Soil & Tillage Research, 2 (1982) 305--309 305 Elsevitr Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

Guest Editorial

TILLAGE RESEARCH IN THE TROPICS

R. LAL

International Institute of Tropical Agriculture, Oyo Road, P.M.B. 5320, Ibadan (Nigeria)

INTRODUCTION

Out of approximately 711 million ha of potentially cultivable land area in Africa, only 22% is currently being cultivated. In comparison, only 13% of the potential arable land area of 596 million ha is cultivated in South America (Schulze and Van Staveren, 1980). There is enormous scope to bring more land under production in the tropics of South America and Africa, resulting in horizontal expansion of land activities. Simultaneously, the expansion is leading to more continuous cropping, thus shortening or even eliminating vital links of productivity restoring fallow periods in the existing shifting cultivation or bush fallow systems. The result is an increased exposure of structurally fragile soils to harsh tropical climates with intense rains of high energy load and year-round high temperatures. The rapid decline in soil productivity often observed is due to accelerated soil erosion, decrease in soil organic matter content, reduction in water retention and transmission properties, severe inhibition in biotic activities of soil fauna, and compaction of the surface soil horizon. As a result of this overall degradation, the pro- ductive capacity of the land resource is shrinking.

Soil surface management seems to be the key towards solving the problems associated with the transition from traditional farming to more productive land use systems in the tropics. The maintenance of soil structure and a favour able level of organic matter content are the basic ingredients in maintaining soil productivity and the stability of the system concerned. Without a good soil structure, other inputs are easily wasted. Soil management research, therefore, provides a useful avenue to tackle this important problem and it is a powerful point of entry to understand the complex problems of main- taining productivity of soils in the tropics (Greenland, 1981).

SOIL MANAGEMENT FOR SOIL AND WATER CONSERVATION

Cultural practices that have proven effective in preventing erosion and in preserving soil organic matter content are based on the principles of main- raining soil and environmental characteristics similar to those under a native vegetation cover which minimise the ecological imbalance caused by reckless deforestation. Protecting the soil surface with organic mulches and the elimi-

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nation of mechanical soil manipulation meet the requirements of maintaining the delicate equilibrium between soil and climate. Crop residue mulch, dead or alive, produced in situ or brought in, is a good substitute for the forest canopy in preserving or restoring soil physical, chemical, and biological properties and in preventing soil erosion. Methods of seedbed preparation and mulching are practical tools in achieving these principles of soil manage- ment and in providing optimum conditions for seedling establishment and crop production (Lal, 1982).

In the humid and sub-humid tropics of Africa and South America, no- tillage systems with crop residue mulch at 4--6 t/ha have proven successful, provided weeds can be effectively controlled and optimum crop stand can be established without causing unnecessary soil compaction. Basic principles of no-till farming are the same for both small-holders and large-scale mechanised farms. Seeds are planted in a narrow slit, opened mechanically or by manually operated equipment and tools, in the killed sod or in previous crop residues without primary or secondary tillage operations. A crop residue mulch is an essential component of this system, while chemicals can be used to control weeds. This system, where applicable, has given effective erosion control through maintenance of soil structure and water retention and transmission properties, and by offering more physical impedance to water runoff. It also has a better soil organic matter content in the surface layer, a better hydro- thermal regime for crop production, and it also stimulates biological activity of soil fauna e.g. earthworms. In fact, earthworms may be the best ploughing implements for these tropical soils. They turn over the soil and incorporate soil organic matter without causing erosion problems for which the plough is notorious (Lal, 1982).

NEEDS FOR RESEARCH IN APPROPRIATE TILLAGE SYSTEMS

Theoretically, the no-tillage system is suitable for all soils and crops. How- ever, in practice there are exceptions to this rule. For the no-tillage system to be successfully adopted in the management system of easily erodible and degraded soils in the tropics, it has to fit into the overall farming/cropping system of the region. A no-tillage approach cannot be adopted in isolation and should be used in conjunction with other practices. These inputs or packages of cultural practices have to be developed and investigated for different soils, crops, and agro-ecological regions. The present system of zero- tillage needs improvement to make it more versatile. More specifically, the problems associated with no-tillage that should be investigated include the development of (1) appropriate equipment for planting and fertilizer place- ment for both large- and small-holders; (2) alternate systems of weed control; (3) suitable crop rotations that include cover crops; and (4) prevention and alleviation of compaction of the soil surface with mechanized no-till systems.

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While the package of cultural practices for no-tiUage is being researched, alternate tillage systems should also be developed. Tillage systems adopted should be problem-oriented, and the farmers should have a choice of different methods according to physical limitations, resources available, and socio- economic constraints. The range of soil management problems, depending on soil texture, soil moisture regime, and different ecologies are broadly summarised in Fig. 1. Accordingly, perhaps the easily compactable soils of

w ~D l-- X IJJ I -

[] Water Erosion, Low Water Holding Capacity

[] Water Erosion - Crusting

[] Water Logging- Water Erosion

PER HUMID SUB SEMI ARID HUMID HUMID ARID

MOISTURE REGIME

Fig. 1. Soil physical constraints to crop production for different ecologies in the tropics.

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the semi-arid savanna require some mechanical tillage operations to loosen the compacted soil and improve its water infiltration. Another limitation for this region would be the scarcity of crop residue mulch. In addition to the mechanical tillage methods of erosion control and soil structure improvement, agronomic research is also needed in developing appropriate packages to procure organic mulches.

Heavy textured soils, and those that contain high-activity clays, have peculiar problems of structural maintenance and water management. In the humid region, poor drainage conditions and trafficability of the soil during the rainy season are serious constraints in realising their yield potential. Unless drainage conditions are improved through proper water management, tillage systems are of secondary importance. There is a strong drainage/tillage interaction. Although these soils can be developed for lowland rice production during the rainy season, the problem of maintenance of a favourable soil structure for the succeeding dryland crops remains to be solved.

In the semi-arid tropics, Vertisols have similar problems as in the humid region. Because the growing period is rather short, it is important that seedbed preparation and planting should be done just before the on-set of rains. Because these soils slake rapidly on quick wetting, understanding the basic aspects of soil water phenomena in terms of soil and water management requires detailed studies of the thermodynamic aspects of soft-water absorption, heat of wetting and its role in structural degeneration.

Vast regions of semi-arid and arid tropics in Africa and elsewhere suffer from severe wind erosion problems. However, there is little research information available in developing appropriate tillage methods for improving soil surface conditions for wind erosion control.

SOIL--TILLAGE INTERACTION

Soils of different physical and chemical properties respond differently to different tillage methods. Also important to be considered while deciding the choice of a tillage method are the initial soil conditions, alterations in soil properties caused by different tillage methods, and the previous and intended land use. Important soil factors apart from texture, structure, and organic matter contents are (1) compaction; (2} soil heterogeneity; (3} topography; {4) erodibility, potential erosion risk, and soil loss tolerance; (5) quanti ty and quality of residue mulch; and (6) hydro-thermal regime. Crop response to alternate tillage methods should be evaluated in terms of prevalent soil properties and climatic factors so that the results can be compared with ex- periments conducted elsewhere, and also extrapolated to similar soils and environments in other regions of the tropics. It should be appropriate to develop a "Parametric" method to assess soil suitability and response to different tillage methods (Lal, 1982).

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TEAM APPROACH

Tillage research requires heavy investment in terms of equipment, technical man-power, and time. In the tropics most of these resources are scarce. And yet, information about appropriate tillage systems is required to maintain soil resources and increase production. There is a need, therefore, to assess and develop physical models that can predict the effects of tillage systems on yield and soil properties -- compaction, water transmission and retention, soil temperature regime etc. -- for a range of soils and crops in the tropics.

The research needs and priorities listed emphasise the importance of a team approach involving a coordinated effort by soil and plant scientists, biologists, engineers, and social scientists. The realization of the potential benefits of appropriate tillage systems depend on understanding and comprehension of all phases of this complex problem.

REFERENCES

Greenland, D.J., 1981. Soil management and soil degradation. J. Soil Sci., 32: 301--322. Lal, R., 1982. No-till farming: Soil and Water Conservation and Management. IITA

Technical Bulletin Series, Ibadan, Nigeria (In press). Schulze, F.E. and Van Staveren, J.M., 1980. Land and Water Development in the Third

World. In: "Land Reclamation and Water Manegement." ILRI Publication 27, Wageningen, The Netherlands, pp. 13--28.