An e-publication by the World Agroforestry Centre
AGROFORESTRY FOR SOIL CONSERVATION
The following is a summary of the conclusions reached in this review. Summaries also have been given in Young (1987b, and in press, a). 'Trees' refers to all woody perennials, including trees, shrubs and bamboos. 'Crops' includes both agricultural crops and pastures.
Soil conservation and sustainability
Sustainability refers to productivity combined with conservation of the natural resources on which production depends. Maintenance of soil fertility forms a major component of sustainable land use.
The primary objective of soil conservation is maintenance of soil fertility. To achieve this, control of erosion is one necessary, but by no means sufficient, condition. Equally important are maintenance of the physical, chemical and biological soil conditions that are favourable for plant growth.
Agroforestry refers to land-use systems in which trees or shrubs are grown in association with crops (agricultural crops or pastures), in a spatial arrangement or a rotation, and in which there are both ecological and economic interactions between the trees and other components of the system.
An agroforestry practice is a distinctive arrangement of components (e.g. trees, crops, pastures, livestock) in space and time. An agroforestry system is a specific local example of a practice. There are thousands of agroforestry systems, traditional and modern, but only some 20 distinct practices. Thus, agroforestry offers a wide range of choice, giving opportunities to design systems suited to a variety of physical environments and social and economic conditions.
Agroforestry practices and systems can be classified according to their components and their temporal and spatial arrangement. The division into rotational, spatial-mixed and spatial-zoned practices is related to the types and degrees of interaction between tree and crop components, and forms a basis for research (see Table 4, p. 12).
Management options for restoring or maintaining soil fertility may be constrained by:
Most non-agroforestry methods suffer from one or more of these constraints. The various agroforestry practices are applicable to a wide range of environmental conditions and do not require inputs that are in short supply or costly. The land requirements of the tree component may be compensated either by higher crop yields or by the value of products from the tree. Thus, agroforestry is widely applicable as a practical management option. One of its greatest potentials is to help solve land-use problems in areas of sloping land.
Trends in soil-conservation research and policy
The earlier approach to soil conservation centred upon rates of soil loss. The requirements of arable cropping were taken as fixed, and hence conservation measures were directed at reducing runoff, through earth structures. On the basis of assessed land capability, much sloping land was regarded as only suitable for non-arable use. In extension, soil conservation was often treated in isolation, and sometimes on the basis of quasi-legal compulsion.
Arising from problems in the earlier approach and from recent research, greater attention is now given to the effects of erosion on soil properties, fertility and crop yields. In conservation, there is greater emphasis on maintaining a soil cover, as compared with checking runoff. Where sloping land is already under arable use, means must be found of making this sustainable. In extension, it is recognized that conservation is only likely to succeed where it is implemented through the willing cooperation of farmers. It must therefore be in their perceived interests, as an integral part of improvements leading to higher production.
Aspects of these recent trends significant to agroforestry are:
Soil erosion is the cause of substantial lowering of crop yields and loss of production. The effect on yields is in general greater on tropical than on temperate soils, and greatest on highly weathered tropical soils. The major causes of such yield reduction are loss of organic matter and nutrients and, in dry areas, loss of runoff and lowering of available water capacity. Hence, agroforestry practices which combine maintenance of fertility with control of soil loss are of particular importance.
Where erosion is treated as simple loss of soil depth, it is frequently difficult to justify conservation in economic terms. Economic justification is frequently possible, however, on the basis of prevention of crop-yield losses. Agroforestry methods usually have lower initial costs than terracing or bunds, and also have the potential for maintaining or increasing crop yields. It is therefore likely, other things being equal, that conservation by means of agroforestry will show more favourable results from economic analysis than conservation by means of earth structures.
Soil conservation by means of an enforced policy frequently does not work. Conservation is likely to be most effective where it is conducted with the active cooperation of farmers, in their perceived interests, and integrated with other measures for agricultural improvement. This situation is in good accord with the diagnosis and design approach to the planning of agroforestry.
Erosion can be controlled through checking downslope flow of water and entrained soil by means of barriers to runoff, the barrier approach, and through maintenance of a ground surface cover of living plants and litter, the cover approach. The effect of soil cover is both to check raindrop impact and to provide dispersed micro-barriers to runoff.
Models for the prediction of erosion are based on the controlling variables of rainfall erosivity, soil credibility, slope (angle and length) and soil cover. A review of these models shows that there arc equal or greater opportunities to reduce erosion by means of the cover approach than by the barrier approach.
Experimental evidence supports that of models in showing the high potential for erosion control of soil cover. The effect of tree canopy cover is relatively small, and may even be negative. Ground litter or mulch, on the other hand, is highly effective; a litter cover of 60% will frequently reduce erosion to low levels, even without additional measures of the barrier type. The potential of agroforestry for erosion control therefore lies in its capacity to maintain a ground surface cover of greatest litter during the period of erosive rainfall.
On the basis of the limited available evidence, the effects of agroforestry on the causative factors of erosion appear to be as follows:
Thus, in the design of agroforestry systems for erosion control, the primary aim should be to establish and maintain a ground surface cover of plant litter. This conclusion is supported by a range of convergent evidence, direct and inferential.
The presence of trees does not necessarily lead to low rates of erosion. What matters is the spatial arrangement of the trees and, especially, the way in which they are managed.
Data on recorded erosion rates under agroforestry are sparse, although more measurements are in progress. The limited existing data support the hypothesis that agroforestry systems have the potential to reduce erosion to acceptable rates.
Hedgerows differ from ditch-and-bank structures in that they are partly permeable barriers. Standard criteria for design of conservation works, based on impermeable earth barriers, are not necessarily transferable without modification to barrier hedges. An advantage arising from partial permeability is that hedgerow barriers are less likely to be destroyed during heavy storms. Research is needed into the effects of hedgerow barriers on runoff and soil movement.
The role of trees and shrubs in erosion control may be direct or supplementary. In direct use, the trees are themselves the means of checking runoff and soil loss. In supplementary use, control is achieved primarily by other means (grass strips, ditch-and-bank structures, terraces); the trees serve to stabilize the structures and to make productive use of the land which they occupy.
The functions of the tree component in erosion control may include any of the following:
Methods of erosion control through agroforestry have been designed, recommended or are being tried in a number of countries, in some cases on the basis of experimental results, at other sites on an empirical or trial basis.
Firm knowledge of the effects of agroforestry practices on erosion is sparse. On the basis of such data as exist, the probable effects may be summarized as follows (see Table 10, p. 76).
Rotational Practices. Improved tree fallow can check erosion during the period of fallow, but erosion control as a whole will depend mainly on practices during the cropping period. For taungya, limited evidence suggests there may be some increase in erosion during the cropping period, as compared with pure tree plantations, but probably not a substantial adverse effect.
Spatial-mixed practices. Plantation crop combinations and multistorey tree gardens, including home gardens, can control erosion through the provision of a dense, regularly renewed, ground surface cover. In the case of multistorey gardens, such control is intrinsic to the nature of the practice. For plantation crop combinations, control depends on management, specifically the maintenance of a ground cover of litter.
Spatial-zoned practices. For hedgerow intercropping (alley cropping, barrier hedgerows) there is substantial inferential, and limited experimental, evidence of potential erosion control through provision of a litter cover on the cropped alleys and a barrier function through the hedgerows. Effective erosion control will not be automatic, and will vary with detailed design and management practices. Given the apparently high potential coupled with the sparsity of experimental data, there is an urgent need for controlled measurements of erosion rates under this practice.
The practice of trees on erosion-control structures involves the supplementary use of the tree component. Tree planting can make productive use of the land occupied, help to stabilize the structures and in some cases add to their protective effects. It also fulfils a psychological function, making it more likely that the structures will be perceived as beneficial and thus maintained. This applies to trees on ditch-and-bank structures, grass barrier strips, and terraces.
Although not covered in this review, the established potential of windbreaks and shelterbelts to control wind erosion may be noted for completeness.
Sylvopastoral practices. Erosion control on grazing land depends primarily on the basic, established practices of pasture management, notably limitation of livestock numbers and rotation of grazing. Sylvopastoral methods alone are unlikely to succeed, but can contribute when carried out in conjunction with other measures for pasture management. A specific potential is for reducing grazing pressure through provision of protein-rich fodder at those times of the year when grass pasture is scarce.
Reclamation forestry and watershed management. There are opportunities to integrate agroforestry with the known benefits of reclamation forestry. A period of reclamation is followed by controlled productive use, retaining part of the tree cover for continued conservation.
Agroforestry can form a component, together with other major kinds of land use, in integrated watershed management.
Soil fertility and degradation
Soil fertility is the capacity of soil to support the growth of plants, on a sustained basis, under given conditions of climate and other relevant properties of land. It is part of the wider concept of land productivity.
Diagnosis of the problem of low crop yields should distinguish between low soil fertility, caused by natural soil conditions, and decline in soil fertility, brought about by past land use. These two causes may call for different kinds of action.
The association between trees and soil fertility is indicated by the high status of soils under natural forest, their relatively closed nutrient cycles, the soil-restoring power of forest fallow in shifting cultivation, and the success of reclamation forestry. More detailed evidence is provided by comparisons of soil properties beneath and outside tree canopies.
Trees maintain or improve soils by processes which:
Some of these processes are proven, others are hypotheses in need of testing (see Table 14, p. 97; Figure 7, p. 98).
Soil organic matter plays a key role in maintaining fertility, particularly, but not only, under low-input conditions. Its main effects are to improve soil physical properties and to provide a reserve of nutrients, progressively released by mineralization.
Herbaceous plant residues applied to the soil initially decompose rapidly, with a half-life in tropical soils of less than six months. Woody residues decompose more slowly. During decomposition there is a loss of carbon and a release of nutrients. The remaining material becomes soil organic matter or humus. There are at least two fractions of humus, labile and stable. It is largely the labile fraction which contributes to nutrient release, and which is directly affected by management. It is not known whether woody residues confer distinctive properties on soil humus.
Taking as a basis the established cycling of organic matter under natural forest and decline under cultivation, it is feasible to construct a cycle under agroforestry which maintains equilibrium in soil organic matter. The following are approximate rates of above-ground biomass production which, if returned to the soil, can be expected to maintain organic matter at levels acceptable for soil fertility:
Humid tropics 8000kgDM/ha/yr
The net primary production of natural vegetation communities is somewhat higher than these values, whilst that from trees used in agroforestry can approach, and occasionally exceed, that from natural vegetation (see Table 20, p. 22).
In agroforestry systems, the requirements to maintain soil organic matter can certainly be met if all tree biomass and crop residues are added to the soil. If the woody part of the tree is harvested, this becomes more difficult, and it is impossible if tree foliage and crop residues are also removed.
The rate of litter decay is influenced by its quality, or relative content of sugars, nutrient elements, lignin and other polyphenols. Rates of decay determine the timing of nutrient release. It is desirable to synchronize nutrient release with plant uptake requirements. Agroforestry systems offer opportunities to manipulate this release, through selection of tree species and timing of pruning.
Nitrogen-fixing trees and shrubs, growing within practical agroforestry systems, are capable of fixing about 50-100 kg N/ha/yr. The nitrogen returned in litter and prunings may be 100-300 kg N/ha/yr, partly derived by recycling of fertilizer nitrogen (see Table 22, p. 131).
The second major role of trees is to improve the efficiency of nutrient cycling. Mechanisms are uptake from lower soil horizons, reduction of leaching loss by tree-root systems, balanced nutrient supply, and improvement in the ratio between available and fixed minerals. For a tree-leaf biomass production of 4000 kg DM/ha/yr, the potential nutrient return in litter, as kg/ha/yr, is of the order of 80-120 for nitrogen, 8-12 for phosphorus, 40-120 for potassium and 20-60 for calcium. These amounts are substantial in relation to the nutrient requirements of crops (see Table 23, p. 136; Figure 12, p. 132; Figure 13, p. 134).
In research, the emphasis on nitrogen fixation has led to a comparative neglect of the effects of agroforestry systems on other nutrients, and on the potential to achieve more closed cycles of all nutrients under agroforestry as compared with agriculture.
There is substantial evidence that trees in agroforestry systems can help to maintain soil physical properties, a major element in soil fertility.
The base content of tree litter can help to check acidification. It is unlikely to be of sufficient magnitude appreciably to moderate the acidity of strongly acid soils, other than in systems which make use of tree biomass accumulated over many years.
As a means of forest clearance, manual and shear-blade methods leave the soil in better condition than bulldozer clearance. The efficiency of rotational systems is necessarily reduced if burning is practised, with consequent loss of most stored carbon, nitrogen and sulphur.
As shown in Part II of this review, agroforestry has a potential for control of soil erosion. Since the major adverse effect of erosion is loss of organic matter and nutrients, the potential to control erosion constitutes a major means of maintaining soil fertility.
There has recently been increasing recognition of the importance of roots as a component of primary production. Root biomass of trees is typically 20-30% of total plant biomass (or 25—43% of above-ground biomass). However, net primary production of roots is substantially more than standing biomass, owing to the turnover of fine roots. Roots form an appreciable store of nutrients, and since they are almost invariably returned to the soil, constitute a substantial clement in nutrient recycling.
Tree root systems, together with their associated mycorrhizae, improve the efficiency of nutrient cycling, defined as the ratio between plant uptake and losses by leaching and erosion. They also contribute to soil physical properties.
The key to making use of root and mycorrhizal systems in agroforestry lies in maximizing these positive effects whilst reducing tree-crop competition for moisture and nutrients. There is a clear need for more knowledge of root growth and functioning in agroforestry systems.
The properties which constitute a good soil-improving tree, and thus the means of recognizing one, are not well established. The following are contributory:
Fifty-five tree and shrub species, belonging to 32 genera, are identified which have a potential to maintain or improve soil fertility (Table 27, p. 159). Species with particularly high potential include:
Agroforestry practices for soil fertility
Most reported indigenous agroforestry systems (other than shifting cultivation) have a spatial-mixed structure, in contrast to the spatial-zoned systems which are the focus of much current research. In the majority of indigenous systems, control of erosion, maintenance of fertility, or both, are an identified function. Use of poor soils and reclamation of degraded land are also found (see Table 28, p. 170).
A substantial body of research results on soil exists only for shifting cultivation and the plantation-crop combination of coffee or cacao with combinations of Erythrina, Inga and Cordia. Data on hedgerow-intercropping systems come mainly from one site, at Ibadan, Nigeria, although further studies are in progress or planned. Soils data on other agroforestry practices are sparse.
Results from soils research on agroforestry practices include the following.
Rotational practices. For shifting cultivation, dependent on natural forest fallow, there is no way of escaping the large land requirement implied by the fallow-to-cropping ratio necessary to restore soil fertility. Owing to population pressure upon land, this formerly stable system is no longer sustainable in many areas.
The potential of improved tree fallows, and more generally the relative effects on soils of rotational and spatial combinations of trees and crops, are not known.
Spatial-mixed practices. Plantation crop combinations of coffee or cacao with Erythrina, Inga and Cordia are characterized by a large return of organic matter and nutrients to the soil, in litter and primings, together with a moderate level of nitrogen fixation. Where fertilized, the nutrient return includes nutrients in fertilizer, demonstrating the efficiency of the system in promoting nutrient retrieval and recycling.
Multistorey tree gardens, including home gardens, through a high rate of biomass production and efficient nutrient recycling, exemplify conditions of sustainability, by combining high productivity with complete conservation of resources.
Spatial-zoned practices. In hedgerow intercropping (alley cropping), a large biomass production can be obtained from hedgerows, together with nitrogen fixation and substantial return of nutrients in primings. It may be possible to design systems in which crop yields, per unit of total area, are greater with hedgerows than in monocropping. The one available soil-monitoring study showed successful maintenance of fertility for six years. Roots are probably a contributory factor (see Table 32, p. 000).
The presence of a given agroforestry practice is by no means sufficient to ensure maintenance of soil fertility. Equally important are: (1) the design of the system in relation to local environmental and socio-economic conditions; (2) good management of the system; (3) the integration of agroforestry with the farming system as a whole.
Modelling soil changes under agroforestry
A computer model has been developed, Soil Changes Under Agroforestry (SCUAF), to predict the effects on soils of specified agroforestry systems within given environments. This is a relatively simple input-output model, covering prediction of changes in erosion, soil organic matter and nutrients. Illustrative outputs are given in Figures 19-23 (pp.209-211). The SCUAF model can be used as an aid to the design of agroforestry research.
In less-developed countries of the tropics and subtropics, there is a large and growing problem of decline in soil fertility. This is caused both by erosion and by other processes of soil degradation. Indirect evidence, together with limited experimental data, indicate that many agroforestry practices have the potential both to control erosion and to check other forms of soil degradation. The combination of a high apparent potential with a scarcity of experimental results points clearly and strongly to the need for research.
Agroforestry research can be conducted at three levels: 'What happens?' or trials of systems, 'Why does it happen?' or studies of elements within systems or interactions between components, and 'How does it happen?' or studies of basic processes. Trials of systems alone (what research) arc inefficient as a means of advancing knowledge, owing to the large number of variables and the site-specific weather and soil conditions. Studies of elements within systems (why research) lead towards the efficient design of prototype systems, which can then be tested over a limited range of variation. A better knowledge of basic processes will help in understanding the functioning of components, their interactions and thereby systems.
Research into soil conservation by means of agroforestry can be considered in two parts: specialized studies and soil aspects of general agroforestry research. Subjects for specialized soil research are listed, together with a suggested minimum set of soil observations to be included in general agroforestry research. A set of ten hypotheses for investigation by specialized soil-agroforestry research is presented p. 218.
Examples of research designs at the why level are given, together with notes on experimental techniques and observations. Further studies of research methods specific to the problems of agroforestry are required.
The general soil-agroforestry hypothesis is that:
Appropriate agroforestry systems control erosion, maintain soil organic matter and physical properties, and promote efficient nutrient cycling.
It is concluded that this hypothesis is essentially true. There is a considerable potential for soil conservation through agroforestry, both in control of erosion and by other means of maintaining soil fertility. This potential applies to many agroforestry practices and over a wide range of climatic zones and soil types (see Table 33, p. 231).
If research succeeds in confirming this conclusion, then agroforestry has the potential to make a major contribution to soil conservation and sustainable land use.