«Henk Breman INTERNATIONAL INSTITUTE FOR SOIL FERTILITY MANAGEMENT IFDC-Africa PO Box 4483, Lomé, Togo, hbreman Network on Agroforestry and ...»
The compromise as described explains why the maximum contribution of trees to crops in the most effective agroforestry system of Sahelian countries, the Acacia Faidherbia parkland, is so limited. The additional N available for growth of sorghum and millet is about 5 kg/ha/yr or less. In other words, the cereals yield at maximum 200 - 400 kg/ha extra grain, a figure obtained theoretically and confirmed by field data (Breman & Kessler, 1997). The maximum is reached on deep soils, at a canopy cover of about 20% of homogeneous distributed trees, and at return to the soil of all organic material produced by the agroforestry system, except for the cereal grain yield. Limiting shade effects is an additional condition. In case of Acacia Faidherbia this is not a problem in view of its exceptional phenology (leaves during the dry season). In case of other species this has to be obtained by assuring that the crown diameter is not more than half the stem height.
Alley cropping and multipurpous trees, two fallicies The principles above explains why alley cropping is not adopted by farmers (Leihner et al., 2001) and why multipurpose trees often are unsuccessful in producing a benefit for the herblayer or crop component of an agroforestry system. The density of woody plants in alley cropping leads to very high competition. Frequent cutting, exporting the twigs, cures this. Not only this is very labour demanding, but also the woody plants do not have the opportunity to create a large stock of nutrients and organic matter in biomass and soil, while effective deep roots are not developing or die off. Multipurpose trees get a better chance to behave really like a perennial and to concentrate a certain amount of nutrients, but due to multiple exploitation, the amount is easily too low to become interesting for accompanying vegetation or crop. Beside, each of the individual functions (erosion control, soil improvement, wood, food and fodder production...) has its own optimum conditions for realisation; in other words, combined in one single plant, the functions have to be realised under sub-optimum conditions. The nice examples of multipurpous trees are from favourable regions as far as agroecological and/or market conditions concern.
It is therefore, that nice examples are found of agrosylvicultural systems in which integration of woody species with crops has as primary goal maintaining/improving soil fertility (the Acacia Faidherbia system above). Sylvopastoral systems could be imagined in comparable semi-arid regions with nutrients as most limiting factor. However, which herdsman has the time to maintain parkland as described for a slight increase in fodder production and/or quality? And where the woody component is growing naturally, livestock will exploit it intensively or it will become a real competitor for the herblayer, decreasing the carrying capacity of the range. The latter is the most frequent and tree killing is one of the most applied management tools for rangeland improvement. If there are reasons to use a woody species for improvement of the fodder situation, separated blocks (e.g. Leucena leucocephala fodder banks), which can be protected and managed separately, appears the practical solution. The soil improvement function is in that case very limited.
Agroforestry and integrated soil fertility management Soil improvement through agroforestry has the best chance in dry land areas that are affected by drought and desertification integrating crops and trees (agrosylvicultural systems). And it is practical possible to go far beyond the present maximum Acacia Faidherbia parkland of 200 – 400 kg/ha/yr cereal yield increase. As shown (par. 2.1), almost the whole benefit is obtained thanks to decreased nutrient losses. In other words, when the availability of nutrients increases, the benefit increases also. The natural sustainable availability of N in regions where the Acacia Faidherbia parkland is used is in the order of 15 – 20 kg/ha/yr (Penning de Vries & Djitèye, 1982). The amount of N protected against losses by the agroforestry system is 5 kg/ha/yr at maximum. In other words, losses in the order of 20 - 25% of nutrients can be avoided. If, due to fertiliser use, the availability of N is, for example, 100 kg/ha/yr, the loss of 20 to 25 kg/ha/yr can be avoided. Or, with the same input of labour to maintain the agroforestry system, 900 – 1800 kg/ha/yr more cereal grain can be harvested. Farmers will be more eager to continue to maintain the parkland with such a benefit than with the actual maximum benefit of 5 kg/ha/yr of extra N!
It is even possible that the benefit is still higher. With the higher availability of N (and of other nutrients required for a balanced fertilisation), the biomass production and the organic matter enrichment of the soil will increase also. As a consequence, the nutrient use efficiency will increase, improving even more biomass production, SOM status and, in turn, nutrient use efficiency as long as potential levels has not been reached yet.
It is the mechanism described in chapter 1 when presenting ISFM. Several processes contribute to higher fertiliser use efficiency due to SOM status improvement (Vanlauwe, in
• improved nutrient and water holding capacity; as far as the first concern, increased cation exchange capacity (CEC) is the main reason;
• buffering of the soil against acidification (decrease of pH), and therefore against Al and Fe toxicity;
• occupation of P-fixation sites by organic matter, leading to increased P availability;
• increased availability of other nutrients than those added through fertiliser.
To enable farmers to benefit from the ISFM mechanism, agricultural intensification through soil improvement should be the main goal of a network combining agroforestry and soil fertility in desertification control, in a continent threatened by overpopulation.
Agroforestry options not viewing improved efficiency and higher accessibility of external inputs have no chance to become successful at the actual level of agricultural intensification. It is recommended to include systems with perennial species in general under agroforestry, widening the number of technical options that can be offered to and developed with farmers. Table 2 presents an overview of the main options. It does not repeat practices treated as individual options in table 1, which can become part of packages based on the options presented in table 2, like crop-residue recirculation, the use of manure, compost, phosphate rock, etc.
Table 2. 'Menu' of integrated agroforestry soil fertility management practices in the framework of desertification control, and their recommendation domains
A question on its own concerns the use of leguminous species in the systems of table 2.
In spite of their N-fixation capacity, they are not automatically the best species to be used,
in view of three particularities:
• the biomass of leguminous species has often a relatively high mineralisation rate, while SOM status improvement demands a medium to low organic matter turnover;
• to be effective, leguminous species have a high P requirement, while P-fertiliser may be very expensive;
• energy may be more limiting than protein for man and/or livestock in a certain environment.
It is therefore recommended to use decision support systems for fertiliser use, like the one developed for West Africa (Breman & van Reuler, in press). Other criteria for the choice of agroforestry species are treated, for example, in the book at the base of this paper.
2.3 Recommendation domains.
Soils and rainfall; West Africa The paper concentrates on the semi-arid tropics, or “dry land areas that are affected by drought and desertification”, the definition of the intervention domain formulated at the Bamako workshop (Anon., 1998). As far as West Africa is concerned, this paper supposes that both the Sahel and the Sudanian savannah are covered by this definition.
However, to make it worthwhile to combine perennials and fertilisers for crop or fodder production, nutrients should be the limiting factor. In the region concerned, for systems including woody species, this is the case for zones with more than 200 mm/yr of infiltrated water. For sandy soils with good and homogeneous infiltration of rain, infiltration is almost equal to the annual rainfall: more than 250 – 350 mm will be enough to create nutrient limiting growth production. For soils with run-off, like loamy-sands and sandy-loams, more than 350 mm is required; depending on texture and degree of soil degradation and soil crusting up to 800 mm or more may even be required. And in depressions with run-on, even 200 mm of annual rainfall or less may be enough.
In view of the fact that the surplus value of trees as far as nutrient availability concerns is very limited at low rainfall, higher limits of minimum annual rainfall are recommended where no important amounts of additional water (run-on or floods) can be exploited. For sandy soils with good and homogeneous water infiltration, 750 mm of average annual rainfall is suggested as minimum. Below that level it will be very difficult to maintain enough vital perennial plants in the system for being effective. It is only in case of perennial fodder banks on loamy soils with strong redistribution of rainwater over limited distances, the 500 mm isohyet is suggested as minimum.
Woody species have their limits
The limits defined concern integrated agroforestry soil fertility management practices, not the use of woody specious as such nor ISFM and fertiliser use! As indicated at the end of paragraph 2.1 and at the beginning of 2.2, woody specious can always play a role in obtaining more stability in production systems, even when only water is the limiting factor.
Soil protection can be reinforced, but soil improvement does not really take place. And the woody species (and other perennials) are themselves a rather unstable element of the agro-ecosystems concerned: the aridity of the West African dry season is so extreme that perennial species have much less chance to be established, to develop and to produce than elsewhere in semi-arid regions at the same annual rainfall. And again and again droughts eliminate woody species and other perennials with a certain frequencies, by lack of water in the underground during the dry season. A Sahelian climax vegetation is dominated by annuals (Breman & de Ridder, 1992).
Fertiliser use can be an instrument of desertification control beyond the limits of agroforestry use. Other ISFM practices can be used to make fertilisers as efficient as possible, e.g. the use of manure and compost, crop-residue recirculation and legumes (combined with sources of P; table 1). Only if revenue on farmers can be improved in this way and overpopulation corrected, woody species will be able to continue to play a role in desert margins (Breman, in press). Desertification control is a condition for their contribution, and not a result of their application under the present conditions.
It is worthwhile to consider external support for allowing farmers to make a decent living out of crop production and/or livestock raising using external inputs like fertiliser, if in this way mining of their natural resources can be avoided and more productive land protected against desertification (Breman, 1987; Breman, 1992). Often, such a way for desertification control will be more effective and cheaper than the constant struggle against the symptoms of desertification, like reforestation, the planting of trees which will not survive in term.
Choice of nutrients and their application
Nor in case of fertiliser use in isfm on base of agroforestry (table 2), nor on base of other options like presented in table 1, it is possible to indicate without local knowledge which nutrients have to enter the system and in what way they should be applied. The limiting nutrients have to be known and the prices of crops and livestock (products). In semi-arid West Africa, N is more often limiting than water, in other words, sources of N are more crucial than sources of P. However, the degree of deficiency of soils concerning N and P is never very different; to solve one in an economically interesting way implies in general that also the other deficiency has to be eliminated (Breman, 1998). It depends on farmgate prices for crops and livestock products, which crop(s) should be fertilised, and/or if N and/or P should enter crop-livestock systems through fertilisation of the crop/fodder component or through direct supplementation of the livestock component (Breman & van Reuler, in press; Breman et al., 1990).
Other African regions
The principles as described are not limited to West Africa. However, the rainfall limits of the recommendation domains can’t be directly transferred to other African semi-arid regions. The important factors required for translation while transferring concerns effective rainfall, rainfall distribution, soil fertility and input and output prices. The latter will not require more details to be understood. The effective rainfall counts while it determines the presence of woody species and their production. In view of both rainfall and potential evapotranspiration, at least 2 mm are required, for example, in semi-arid West Africa against one single mm in Mediterranean North Africa. Where one rainy season exists, the indicated limits (paragraph 2.3) can be corrected using a ratio based on effective rainfall. But where two instead or one rainfall season is normal, the situation is more complicated: the dry seasons are less arid and at the same total annual rainfall a much higher canopy cover can be reached. The same is true where soils are more fertile than in semi-arid West Africa. As a first estimate, one could use the same natural canopy cover to guestimate the minimum rainfall for integrated agroforestry soil fertility management practices in the framework of desertification control. A natural average cover of 20% has been used to indicate the 750 and 500 mm isohyets for West Africa (respectively for sandy soils, and for crops and improved fallows on the one hand, and for loamy soils and fodder banks on the other). It is the maximum cover to be respected, avoiding competition for nutrients and water triggered by shade (par. 2.2).
3. Enabling socio-economic and policy environments