Agroforestry Systems 5, 277-291 (1987) 277 © Martinus Nijhoff Publishers, Dordrecht - Printed in the Netherlands

Soil productivity, soil conservation and land evaluation

ANTHONY YOUNG*

Abstract. ICRAF's main contributions to research related to soils have been a symposium, Soils Research in Agroforestry; a review of soil productivity aspects of agroforestry; a further review &the potential of agroforestry for soil conservation, covering both erosion control and maintenance of fertility; the construction of a computerized model to predict soil changes under agroforestry systems; and a handbook of practical methods of agroforestry for soil and water conservation in dryland Africa. In research on land evaluation, an environmental data base has been established, leading to a capacity to obtain information, for environmental conditions similar to those of a given site or area, on publications, multipurpose trees, crops, existing agroforestry systems and current experimental work. Land evaluation for agrofore- stry cannot be achieved merely by synthesis of methods from agriculture and forestry, but will require more data on the performance of agroforestry land utilization types. Recognition of problems of environmental degradation has become an integral part of planning for agrofore- stry research. By means of a partial synthesis between land evaluation and diagnosis and design, a procedure of site selection for agroforestry research and development has been established.

1 Soil productivity and conservation

1.1 The 1979 symposium on soils research in agroforestry

One of the first research activities undertaken by ICRAF was to convene an international conference on soils research in agroforestry, held in Nairobi in March 1979. The report of this meeting is the first in its published sets of proceedings [10]. The Preface states the essential feature of the meeting, that it reviewed "soils research that can be of value in the study of agroforestry systems", and proposed research methods.

In an opening address, the then Director, K.F.S. King, put forward the view common in the early years of agroforestry, that it can be of particular value in areas of "fragile ecosystems" in the tropics, of which he identified four: the semi-arid zone, the savannas, sloping lands, and areas presently under shifting cultivation. He also proposed sustainability as a key element of agroforestry land use systems.

The succeeding 21 scientific papers are typical of the early years of agroforestry research, in that they take information derived primarily from research in other disciplines and show its potential relevance. They include reviews of problems in the maintenance of soil physical, chemical and biological conditions, accounts of nitrogen fixation, mycorrhiza and soil

* Principal Scientist, ICRAF

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microorganisms, the relevance of studies of forest fallows, and the role of trees in watershed management. Notable in the light of subsequent develop- ments is the sketching out of a plant-soil carbon cycle under forest fallow [1, cf. also 2] and the identification of the key role of root growth and exudation (pp. 386-9). Several papers point to the fact that present methods of land use are leading to soil degradation, and note the potential of trees to stem or reverse this situation, for example through the fertility-enhance- ment properties of Acacia senegal and A. albida in the semi-arid zone and reclamation forestry on the "wasted lands" of India. There is also a sugges- tion to use trees for improvement of degraded pastures.

A summary paper by Lundgren gives the first of many accounts by ICRAF of the ways in which trees improve soils (pp. 526-7), and sets out a strategy for soils research in agroforestry. The latter includes further synthesis of existing relevant knowledge, the identification of aims of soil management under agroforestry systems, and of the distinctive soil pro- blems which these present. What is necessarily absent from a review at this date is experimental results from field research on agroforestry.

1.2 Soil productivity

A wide-ranging review of soil productivity aspects of agroforestry appeared in 1984, as the first in the series, "Science and practice of agroforestry" [11]. It spans the gap between the pioneer and modern eras of agroforestry research, in being based partly on drawing evidence from existing land use practices, including non-agroforestry, and partly on reviewing results of field research directly into agroforestry. There are two related shorter acc- ounts [9, 12].

Five "land use systems related to agroforestry" are identified: shifting cultivation, the taungya system, plantation agriculture, plantation forestry and multiple cropping. For each, a short system overview is given, followed by more detailed discussion of soil management. Shifting cultivation is the oldest agroforestry practice, sustainable under low population densities but degrading where the cultivation period, as a percentage of the total cul- tivation-fallow cycle (the R factor), rises above a critical level, which varies with climate, slope and soil type [1, 20]. Additions of biomass and nutrients to the soil from clearance of forest fallows can be substantial, although C, N and S are largely lost if the felled vegetation is burnt. Apart from demonstrating the soil-improving role of trees, an obvious direct develop- ment is to replace natural fallows with planted fallows of fast-growing woody perennials, which might have the potential to restore soil fertility more rapidly and at the same time provide one or more useful products [13].

The taungya system, in which food crops are intercropped during the first

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few years of establishment of timber plantations, has been a success in some countries from social and economic aspects, as a way of obtaining food from forest land and cheaply establishing forests, but is probably neutral to negative from a soils aspect, with a danger of erosion during the cropping period. Agricultural plantations (e.g. oil palm, cocoa, coffee) demonstrate how trees and shrubs, particularly if in multi-layered arrangements, can conserve soils and maintain productivity even in environments with high erosion and leaching hazards, namely sloping lands in the humid tropics. This capacity can be adapted to agroforestry, through systems of plantation crop combinations, either by trees above the main agricultural crop or by shrub or herbaceous crops below. An example is the coffee-Cordia-Erythrina system found in latin America, in which soil fertility is maintained through litter and prunings from the nitrogen-fixing tree Erythrina poeppigiana.

The next part of this review sets out the role of trees in soil productivity and conservation, giving as the major functions, to maintain soil organic matter, through addition of leaf litter; to maintain nitrogen, through fixa- tion by leguminous trees; to reduce leaching losses and make the plant-soil nutrient cycle more closed; to improve soil physical conditions, through maintenance of organic matter and the role of roots; and to check soil erosion, by water and wind.

The third part of the review summarizes field research into soils aspects of agroforestry. Hedgerow intercropping (alley cropping) is capable of maintaining soil fertility and crop yields under conditions where control plots under annual crops alone show soil degradation and declining yields. Plantation intercropping leads to high biomass production and relatively closed nutrient cycles. Acacia albida and Prosopis spp. have a demonstrated capacity to improve soil fertility in semi-arid to dry savanna regions. Meth- ods of clearing vegetation prior to agriculture are important: manual slash- and-burn leads to less loss of stored fertility than bulldozer clearance; where mechanical methods are economically necessary, the shear-blade, by which trees are sawn off close to ground level, is preferable.

The basic messages of this review are first, that by making use of the soil-improving capacity of trees, the potential exists for agroforestry systems to be productive and at the same time sustain soil fertility; and secondly, that the design of agroforestry systems should take account of these features in soil management.

1.3 The potential of agroforestry for soil conservation

A further review was carried out in 1985-87, with the focus on soil conserva- tion. The term soil conservation was interpreted in its wider sense, to include

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both control of soil erosion and maintenance of soil fertility. This review is in course of publication.

The first part covers the potential of agroforestry for control of soil erosion [28, 29], drawing upon an earlier externally-produced summary [18]. Trends in soil conservation research and policy are reviewed and appraised with respect to their significance for agroforestry. Formerly, attention was concentrated on absolute rates of soil loss, and the effects of erosion viewed in terms of loss of soil profile depth leading to total removal. Based on the approach of land capability classification, many sloping lands were declared unsuitable for arable use. Extension was carried out on the basis that erosion control must come first, often applied in isolation from other agricultural improvements and sometimes enforced by a prohibitive policy.

More recently, it has been shown that the major ill effect of erosion is in lowering soil fertility, through removal of nutrients and organic matter in eroded sediment. For a given amount of erosion, the loss of productivity is greater in tropical than temperate soils, and greatest on "old", highly-weath- ered tropical soils. It is now recognized as unrealistic to prohibit cultivation of all steeply-sloping lands, on which many farmers depend for their liveli- hood; ways must be found of making the productive use of such lands environmentally acceptable. Finally, it is now recognized that erosion con- trol can only be lastingly achieved with the willing cooperation of farmers; it follows that they must be able to see tangible benefits from conservation.

The functions of agroforestry in erosion control are: i. to reduce erosion by increasing soil cover, through tree litter; ii. to act as a runoff barrier, by closely-planted hedgerows and the litter that

accumulates against them; iii. to strengthen and stabilize earth structures for erosion control, through

the binding action of tree roots; iv. to make productive use (e.g. for fuelwood, fodder, fruit) of land taken

up by conservation structures; v. a partly psychological function, that of linking erosion control practices

with increased production, thus helping to make these practices an integral and permanent part of the farming system.

One finding of particular importance from the point of view of agrofore- stry design is that the potential of trees and shrubs to limit erosion arises largely from the ground surface cover of litter. A tree canopy alone frequent- ly reduces erosion only slightly and may sometimes increase it [18, 19].

The following agroforestry practices have been successfully employed in erosion control: - barrier hedges - trees on soil conservation works

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- alley cropping - multistorey tree gardens - plantation crop combinations.

There may also be a potential to reduce erosion on pastures, but only if trees are employed in conjunction with standard methods of pasture man- agement.

The second part of the review covers the potential of agroforestry for maintenance of soi! fertility, and restoration of fertility on degraded soils [25, 26, 29, 31]. Some of the concepts and material are derived through collaboration with the Tropical Soil Biology and Fertility (TSBF) project [15, 16] and the Commonwealth Science Council programme on Ameliora- tion of Soils by Trees [3].

Agroforestry offers a practical means of achieving productive output whilst at the same time maintaining soil fertility: that is, it meets the joint criteria of productivity, sustainability and practicability. The productive element arises first, from the capacity to sustain, or even augment, crop or pasture production in the presence of trees; and secondly, through produc- tion from the trees themselves. Practicability stems from the fact that most agroforestry land use systems require neither costly external inputs nor complex technology.

Sustainability arises from the capacity of trees to maintain or restore soil fertility. Soil-plant cycles of organic matter and nutrients lead to declining soil fertility under permanent annual cropping in most tropical environ- ments; the corresponding cycles under natural forest or woodland vegeta- tion sustain fertility, as shown by the high initial productivity of newly- cleared soils. Some of the mechanisms by which trees improve soils are proven, others are hypotheses. These processes, and the state of knowledge with respect to them, are shown in Table 1. Research into some of the hypotheses for processes will require specialized studies, including by means of isotope-labelling techniques.

The following agroforestry practices have a substantial and proven capac- ity for the tree component to augment soil fertility: - - alley cropping - - multistorey tree gardens - - plantation crop combinations - - biomass transfer (transport of tree foliage from forests to cropland) - - planted tree fallows - - trees on cropland (e.g. Acacia albida) - - trees on pastures

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Table l. Processes by which trees maintain or improve soil fertility. Based on references 11, 25, 26, 29, 31.

PROCESS STATE OF KNOWLEDGE

A. Processes which augment additions to the soil." 1. Photosynthetic fixation of carbon

and its transfer to the soil via litter and root decay

2. Nitrogen fixation

3. Improved nutrient retrieval by tree roots; including through mycorrhiza, and from lower soil horizons

4. Providing favourable conditions for the input of nutrients from rainfall and dust B. Processes which reduce losses from the soil."

5. Control of erosion, by combination of cover and barrier effects, especially the former (see text p. 278)

6. Root uptake of nutrients that would otherwise have been lost by leaching

C. Improvement of soil physical properties: 7. Soils under trees have favourable

structure and water-holding capacity, through organic matter maintenance and root action

Proven, including by quantitative measurement under natural vegetation and some agroforestry systems

Proven, including some quantitative measurements. By no means all leguminous trees are N-fixing, whilst a few non-leguminous species are, e.g. Casuarina spp.

Effects of mycorrhiza well demon-strated; retrieval from depth not specifically demonstrated; indirectly indicated by the foliar nutrient content of trees growing on poor soils

Not specifically demonstrated; nutrients in throughfall and stemflow originate partly from this source

Experimentally demonstrated; including magnitude of nutrient losses in erosion, and thus saving by control

Not specifically demonstrated

Well documented, including effects of soil physical properties on productivity

D. Processes which affect the properties (quality) of plant residues and the timing of their transfer to the soil:

8. Provision of a range of qualities of plant litter, woody and herbaceous

9. Growth-promoting substances in the rhizosphere 10. The potential, through management of pruning and relative rates of leaf litter decay, to synchronize the timing of release of nutrients from litter with demand for their uptake by crops 11. Effects of tree shading on microclimate, and thereby rates of litter decay

Specific favourable effects upon soils an unproven hypothesis [17] Demonstrated in a few cases

Exists as a management option; a basic hypothesis in the Tropical Soil Biology and Fertility programme [15, 16]

Potential exists, research needed into mechanisms

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In research, particular attention has been given to alley cropping. Provided that prunings of tree herbaceous matter are placed on the soil, alley cropping systems contribute to soil fertility through maintenance of organic matter (and thereby soil physical properties), nitrogen fixation and improved cycling of phosphorous and other nutrients. In at least three African countries, the capacity has been demonstrated for certain alley cropping combinations of tree species and spacing to give cereal yields higher than those obtained from sole cropping without trees. By contrast, many current studies in India are showing some reduction in crop yields associated with the presence of tree rows, a likely cause being the removal of tree prunings to feed to livestock.

Removal of tree leaf litter for other purposes substantially reduces the soil improvement capacity of any form of agroforestry. Partial compensation is possible where the foliage is fed to livestock and farmyard manure returned to the soil. The optimum agroforestry system in economic terms is unlikely to coincide with that which is optimal for soil fertility, and compromise in design is necessary.

One common feature of these findings is of particular significance: the major contribution of trees to the control of soil erosion is through main- tenance of a surface litter cover; and this cover is at the same time a source of organic matter and nutrients for maintenance of soil fertility. In the cases of barrier hedges and alley cropping, the potential to add a barrier effect by aligning tree rows along the contour on sloping land adds a third component of soil conservation. Thus in the design of agroforestry systems, special attention should be given to the maintenance of a surface cover of litter during the period of erosive rains and early crop growth, with the combined aims of checking erosion and providing nutrients released through mi- neralization.

As a further part of the review, a computerized model was constructed, Soil Changes Under Agroforestry (SCUAF) [32]. This models the cycling of organic matter, represented as carbon, together with soil erosion. The objectives of the SCUAF model are:

i. to provide approximate predictions of changes in soil organic matter under specified agroforestry systems in given environments;

ii. to aid in the design of agroforestry systems, for research or development; iii. to show what data, and improvements in knowledge, are required from

agroforestry research in order to improve the accuracy of such predic- tions.

The SCUAF model is made available by ICRAF to interested users. It permits the rapid investigation of the likely impact on soils of a range of

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agroforestry systems; in particular, it is possible to study the effects of changing one or more critical variables, e.g. the ratio of trees to crops, or the retention or removal of tree prunings.

There are eight sets of inputs: environment, the spatial or temporal struc- ture of the agroforestry system, initial soil conditions, erosion, plant growth (initial), removals, organic additions, and soil processes. Where data are lacking, the model provides best estimates by a system of default values. The outputs are estimates, for any period specified by the user (e.g. 20 years) of: - changes in soil organic carbon; - changes in soil erosion; - changes in plant growth, as brought about by soil; - changes in harvest.

An example of the output of soil carbon changes is shown in Figure 1.

o

o

O

M O N O C U L T U R E

30':

20

i0

REPLACED BY AGROFORE S TRY

Lowland humid climate, moderate slope

MAIZE MONOCULTURE l I AGROFORESTRY

2

A 3

i0 12

Year

. . . . . . ', | 14 16 18 20

Fig. 1. An example of output from the model, Soil Changes Under Agroforestry (SCUAF), showing changes in soil humus carbon. A soil-degrading system of continuous arable cultiva- tion is replaced by a spatial agroforestry system with 40% tree and 60% crop cover [32]. 1. Soil carbon. 2. Carbon erosion (× 10). 3. Litter additions to humus carbon.

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1.4 Handbook of agroforestry for soil and water conservation in dryland Africa

Concurrently with the above review of scientific aspects, a handbook has been produced, which describes practical methods of agroforestry for soil and water conservation, with particular reference to dryland Africa (semi- arid and dry savanna zones) [14]. This is directed at practical extension workers.

The handbook commences with a summary of how to plan agroforestry interventions, based on local site conditions, problems of the farmers and of the land, and local knowledge of plants, animals and conservation methods.

The main part of the text consists of descriptions of specific agroforestry techniques, including alley cropping, dispersed trees, windbreaks, improved fallows, living fences, boundary planting, trees on pastures, and multipur- pose woodlots. Each description covers site preparation, planting, managing the plants and the site, and management organization.

2 Land evaluation for agroforestry

2.1 The environmental data base

Agroforestry is linked to conditions of the physical environment through plant growth, system management and tree/crop interactions. First, it de- pends on plants: trees, crops and pasture plants. For survival and growth, plants require radiation, suitable ranges of temperature, moisture, nutrients, oxygen (and thus soil drainage), conditions for development of a root system, absence of toxicities (e.g. salinity), and limits to the virulence of weeds, pests and diseases. Secondly, there are environmental requirements called for by management operations e.g. slope limits in relation to erosion control. Thirdly, an essential feature of agroforestry is the existence of ecological relations between the tree and non-tree components, or tree/crop interactions. These take place through the medium of microclimate and soil, for example shading, soil moisture relations and nutrient cycling.

Given this intimate linkage to site conditions, ICRAF in 1983 established an Environmental Data Base (EDB) [21]. This consists of an identified set of environmental variables, and classification systems, by which to describe the conditions of a site or area; and a computerized system for input, storage and retrieval of such data. It contains information on all the major factors of the physical environment. Based on the nature of the dependence on, and links with, plant growth, management and tree/crop interactions, detailed

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information is given on climate and soils, less detailed but important basic features of landforms (especially slope), hydrology (especially drainage and depth to water table), and vegetation (particularly required for sylvopas- toral systems), and summary features of geology and climate. The K6ppen climatic classification and the FAO soil classification [8] were adopted as standard. There are three levels of detail within the data base, to meet the requirements of different users: summary, intermediate and detailed levels.

Also in 1983, common features from the EDB were incorporated into the environmental description sections of the multipurpose trees and agrofore- stry systems data bases, and climatic ecozones and features of landforms, soils and vegetation are used as indexing terms in the library data base. Subsequently, the EDB was used as part of the input to the data base on current agroforestry experiments. In practice, relatively little data has been stored in the EDB itself, but the way of utilizing it can now be achieved through combined use with other data bases and, in the case of crop requirements, external information. Thus the scenario envisaged in 1983, shown in Table 2, has become a practical possibility in 1987.

2.2 Land evaluation

Land evaluation is the process of assessment of land potential for specific purposes. It sets out to answer questions of two kinds: 1. Given a specific area of land, what is the best kind of land use on it? 2. Given a specified kind of land use, where are the best areas on which to

practise it? The foundations of modern land evaluation were established by FAO [4].

The tasks to be accomplished in constructing a system of land evaluation for agroforestry are the formulation of appropriate agroforestry land utilization types, determination of land use requirements for these, biophysical models of interactions, the assessment of environmental impact and sustainability of agroforestry systems, and a set of methods for comparison between agroforestry and other land use systems [23]. One biophysical model for assessment of sustainability, SCUAF, has been described above. The com- parison between agroforestry and other land use systems can be made in terms of performance, environmental impact, economics and social criteria. Such comparisons have been carried out in many ICRAF collaborative projects.

It might have been supposed that a system of land evaluation for agroforestry could have been constructed by a synthesis from the detailed FAO guidelines on evaluation for rainfed agriculture, forestry and extensive

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Table 2. Using environmental information on agroforestry. Modified from references 21, 30.

QUESTION ACTION

WHAT CAN WE DO TO HELP DESIGN AN AGROFORESTRY SYSTEM FOR THIS AREA?

What kind of ENVIRONMENT does it have?

What has been PUBLISHED on similar environments?

What TREES grow well in such conditions?

What CROPS grow well in such conditions?

What AGROFORESTRY SYSTEMS are found in similar environments?

What AGROFORESTRY RESEARCH is being conducted in similar environments?

Collect data for the Environmental Data Base, from maps, remote sensing imagery, publications and in the field

Use environmental descriptors of the Library Data Base

Use Multipurpose Tree Data Base

Use external sources, e.g. FAO, CSIRO

Use Agroforestry Systems Data Base

Use Data Base on Agroforestry Experimental Work

Basic data have been assembled. Use SKILL, EXPERIENCE and JUDGEMENT to analyze data

CONTRIBUTE ENVIRONMENTAL ASPECTS TO AGROFORESTRY SYSTEM DESIGN

grazing [5, 6, 7]. Experience has shown that the task is more complex than

this.

There are two fundamenta l reasons. First, in convent ional land evalua-

tion, e.g. for mechanized maize cultivation, or pine plantat ions, the perfor- mance of the land utilization types is relatively well known, and thus

envi ronmenta l requirements can be established. This is not the case with

mos t agrofores t ry systems. Whilst it is self-evident that any agrofores t ry

system, specified in detail (as to tree and crop species, spacing, management ,

etc.) will pe r fo rm best in one set of envi ronmenta l condit ions and less well

or unsatisfactori ly in others, these land use requirements have rarely if ever

yet been established. The second reason is that the suitability of given areas

for agrofores t ry by no means depends on envi ronmenta l condit ions alone, but also on the problems of the farmers; it is possible to envisage two areas

with closely similar envi ronments but different land use problems, for which different agrofores t ry interventions would be mos t suitable.

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A more specific case is that of site selection for multipurpose trees [22]. Here, the approach of specifying climatic and soil requirements for par- ticular species is directly applicable. Problems have arisen in the very wide range of environmental conditions reported for some trees.

An attempt at a synthesis of appropriate land use systems has been made for one distinctive set of environmental conditions, that of sloping lands [27]. The intrinsic land use problem is that of soil erosion; other problems commonly found in densely-populated sloping areas are decline in soil fertility, forest clearance, pasture degradation, shortages of fuelwood and fodder, and a low cash income. Agroforestry practices particularly appro- priate to such environments include barrier hedges, trees on soil conserva- tion works, alley cropping (specifically designed to achieve erosion control), multistorey tree gardens, plantation crop combinations, and multipurpose trees for fuelwood, fodder and soil fertility improvement.

2.3 Relations between land evaluation and agroforestry diagnosis and design

The approach of agroforestry diagnosis and design was initially devised with the emphasis on social aspects, specifically on the problems faced by the farmer. Environmental degradation was treated as part of the causal chain in such problems, e.g. decline in soil fertility as a cause of shortage of food, degradation of pastures a cause of fodder shortage.

Subsequently, the identification of problems of the land has come to be recognized as a specific element in the diagnosis and design procedures. An outline of the main environmental problems for which there is a potential for agroforestry to assist is given in Table 3. Some, such as drought or soils of intrinsically low fertility, and features of the natural environment, but many are caused by environmental degradation, brought about fundament- ally by increase in population without appropriate adaptations to land use technology.

Viewed in perspective, both land evaluation and agroforestry diagnosis and design are basically methods intended to find the best system of im- proved land use for given areas. They have many activities in common, but land evaluation is stronger in its treatment of environmental aspects, diag- nosis and design in social aspects. There must be benefits to be derived by inclusion of features of one approach within the other.

A partial reconciliation of the two sets of procedures was therefore made [24]. With respect to land evaluation, the main additions are inclusion of a stage of identification of land use problems, the use of the diagnosis and design approach as a means of formulating improved land utilization types,

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Table 3. Environmental problems which agroforestry has a potential to alleviate (based on reference 29).

ENVIRONMENTAL PROBLEM POTENTIAL AGROFORESTRY PRACTICES AND FUNCTIONS

Soil erosion by water Agroforestry practices for erosion control (see text)

Windbreaks and shelterbelts

Agroforestry practices for maintenance and improvement of soil fertility (see text)

Soil erosion by wind

Low natural soil fertility] !

Soil fertility decline (physical, chemical and |

biological degradation) )

Forest clearance and degradation

Pasture degradation

Drought hazard

Degradation of river flow

Pest attack

On-farm production of fuelwood

Fodder production from trees; pasture improve- ment through trees

Agroforestry practices for microclimatic modif- ication and moisture conservation; role of deep- rooting trees

Agroforestry as an element in watershed man- agement

Trees for pest inhibition

and provision for improving knowledge of the land utilization types through inclusion of a research programme.

With respect to diagnosis and design, the additions are inclusion of a stage of site selection, and of an element of comparison between alternative land use systems (agroforestry and non-agroforestry) in the e x a n t e stage of evaluation.

Now that agroforestry is being widely considered as a possible form of development, a procedure for site selection has become essential. The re- sources available for agroforestry research and development cannot be invested everywhere at once: a system of determining priorities is needed, to select those areas where the potential benefits (environmental, economic and social) from such development are likely to be greatest. The basic steps in such site selection are [24]: i. Identify regions of relative environmental homogeneity. ii. Identify the major existing land use systems within each region. iii. For each system, estimate the nature and severity of its problems. iv. Assess the potential of agroforestry to assist in solution of these pro-

blems. v. Select for further analysis those regions and land use systems for which

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the potential benefits from agroforestry research and development app- ear to be greatest.

This procedure of site selection has been applied in the planning phase of the ICRAF agroforestry research network for Africa.

References

1. Ahn, P.M. 1979. Optimum length of planned fallows. In: Soils research in agroforestry (ed. H.O. Mongi and P.A. Huxley, ICRAF, Nairobi), 15~40.

2. Brunig, E.F. and Sander, N. 1983. Ecosystem structure and functioning: some interac- tions of relevance to agroforestry. In: Plant research and agroforestry (ed. P.A. Huxley, ICRAF, Nairobi), 221 247.

3. Prinsley, R.T. and Swift, M.J. 1986. Amelioration of soils by trees: a review of current concepts and practices. Commonwealth Science Council, London.

4. FAO 1976. A framework for land evaluation. FAO Soils Bull. 32. 5. FAO 1983. Guidelines: land evaluation for rainfed agriculture. FAO Soils Bull. 52. 6. FAO 1984. Land evaluation for forestry. FAO For, Paper 48. 7. FAO (in press). Guidelines: land evaluation for extensive grazing. FAO Soils Bull. 8. FAO-Unesco 1974. Soil map of the world. Vol. 1. Legend. Unesco, Paris. 9. Lundgren, B. and Nair, P.K.R. 1985. Agroforestry for soil conservation. In: Soil erosion

and conservation (ed. S.A. E1-Swaify, W.C. Moldenhauer and A. Lo, Soil Conserv. Soc. Am., Ankeny, Iowa), 703 717. Reprinted as Icraf Reprint 27 (1985).

10. Mongi, H.O. and Huxley, P.A. (eds) 1979. Soils research in agroforestry. ICRAF, Nairobi.

11. Nair, P.K.R. 1984. Soil productivity aspects of agroforestry. Science and Practice of Agroforestry 1, ICRAF, Nairobi.

12. Nair, P.K.R. (in press). Soil productivity under agroforestry. In: Realities, potentials and practices (ed. H. Gholz, Nijhoff, The Hague).

13. Nair, P.K.R. and Fernandes, E. 1984. Agroforestry as an alternative to shifting cultiva- tion. FAO Soils Bull. 53, 169-182. Reprinted as ICRAF Reprint 17 (1985).

14. Rocheleau, D. and Weber, F. (in press). Agroforestry for soil and water conservation in dryland Africa. Science and Practice of Agroforestry, ICRAF, Nairobi.

15. Swift, M.J. (ed.) 1984. Soil biological processes and tropical soil fertility. Biol. Internat. Spec. Issue 5.

16. Swift, M.J. (ed.) 1985. Tropical soil biology and fertility (TSBF). Planning for research. Biol. Internat. Spec. Issue 9.

17. Swift, M.J., Heal, O.W. and Anderson, J.M. 1978. Decomposition in terrestrial ecosys- tems. Blackwei1, Oxford.

18. Wiersum, K.F. 1984. Surface erosion under various tropical agroforestry systems. In: Symposium on effects of forest land use on erosion and slope stability (ed. C.L. O'Lough- lin and A.J. Pearce, East-West Center, Honolulu), 231 239.

19. Wiersum, K.F. 1985. Effects of various vegetation layers in an Acacia auriculiformis plantation on surface erosion in Java, Indonesia. In: Soil erosion and conservation (ed. S.A. E1-Swaify, W.C. Moldenhauer and A. Lo, Soil Conserv. Soc. Am., Ankeny, Iowa), 79 89.

20. Young, A. 1980. Rest period requirements of tropical and subtropical soils under annual crops. In: Report of the second FAO/UNFPA expert consultation on land resources for the future (FAO, Rome), 197-268.

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21. Young, A. 1983/5. An environmental data base for agroforestry. ICRAF Working Paper 5, 1983. Revised edition, 1985.

22. Young, A. 1984a. Site selection for multipurpose trees. ICRAF Working Paper 23. 23. Young, A. 1984b. Land evaluation for agroforestry: the tasks ahead. ICRAF Working

Paper 24. 24. Young, A. 1985. Land evaluation and agroforestry diagnosis and design: towards a

reconciliation of procedures. Soil Survey and Land Evaluation 5, 61-76. Reprinted as ICRAF Reprint 30 (1986).

25. Young, A. 1986a. The potential of agroforestry as a practical means of sustaining soil fertility. In: Amelioration of soils by trees: a review of current concepts and practices (ed. R.T. Prinsley and M.J. Swift, Commonwealth Science Council, London), 121-144. Re- printed as ICRAF Reprint 36 (1987).

26. Young, A. I986b. Effects of trees on soils. In: Amelioration of soils by trees (Common- wealth Science Council, Tech. Publ. 190), 28~41. Reprinted as ICRAF Reprint 31 (1986).

27. Young, A. 1986c. Evaluation of agroforestry potential in sloping areas. In: Land evalua- tion for land-use planning and conservation in sloping areas (ed. W. Siderius, ILRI Publ. 40, Wageningen), 106-132. Reprinted as ICRAF Reprint 33 (1986). Published in draft as ICRAF Working Paper 27 (1984).

28. Young, A. 1986d. The potential of agroforestry for soil conservation. Part I. Erosion control. ICRAF Working Paper 42.

29. Young, A. (in press, a). The potential of agroforestry for soil conservation and sustainable land use. Presented to Land and Water Resources Management Seminar, Econ. Dev. Inst. (World Bank), Wash. DC, 1986. To appear in proceedings.

30. Young, A. (in press, b). The environmental basis of agroforestry. In: Proceedings of the ICRAF/WMO/UNEP Workshop on the Application of Meteorology to Agroforestry Systems Planning and Management (ed. W. Reifsnyder and T. Darnhofer, ICRAF, Nairobi).

31. Young, A. (in press, c). The potential of agroforestry for soil conservation. Part II. Maintenance of fertility. ICRAF Working Paper 43.

32. Young, A., Cheatle, R.J. and Muraya, P. 1987. The potential of agroforestry for soil conservation. Part III. Soil changes under agroforestry (SCUAF): a predictive model. ICRAF Working Paper 44.