Chapter 8
Conservation tillage for increased crop production

O.A. Opara-Nadi, College of Agriculture and Veterinary Medicine, Imo State University, Okigwe, Nigeria

Tillage aims to create a soil environment favourable to plant growth (Klute 1982). Definitions of tillage vary. According to Lal (1979a, 1983) it is defined as physical, chemical or biological soil manipulation to optimize conditions for germination, seedling establishment and crop growth. Ahn and Hintze (1990), however, define it as any physical loosening of the soil carried out in a range of cultivation operations, either by hand or mechanized. Soil manipulation can change fertility status markedly and the changes may be manifested in good or poor performance of crops (Ohiri and Ezumah 1991). In addition, tillage operations loosen, granulate, crush or compact soil structure, changing soil properties such as bulk density, pore size distribution and composition of the soil atmosphere that affect plant growth.

Appropriate tillage practices are those that avoid the degradation of soil properties but maintain crop yields as well as ecosystem stability (Lal 1981b, c, 1982, 1984b, 1985a; Greenland 1981). Conservation tillage provides the best opportunity for halting degradation and for restoring and improving soil productivity (Lal 1983; Parr et al. 1990). In recent years interest in conservation tillage systems has increased in response to the need to limit erosion and promote water conservation (Hulugalle et al. 1986; Unger et al. 1988).

Conservation tillage, by most definitions, embraces crop production systems involving the management of surface residues (Unger et al. 1988; Parr et al. 1990). According to the Conservation Technology Information Center in West Lafayette, Indiana, USA, conservation tillage is defined as: "any tillage or planting system in which at least 30% of the soil surface is covered by plant residue after planting to reduce erosion by water; or where soil erosion by wind is the primary concern, with at least 1120 kg ha^-1 flat small grain residue on the surface during the critical wind erosion period." No tillage, minimum tillage, reduced tillage and mulch tillage are terms synonymous with conservation tillage (Willis and Amemiya 1973; Lal 1973, 1974, 1976b; Phillips et al. 1980; Greenland 1981; Unger et al. 1988; Antapa and Angen 1990; Opara-Nadi 1990; Unger 1990; Ahn and Hintze 1990). The reasons for current interest in conservation tillage vary from soil to soil, crop to crop, and from one agro-ecological region to another. One major reason is its effectiveness for controlling erosion. Closely allied are the water conservation benefits, not only in semi-arid and sub-humid regions but also in humid regions (Unger et al. 1988). According to Greenland (1981), Lal (1973, 1974, 1975, 1976b), Rockwood and Lal (1974), Lal and Hahn (1973), the no-till system of cultivation with crop residue mulches forms a basis for conservation farming because it conserves water, prevents erosion, maintains organic matter content at a high level, and sustains economic productivity. In addition, there are savings in machinery investment and in the time required for seedbed preparation (Lal 1974, 1985b; Rockwood and Lal 1974). Antap and Angen (1990) report that retaining crop residues on the soil surface with conservation tillage reduces evapotranspiration, increases infiltration rates, and suppresses weed growth.

Since the late sixties, many studies of the effects of conservation tillage systems on soil properties and crop yield have been conducted in many parts of the world. A complete review is beyond the scope of this presentation, the object of which is to give an overview of the early studies on conservation tillage systems, discuss some results from present-day studies and outline research needs and goals for the future aimed at enhancing and sustaining crop production through conservation tillage systems.

CONSERVATION TILLAGE VERSUS CONVENTIONAL TILLAGE: DEFINITIONS AND BASIC CONCEPTS

Tillage includes all operations of seedbed preparation that optimize soil and environmental conditions for seed germination, seedling establishment and crop growth (Lal 1983). Tillage is defined as the soil-related actions necessary for crop production (Boone 1988). According to Antapa and Angen (1990), tillage is any operation or practice taken to prepare the soil surface for the purpose of crop production. The definition by Ahn and Hintze (1990) states that tillage is any physical loosening of the soil as carried out in a range of cultivation operations, either by hand or mechanized. The overall goal of tillage is to increase crop production while conserving resources (soil and water) and protecting the environment (IBSRAM 1990). The benefits of tillage are:

1. seedbed preparation,
2. weed control,
3. evaporation suppression,
4. water infiltration enhancement, and
5. erosion control. These benefits together result in increased and sustained crop yields. The definitions of tillage, as given above, embrace the concepts and features of both conservation and conventional tillage systems.

Conservation Tillage Systems

Conservation tillage as defined by the Conservation Tillage Information Center (CTIC) excludes conventional tillage operations that invert the soil and bury crop residues. The CTIC identified five types of conservation tillage systems:

1. no-tillage (slot planting),
2. mulch tillage,
3. strip or zonal tillage,
4. ridge till (including no-till on ridges) and
5. reduced or minimum tillage.

No-tillage

The no-till system is a specialized type of conservation tillage consisting of a one-pass planting and fertilizer operation in which the soil and the surface residues are minimally disturbed (Parr et al. 1990). The surface residues of such a system are of critical importance for soil and water conservation. Weed control is generally achieved with herbicides or in some cases with crop rotation. According to Lal (1983), no-tillage systems eliminate all preplanting mechanical seedbed preparation except for the opening of a narrow (2-3 cm wide) strip or small hole in the ground for seed placement to ensure adequate seed/soil contact. The entire soil surface is covered by crop residue mulch or killed sod. A review of tillage studies in Nigeria (Opara-Nadi 1990) shows that no-tillage with residue mulch is appropriate for Luvisols in the humid tropics. No-tillage is used in mechanized wheat farming in northern Tanzania and for some perennial crops, for example coffee plantations (Antapa and Angen 1990). Several studies (Smika and Unger 1986; Unger et al. 1988; Parr et al. 1990) have reported the success of no-tillage systems in many parts of the USA. Though the use of no-till is increasing, adoption has been slow. Parr et al. (1990) report that in the USA, no-till is practised on less than 10% of the farmland that is in some form of conservation tillage.

No-till fallow is a type of no-tillage system which is used in the dryland areas in the USA. No-till fallow has been most successful in summer rainfall areas (Parr et al. 1990). A major goal of fallowing is to recharge the soil profile with water so that the risk of failure for the next crop is greatly reduced (Unger et al. 1988). According to Parr et al. (1990), the potential benefits of no-till fallow, compared with other tillage systems, are more effective control of soil erosion, increased water storage, lower energy costs per unit of production and higher grain yields. A major disadvantage of no-till fallow (sometimes referred to as chemical fallow) is its heavy use of herbicides for weed control.

Mulch tillage

Mulch tillage techniques are based on the principle of causing least soil disturbance and leaving the maximum of crop residue on the soil surface and at the same time obtaining a quick germination, and adequate stand and a satisfactory yield (Lal 1975, 1986b). Lal further reported that a chisel plough can be used in the previously shredded crop residue to break open any hard crust or hard pan in the soil; care should be taken not to incorporate any crop residues into the soil. The use of live mulch and crop residue in situ involves special mulch tillage techniques or practices. In situ mulch, formed from the residue of a dead or chemically killed cover crop left in place (Wilson 1978a, b), is generally becoming an integral component of mulch tillage techniques.

Stubble mulch tillage or stubble mulch farming (sub-tillage) is a crop production system involving surface residues that was first used by a farmer in Georgia, USA, in the early 1930s for controlling water erosion (Unger et al. 1988). It was developed primarily for controlling wind erosion, but its value for reducing runoff and controlling water erosion was also soon apparent (Smika and Unger 1986). This practice is carried out by small- and large-scale farmers in Tanzania for perennial crops like coffee and banana, as well as annual crops such as wheat and barley (Antapa and Angen 1990).

Strip or zonal tillage

The concept of strip or zonal tillage is described by Lal (1973, 1983). The seedbed is divided into a seedling zone and a soil management zone. the seedling zone (5 to 10 cm wide) is mechanically tilled to optimize the soil and micro-climate environment for germination and seedling establishment. The interrow zone is left undisturbed and protected by mulch. Strip tillage can also be achieved by chiselling in the row zone to assist water infiltration and root proliferation.

Ridge till

In this system, the soil is left undisturbed prior to planting but about one-third of the soil surface is tilled at planting with sweeps or row cleaners; planting of row crops is done on preformed cultivated ridges, while weeds are controlled by herbicides. Ridge till has been gaining popularity as a conservation practice for maize and soybean production in the USA (Parr et al. 1990).

Reduced or minimum tillage

This system covers other tillage and cultivation systems not covered above but meets the 30% residue requirement (Laryea et al. 1991). In Africa, the term minimum tillage is not always employed with the same meaning as in temperate countries, and may also be used differently in the different contexts of shifting cultivation (still the dominant system in most of Africa) and mechanised agriculture (Ahn and Hintze 1990).

Conventional Tillage Systems

Mechanized systems

These involve the mechanical soil manipulation of an entire field, by ploughing followed by one or more harrowings. The degree of soil disturbance depends on the type of implement used, the number of passes, soil and intended crop type.

Traditional tillage

In the humid and sub-humid regions of West Africa, and in some parts of South America, traditional tillage is practised mostly by manual labour, using native tools which are generally few and simple, the most important being the cutlass and hoe which come in many designs depending on function (Morgan and Pugh 1969). To facilitate seedbed preparation and planting, forest undergrowth or grass is cleared with a cutlass and trees and shrubs left, but pruned. The cut biomass and residues are disposed of by burning in situ. This type of clearing is non-exhaustive, leaving both appreciable cover on the soil, and the root system which gives the topsoil structural stability for one or two years (Aina et al. 1991).

FACTORS AFFECTING THE CHOICE OF TILLAGE PRACTICES

Tillage is a labour-intensive activity in low-resource agriculture practised by small land-holders, and a capital and energy-intensive activity in large-scale mechanized farming (Lal 1991). For any given location, the choice of a tillage practice will depend on one or more of the following factors (Lal 1980; Unger 1984):

According to Unger et al. (1988) conservation tillage systems to protect the soil and water reserves often have limited appeal to producers unless they offer economic advantages. Economic factors contributing to interest in conservation tillage include:

1. high costs of fuel, labour, tractors, and other equipment;
2. high equipment inventories and maintenance costs;
3. ability to use land at risk of erosion for more intensive crop production (rather than for pastures or in long-term rotations);
4. the opportunities offered for more intensive cropping, avoiding long fallow periods, because of greater water conservation; and
5. in many instances, higher crop yields.

CONSERVATION TILLAGE - THE EFFECTS ON SOIL PROPERTIES AND CROP YIELD

In Nigeria, scientists at the International Institute of Tropical Agriculture, Ibadan, started research on no-tillage or mulch-tillage systems in 1970 (Rockwood and Lal 1974; Lal 1973, 1974, 1976b, 1979a). Other scientists working in national research institutes and universities in Nigeria also started studies on a range of soils in the 1970s to compare the effects of different tillage methods on soil properties, crop growth and yield (Agboola and Fayemi 1972; Aina et al. 1976; Aina 1979; Wilkinson and Aina 1976). Similar studies were also initiated in other African countries including Ghana (Kannegieter 1967, 1969; Ofori and Nanday 1969; Ofori 1973), Liberia (Lal and Dinkins 1979) and Senegal (Nicou and Chopart 1979).

In the USA, Unger et al. (1988) report that except for stubble mulch tillage, there was limited interest in crop production systems involving surface residues until the late 1960s or early 1970s when interest became widespread. Several more recent studies have shown that no-tillage systems with crop residue mulch can

1. maintain the productivity of upland soils by reducing erosion (Mensah-Bonsu and Obeng 1979; Lal 1981a, b, 1984a, b; Aina 1988);
2. maintain a favourable soil temperature (Hulugalle et al. 1985, Lal 1986a);
3. improve water-retention capacity (Aina 1979; Opara-Nadi and Lal 1986, 1987a, b, c; Hulugalle et al. 1990);
4. improve water use efficiency (Osuji 1984; Osuji et al. 1980), and
5. increase nutrient use efficiency (Lal 1979a, b, c; Hulugalle et al. 1985). The no-till system seems to have a broad application in humid and sub-humid regions, for which 4-6 tons ha^-1 of residue mulch appears optimal (Lal 1975; Aina et al. 1991). The beneficial effect of conservation tillage systems on soil loss and runoff have been demonstrated in Ghana (Table 18) and Chaguanamas, Venezuela (Table 19).

TABLE 18
Effects of tillage systems on soil loss and runoff in Ghana (1976) (Mensah-Bonsu and Obeng 1979)

In Ibadan, Nigeria, Lal (1976a) reported an annual saving of 32% of rainfall for a crop residue mulch of 6 tons ha^-1. In Côte d'Ivoire, Roose (1988) reported drastic reductions in runoff and soil erosion from a mulched pineapple field on a 20% slope. According to estimates by the CTIC, the average reduction in soil erosion for conservation tillage systems is about 50% of that for most conven-tional tillage practices (Brosten 1988). Rockwood and Lal (1974) report that a thin layer of dead crop residue 1-2 cm thick on the soil surface of no-till plots decreases the minimum soil temperature (Table 20) and improves soil moisture conditions (Table 21). Smika (1976) summarizes the effects of clean (little or no residue retention) or stubble-mulch (sub-surface) tillage practices on soil water gain during fallow periods at several Central Great Plains (USA) locations (Table 22). At wheat planting time at the end of the fallow period, the average water content was 27 mm greater with stubble mulch than with clean tillage. Rockwood and Lal (1974) observed that biological (for example, earthworm) activity was stimulated by more favourable water and temperature regimes in no-tillage plots than in conventionally tilled plots. As many as 2400 earthworm casts per square metre were counted in no-tillage plots, compared with fewer than 100 casts per square metre for the ploughed plots. In a study in Western Nigeria, Osuji (1984) observed that water-use efficiency and maize grain yields during the early season of 1978-80 were significantly higher under zero tillage than under other tillage treatments (Table 23). Lal (1985c) showed that soil physical properties and chemical fertility were substantially worse in ploughed watersheds after six years of

TABLE 19
Soil loss for three tillage systems on a Luvisol, Chagnaramas, Venezuela (Casanova et al. 1989)

TABLE 20
Effect of tillage on maximum soil temperature at 5 cm depth under different crops two weeks after planting (1 May 1973) (Rockwood and Lal 1974)

TABLE 21
Effect of tillage on soil moisture retention at 0-10 cm depth under different crops two weeks after planting (Rockwood and Lal 1974)

TABLE 22
Net gain in soil water during fallow with clean and stubble mulch tllage at seven Central Great Plains locations (USA) (Smika 1976)

continuous mechanized farming and twelve crops of maize, while the decline in the soil properties was decidedly less in the no-tillage watershed. The lower maize yields of the ploughed watershed are related to erosion, compaction, fall in organic matter content and fall in pH (Table 24). After 10 years of continuous comparative no-tillage and conventional tillage trails in Southwest Nigeria, Opara-Nadi and Lal (1986) observed that total porosity, moisture retention, saturated and unsaturated hydraulic conducti-vity, and the maximum water-storage capacity increased under no-tillage with mulch.

TABLE 23
Effect of tillage practices on water use, maize yield and water-use efficiency (early season) (Osuji 1984)

¹Means followed by the same letter in the same column are not significantly different at the 5% level.

TABLE 24
Effects of mechanized tillage methods on soil chemical properties 6 years after imposing the tillage treatments (Lal 1985c)

Earlier studies on the effects of mechanized tillage systems on maize yield (Couper et al. 1979) showed that maize grain yields for six consecutive years were higher on no-till than on ploughed plots (Figure 32). At the end of six years in 1980, no-till plots yielded 3 times the ploughed plots (3 t ha^-1a^-1 and 1 t ha^-1a^-1 respectively); however, maize yields declined from 1979 in both plots. In a study in Liberia, Lal and Dinkins (1979) demonstrated that no-tillage is effective for the production of grain crops but the yield of cassava was higher in plough than no-till plots (Table 25). Under rainfed conditions in arid and semi-arid climates, Unger and Wiese (1979) showed that managing the residue from irrigated winter wheat and

TABLE 25
Effects of tillage methods on crop yield on an acid soil in Liberia (Lal and Dinkins 1979)

F_o = without fertilizer; F₁ = with fertilizer

TABLE 26
Tillage effects on water storage, sorghum grain yields and water-use efficiency in an irrigated winter wheat-fallow-dryland-grain sorghum cropping system, Bushland, Texas, USA (1973 to 1977)
(Unger and Wiese 1979)

¹Precipitation averaged 347 mm during fallow
²Water use efficiency based on grain yield, growing season precipitation and soil water changes.
³ Column values followed by differnt etters ar significantly different at P = 0.05 level

using no tillage resulted in greater soil water storage during fallow, higher grain yields, and greater water-use efficiency of a following dryland (non-irrigated) grain sorghum crop than where disk or sweep tillage methods were used (Table 26). For an irrigated winter-wheat - fallow - dryland-sorghum - dryland sunflower (Helianthus annuus L.) rotation, water storage during fallow after wheat, as well as sorghum grain yield again were highest with no tillage (Table 27) (Unger 1984). Effects of no-till and conventional till were studied in a maize-soybean rotation on a clay loam over a 4-year period from 1984-1988, and in a sorghum-groundnut tillage trial on a sandy clay loam for one season (1988) (Thiagalingam et al. 1991). The results in Table 28 show that the 4-year maize yield for no-till was 41% higher than for conventional till. However, the increase in maize yield under no-till was 130 and 110% higher than conventional till in the drier years of 1985-1986 and 1987-1988, compared with 11 and 8% during the more favourable years of 1984-1985 and 1986-1987. Average soybean yields were 20% higher under no-till during the 4-year period and it was observed that emergence of soybean seedlings was better under no-till than conventional till, where severe crusting occurred. Thiagalingam et al. (1991) further report that applications of 30, 60 and 120 kg N ha^-1 significantly increased flag leaf area by 24, 41 and 47% for no-till and 11, 24 and 44% for conventional till (Table 29). The response under no-till was much higher at low levels of nitrogen (30 and 60 kg N ha^-1) than under conventional cultivations. In a study on the effect of tillage methods on soil properties and yield of cassava, Ohiri and

TABLE 27
Effect of tillage methods on average soil water storage during fallow¹ after irrigated winter wheat and on subsequent rainfed grain sorghum yields, total water use and water-use efficiency at Bushland, Texas, USA (1978-1983) (Unger 1984)

¹Fallow duration of 10 to 11 months. Fallow precipitation averaged 316 mm
²Includes average growing season rainfall of 301 mm
³Water use efficiency based on grain yield, growing season precipitation, and soil water changes
⁴Based on fallow period precipitation stored as soil water
⁵Column values followed by the same letter or letters are not significantly different at P + 0.05 level

TABLE 28
Effect of no-till and conventional till on the yield of maize, soybean, sorghum and groundnut

¹Average yield over four years
² One year yield

Ezumah (1990) reported that tillage did not affect total bio-mass yields in the first year, but in the second year significant differences were obtained in the yield of tops but not of fresh roots (Figure 33). No-till and minimum tillage yielded 40 and 23% more tops than convention-al tillage.

TABLE 29
Effect of nitrogen and tillage on sorghum flag leaf area

CONCLUSIONS

Tillage operations are needed for seedbed preparation, weed con-trol, management of crop resi-dues, mixing fertilizer in the soil, improving soil aeration, alleviating compaction and optimizing soil temperature and moisture regimes. The choice of tillage practice depends on soil, climatic, crop and socio-economic factors (Lal 1980; Unger 1984).

Conservation tillage, a crop production system involving the management of surface residues, prevents degradative processes and restores and improves soil productivity. The experimental data presented in this review show that conservation tillage has a wide application for sustainable crop production on a range of soils in the humid and sub-humid tropics. Major goals of conservation tillage are improved maintenance of surface residue for erosion control and efficient water conservation in the different agro-ecological regions. A limitation is its heavy dependence on herbicides and pesticides, which can lead to serious water pollution.

Conservation tillage procedures must be related to the particular site. Their successful application and use over a wide range of soil conditions depends on matching the procedure to soil type, crop cultivar, climatic factors and other aspects of the environment. Appropriate recommendations to farmers should be based on scientific data from well-designed and adequately equipped long-term experiments. The priorities for the development of conservation tillage systems include:

1. the development of cheap alternative methods of weed control, especially in tropical Africa for farmers with few resources;
2. the development of effective and specific herbicides to control weeds for countries where the farmers can afford them (such herbicides should not harm subsequent crops);
3. the development of suitable crop rotations including cover crops, and improved cropping sequences that result in more effective storage of rainfall and efficient utilization of soil available water,
4. provision of appropriate equipment for planting and fertilizer application, and
5. the breeding of crop cultivars that are adaptable to conservation tillage systems and also have characteristics that aid in erosion control as well as improve soil fertility.