swelling and shrinking), and can be destroyed by working the soil.

A disadvantage is that by intensive soil preparation, especially with the mouldboard plow, the soil is more intensively aerated and warmed, the decomposition of organic matter is accelerated and moisture loss causes higher evaporation. Plowing means, in addition, an over-loosening: the loosened structure is not initially suited for cropping and it takes time for restabilization of the soil. Mechanical loosening by means of soil preparation possesses only limited stability. After a sort time the bulk density can already be greater than for no-tillage and in the long term it can be higher than the latter (Armon and Lal, 1979 in: Dehn, 1981). The looser the soil is after tillage, the more sensitive it is to compaction. This applies especially for a sandy soil having little organic matter.

After a some recompaction higher moisture capacity will is achieved. Many soils become depleted with prolonged cultivation. Due to compaction of the topsoil when uncovered or the creation of compaction horizons (e.g. plow sole) they become less permeable and more susceptible to surface water runoff and soil loss. Intensive soil preparation, especially the establishment of a fine-crumbed structure, contributes to a reduction of infiltration due to a decline of aggregate stability and surface sealing. Water drainage can take place unhindered if the surface is uncovered. No resistance is provided against wind erosion. A coarse seedbed preparation, as for example with the ard, therefore brings with it a reduction of risk against erosion. Smallholder agriculture also contributes to damage caused by erosion, particularly due to the penetration of hilly terrain (figure E 18).

The individual crops have an varying impact on the amount of soil loss; the following ranking have been determined for humid regions (table E 5).

In the various cropping regions the sequence is adjusted to concur with the seeding date, since the impact of erosion tends to vary in the course of the year.

Soil preparation is minimized or totally omitted for no-tillage under mulch cover. The soil is covered with organic material. The no-tillage method is referred to when no soil preparation has been carried out over several vegetation periods. Minimum tillage or the no-till approach are less suited for soils that tend to become crusted or compacted, are poorly drained or undergo little biological activity (Hartmans and Kuile, 1983). Weed control remains a constraint for no-tillage under wetter tropical conditions.

4.5.2 Soil preparation in semihumid/semiarid climates

Tillage at the beginning of the cropping period in the wet season In dry regions (dry savanna, semi-desert) agriculture is at risk due to a scarcity of water. Here, a humus-conserving, water-saving soil preparation is critical. Turning the soil leads to a loss of moisture. Therefore, minimum soil tillage with the chisel plow or no-till methods are applied, followed by a breaking of the soil capillarity during weed control. Traditionally, ards are often used for this purpose. The use of the plow is not recommendable in these zones due to the risk of erosion (wind, water) and the low area performance. The organic matter required for mulching is difficult to produce here, since the cropping of green manures, for example, is not possible because of the scarcity of moisture. Harvest residues are usually no longer available to cover the soils, as they are necessary for animal fodder.

Fieldwork is generally begun after the first rains. In Morocco, for example, the ard is utilized for surface tillage. Due to the short duration of the growth period in many regions in West Africa and Northeast Brazil soil preparation is only carried out on the surface and directly after sowing or no-till operations. Sowing must take place as rapidly as possible after the first rains, otherwise the yield declines drastically. No-tillage is favoured by the occurrence of sandy soils and low risk of weeds in the Sudan zone of Africa. Working in of organic matter at this point becomes superfluous (figure C 7). In order to perform plowing in the wet season the first rains must moisturize the soil to a sufficient depth. Subsequently, 4 to 5 days of work (25 hours) are necessary to plow one hectare with a team of oxen (Bordet et al., 1988.)

Where soils tend towards compaction, as in most of Senegal, soil preparation could be more favourable than soil-conserving no-tillage to achieve a better root penetration and thus a higher yield. Simultaneously, the infiltration could be improved. Studies in dry regions showed an increase in yield by means of soil preparation; rice improved the most while groundnuts the least: sequence -rice, sorghum, maize, cotton, millet, groundnut. (Charreau, 1974: in Pingali et al., 1987)

Other trials in sandy Alfisols in Senegal have proved the positive effect of superficial soil preparation with the hand hoe as well as deep plowing, in comparison to no-tillage (Nicou, 1972 in: Sanchez, 1976). In this case there was no difference in yields between the fields cultivated manually and those with the tractor. Considering an economic assessment the result would be an increase of profits in West Africa, particularly for cotton, rice, groundnut and maize (in this order). (Pingali et al., 1987)

Although various studies (e.g. Chopart, 1981) have proved the positive effect of plowing on the yield and these findings have become priority areas for the extension services, plowing is not accepted in some regions, e.g. in Senegal. Plowing with animal traction is only beneficial in rainfed cropping in the Sudano-Sahel zone when precipitation is above 900 mm per annum. This statement must be modified corresponding to the type of soil and practices accompanying plowing: The heavier the soils and the higher the moisture uptake or retention capacity, the more recommendable is plowing. (Bordet et al., 1988)

In order to overcome the limitation of tillage because of the short vegetation period, the time of soil preparation could then also be selected at the end of the vegetation period or during the dry season. Both of the two methods would be suitable for increasing the water uptake during the first rains of the rainy season. The procedures are discussed further below.

Tillage at the end of the cycle or during the dry season Soil preparation at the end of the cycle, e.g. with the chisel plow and rigid tines, would loosen the soil and thus increase the infiltration during the first rainfalls. At the same time, harvest residues could be worked in. This method, as recommended by research, has proved to be impracticable.

The following reasons speak against this approach:

- It requires a repetition of soil preparation for seedbed preparation at the beginning of the rainy season with the above described negative effects, and thus means an extra work operation.

- It competes with the harvesting operations and requires the removal of harvest residues grazed throughout the dry season.

- The agronomic effect is disputed, since the effect is possibly no longer evident by the time the rainy season begins (Bordet et al., 1988).

A further useful method is soil preparation during the dry season with the chisel plow. Various tines have been developed in the Sahel zone for soil preparation where precipitation is under 900 mm (figure E 19). (Bordet et al., 1988; Sene, 1988)

The work in dry seasons is only possible on very sandy soils. But even for numerous light soils in Senegal this is not possible because of the required high draft power due to compaction. The first operations showed that the necessary draft power overloaded the oxen teams. A further developed tine, which was pulled by 2 oxen of 400 kg in good condition, required a draft power of ca. 90 kp in light soil (clay content of 12 -15 %). The working depth was 9 cm; the infiltration profile was deeper than without tillage (Le Thiec and Bordet, 1988). However, the operations did not go beyond the bounds of the experimental station.

4.5.3 Soil preparation in transitional zones

Cropping on ridges is widespread in transitional regions of the semihumid/semiarid climate, e.g. in Casamance in south Senegal (1000 -1300 mm rainfall) and in the Savanes region of Togo (1000 - 1100 mm rainfall). This is practised primarily in Africa (88 % of the cases) according to our survey; in South America it is often used for some crops (potatoes, tobacco). To a great extent the ridger is used exclusively for preparing these fields. Ridged cropping offers, aside from its application in irrigation systems, particular advantages in the regulation of the moisture supply:

- With suddenly occurring high quantities of rainfall in this generally drier region plants stand above water and ridged soil drains well.

- Ridged cropping reduces the surface runoff and increases infiltration. Therefore, storage of water in the deeper layers is greater than cropping on flat soil.

- The soil is only partially tilled, and narrow unworked strips remain under the ridges.

- The ridged soil is loose, favouring the growth and harvesting of tubers and groundnut.

- In cold mountainous climates the ridges offer protection against light frost due to their influence on the microclimate.

The increase of water storage is particularly important for many of the semihumid/semiarid-occurring Alfisols and Ultisols, whose storage capacity is low. Ridged cropping has advantages if the dry season sets in at a later part of the growth period and the roots have penetrated to a deeper level. The crops are protected against waterlogging caused by heavy rainfall. During dry periods in later growth phases the plants can protect larger water reserves in lower layers. By shifting the ridges for the subsequent crop an efficient weed control is achieved. (Dehn, 1981) To control evaporation a compacted, smooth or a loose surface of the ridges is desirable, depending upon the climate and the soils.

Cropping on ridges promotes a more rapid mineralization. Frequently the harvest residues are placed into the furrows, the ridges are flattened, covering the residues. In Senegal (south of Sine-Saloum) methods are used to rebuild ridges by cutting perpendicularly to the old ones. This facilitates soil preparation when low amounts of precipitation occur at the beginning of the rainy season. Ridged structures provide protection against erosion, as long as the rainfall is not so great that it causes the ridges to burst on hilly terrain. A system of tied ridging (figure E 20) has been developed to reduce soil losses, which can be substantially greater than on flat seedbeds. Wind erosion also is reduced by cropping on ridges (Fryrear, 1984: in Klaij and Sarafini, 1988).

According to Bouchet, director of SEMA in Boulel, Senegal (cited by Gaudefroy-Demonbynes, 1957: in Bordet et al., 1988) cropping on ridges increases yields by upto 20 % where high precipitation occurs (more than 1000 mm per annum). In these wet areas more time is available for soil preparation and weed control presents more serious problems. According to our survey ridged cropping however is also frequently practised where low average precipitation occurs (between 500 and 1000 mm).

4.5.4 Soil preparation in humid climates

In the wetter regions usually only the migrants from the savanna zones practise cropping on ridges, e.g. in the Centrale region of Togo (1200 - 1300 mm rainfall). This leads to the conclusion that the ridges originated from the transition from semihumid to subhumid climate, where due to the high humidity already a greater importance is attached to weed control than in the dry savanna. On the other hand, cropping on mounds is widespread in humid climates, especially where the land is used less intensively. The topsoil is accumulated in mounds and thus nutrients are collected. The cropping area on mounds is small. Weed control plays a lesser role in this system and tree stumps are not an obstacle.

Covering the soil represents an essential measure for conserving soil fertility in the tropics and subtropics. According to Rockwood and Lal (1974) the main advantage of mulching, in combination with minimal soil tillage or no tillage, lies in the assured and cheap reduction of erosion. The effect of the mulch consists in protection from the impact of raindrops, which causes surface sealing. This advantage has an effect especially in regions as e.g. in South Brazil where 60 mm per hour or 250 mm per day at seeding time are not unusual. Here, an effective erosion control is only assured by means of a permanent soil covering (Derpsch et al., 1988).

Soil fertility is influenced positively by means of no-tillage under mulch cover whereby soil temperature fluctuations are reduced and higher temperatures are avoided. A slower mineralization occurs due to the low cultivation intensity (Lal, 1975). A higher moisture availability is achieved by a reduction of evaporation and higher infiltration rates in no-tillage under mulch cover. The biological soil activity is increased with prolonged application of mulching under the no-till method.

Despite the positive impact of no-tillage under mulch cover reported by many authors (e.g. for South Brazil: Monegat, 1985; Derpsch et al., 1988) this method in the humid regions is not widespread in practice in terms of animal traction, especially in Africa. The no-till technique places high demands on the management and the cropping of green manure for the production of necessary mulch means an extra investment. Significant problems such as weed control without extra inputs as herbicides, as well as nutrient dynamics remain unsolved. Appropriate draft-animal implements for sowing on unprepared soil is not yet ready to be put into practice.

In contrast to the semihumid/semiarid regions weed control and the working in of organic matter represent the main constrain in humid areas. More time is available for soil preparation due to the longer vegetation period. Therefore, soil preparation with the plow predominates here on the level of animal traction.