«Henk Breman INTERNATIONAL INSTITUTE FOR SOIL FERTILITY MANAGEMENT IFDC-Africa PO Box 4483, Lomé, Togo, hbreman Network on Agroforestry and ...»
SOIL FERTILITY MANAGEMENT THROUGH AGROFORESTRY TO COMBAT
RESPECTIVE AND COMMON RESPONSIBILITIES
RURAL DEVELOPMENT TRIANGLE ***
INTERNATIONAL INSTITUTE FOR SOIL FERTILITY MANAGEMENTIFDC-Africa PO Box 4483, Lomé, Togo, firstname.lastname@example.org Network on Agroforestry and Soil Conservation of the African Regional Action Programme to Combat Desertification, Lomé (TOGO), June 13 – 15, 2001
1. Introduction To win a battle, the enemy must be known. The present paper is based on the conviction that among the three main causes of desertification: lack of knowledge, lack of interest and lack of choice, the last is by far the most important. Overpopulation compels people to take more out of the land than acceptable from the point of view of sustainable resource management. Indigenous knowledge, developed by generations in close contact with nature, enables farmers to go beyondthe carrying capacity of the land and to undermine in term their future (Breman, 1990).
Overpopulation is a relative concept. Linked to agriculture, it is defined in relation to the limiting factors determining plant and animal production. But such factors can be corrected using external inputs; e.g. fertilisers can cure lack of nutrients, irrigation can cure lack of water, and pesticides can control plagues and diseases. However, if such means are not available or not accessible (e.g. unfavourable cost:benefit ratio, or lack of credit), while the exploitation intensity leads to decreasing levels of the limiting factor, overpopulation is a reality. It is therefore that the socio-economic and policy environments are crucial for desertification control. The external inputs required have often a lower use- efficiency on marginal land, most threatened by desertification, while the cost:benefit ratio is already unfavourable simply by high input and low output prices at farm gate.
*** Paper in particular based on Breman & Kessler, 1995. Where references are missing in the paper, background information and justifications can be found in this book.
It is characteristic of Africa that in most of the continent the resource base for agriculture is so low that overpopulation takes place at low population density in an absolute sense.
As a consequence, the use of external inputs is required while the conditions are still unfavourable: the road density is low, transport and distribution systems are poorly developed, the domestic market is small and does not grow fast, and labour for alternative employment is not very scarce and therefore relatively expensive. This explains, together with unfavourable socio-economic and policy environments, why the so called “green revolution” never generalised in Africa (Breman, 1990). Where only thirty years ago, Africa, India and China used 10 kg/ha or less fertilisers (N+P2O5+K2O), today Africa is using 8, India 120 and China 240 kg/ha (CGIAR, 2000).
To complete the picture, the most limiting factor in most of Africa is not water but nitrogen (N) and/or phosphorus (P). Even in the Sahel, soil poverty is a bottleneck more serious than drought. In reality, most water is lost by lack of plant nutrients when crops, fodders and trees most need it, during the growing season. In the Sahel only 10 – 15% of the rainwater is used by plants for growth; this can become 50% through soil fertility improvement (Penning de Vries & Djitèye, 1982; Bationo et al., 1996). It is therefore that soils indeed require major attention in desertification control. It has therefore been a good choice to link agroforestry with soil conservation in one single network. However, it would have been preferable to speak about “soil improvement” instead of about “soil conservation”. The latter suggests that the soils as such are good enough. But, as has been mentioned above, soils are not poor because farmers overexploit them; farmers overexploit them because they are poor, because the “green revolution” did not take off.
Soils of marginal land threatened by desertification have to be improved (Breman, 1998).
2. Agroforestry and integrated soil fertility management
2.1 Principle Menu of integrated soil fertility management Indeed, agroforestry can play a key role in soil conservation. More interesting, however, is its role in soil improvement. Agroforestry is one of the technical options for integrated soil fertility management (table 1; e.g. Breman, in press; Anon., in press). It concerns technologies, which combine the use of soil amendments [organic matter, phosphate(rock), lime, ….] and inorganic fertilisers. Due to the soil amendments, the soil organic matter status and/or the availability of P and/or the pH improve and the use of fertilisers (and of other production factors like water and labour) becomes more efficient. In other words, the use of fertilisers and other inputs becomes economically (more) feasible. E.g., at least doubling of the agronomic efficiency of fertiliser-N appears possible (Breman, 1998; Breman, in press.).
Agroforestry is only a limited part of the menu of technologies for integrated soil fertility management (ISFM; table 1), which IFDC is developing in cooperation with TSBF (Nairobi) and supported by IFAD (Rome). However, it can be hidden under “crop-livestock integration” and under “leguminous species”, and it can be combined with phosphate rock (PR). The latter is rather logic in view of the dominance of N over P in soils under woody canopies of agroforestry systems. The average N concentration in the topsoil under woody canopies is 1.9 times the N concentration in the open field. For P this so called “enrichment factor” is only 1.3.
The other reason to combine agroforestry with the use of PR, when at least such use is more economic than the use of P-fertiliser, is the fact that perennial species are more efficient users of P from PR than annual crops. In other words, the integrated use of woody species and PR will lead in time not only to soil organic matter (SOM) increase, but also to better SOM, to SOM with a higher P content. This P will in time become available for the annual plants to be stimulated thanks to the integration/stimulation of woody species in the production system.
Table 1. 'Menu' of integrated soil fertility management practices and their recommendation domains
Woody species, perennials in optima forma The network will have to decide if it follows ICRAF in regarding systems with a perennial plant component as agroforestry systems, even if the perennial not really gets a woody stem (e.g. improved fallows). If it does, even more agroforestry systems are hidden under “crop-livestock integration” and “leguminous species” of table 1. As far as their role in ISFM concerns, woody species can be regarded as perennials in optima forma. As already mentioned, otherwise than annuals, perennials concentrate nutrients: widespread and/or deep penetrating roots, atmospheric deposition and interception, and deposition of animals (livestock, birds...) enable perennials to absorb more nutrients than annuals. At the end of each growing season, the nutrients of annuals are lost to the environment, but perennials succeed to safeguard an important part of the nutrients in perennial structures;
woody perennials more than herbaceous perennials. As a consequence, next growing season the perennial produces more biomass than the annual, using its own nutrient reserves and nutrients absorbed from the environment. Even those absorbed from the environment may be more than in case of annuals, because of the perennial root system, absorbing nutrients again as soon as the first rain humifies the topsoil.
Another reason is the growing amount of organic matter, both as plant material as such and as mulch and SOM, due to annual recirculation of a fraction of the annual production (leaves, fruits...). It is the increasing amount of mulch and SOM that leads to increasing availability of nutrients and water, and to increased efficiency of use of both nutrients and water. In other words, again more nutrients are absorbed by perennials than annuals.
Concentrating on the limiting nutrient, Breman & Kessler (1995) show that in the West African Sahel, with 100 – 600 mm of annual rainfall and a canopy cover of 2 to 20%, the real increase of available N thanks to agroforestry is negligible. The woody species
succeed, however, in keeping somewhat more N in circulation that the annual herb layer:
the losses by wind and water erosion and leaching are less due to more mulch and SOM, increasing both the absorption and the storage capacity for water and nutrients. For the Sudanian savannah, with 600 – 1200 mm of annual rainfall and a canopy cover of 15 to 30%, an increase of N in comparison with herbaceous annuals is a reality, but the difference will be at maximum in the order of 5 kg/ha/yr. The extra amount of N kept in circulation will be at maximum some 10s of kg/ha/yr. More nutrients can be kept in circulation due to reduction of losses and redistribution/concentration to the benefit of the woody plant. Real enrichment is due to uptake by deep roots, N-fixation and rhizosphere interaction, for P in particular.
Woody species and water As mentioned above, even in a semi-arid region like the Sahel, nutrients are more limiting than water (Penning de Vries & Djitèye, 1982). The same is true for semi-arid parts of the Australia, Israel, South Africa, the USA, etc. (Breman & de Wit, 1983). This, as well as the request of the organisers to treat the soil fertility aspects of desertification control, is the reason that this paper concentrates on the surplus value of woody species (and perennial species in general) in relation to the availability of nutrients. However, this does not mean that the positive influence of woody species on water availability does not count. On the one hand, the improved availability of nutrients is closely linked to the improved availability of water: less run-off, increased storage capacity of the soil for both nutrients and water, and absorption of water with nutrients from deeper soil layers. On the other hand, in real arid regions, like the extreme Northern Sahel ( 300 mm of annual rainfall) and other direct desert margins, water is more limiting than are nutrients.
However, like in case of nutrients, the positive influence of woody species on water availability decreases more than linear with decreasing rainfall. In the Sahel, the increase of available water in comparison with annual plant species is at most some tenth of mm per year. In the Sudanian savannah the potential benefit concerns some hundreds of mm annually, the dominant mechanisms being improved soil structure thanks to increase of mulch and SOM, deep roots and stemflow.
2.2 The practice
Biomass, production and productivity The surplus value of woody species related to nutrients, in comparison to annuals, concerns on the one hand the ability to keep more nutrients in circulation, and on the other hand the slight absolute increase of nutrients available for annual production.
Both abilities together lead to much higher biomass than in case of annuals. And as long as the plants concerned are not or slightly exploited, more and more nutrients are stored, together with organic matter, in the biomass of the living plant and in the surface layers of the soil profile. Therefore, through well-chosen species, the soil can much better be protected against erosion by perennials than through annuals. Woody plants lead to more stability of production systems!
All other uses are confronted with a complicated bottleneck: the tree component is exploited, threatening the surplus value as described. The more is taken from the annual production of tree or shrub, the less becomes the redistribution/concentration, the decrease of losses and the real enrichment of nutrients, because internal storage and external recirculation diminish. In case the production system concerns the woody species as such, but the exploitation concerns only the fruits, the surplus value can be maintained for long. However, in dry land areas that are affected by drought and desertification, fruit production of woody plants alone will never be enough to assure food security and/or income of the local population. And when also the leaves are harvested as fodder, overexploitation becomes easily an issue; the more so if also wood is exploited.
Therefore woody plants and perennials more in general disappear and are replaced by annuals under increased population pressure (Breman & de Ridder, 1992).
When woody species will have to play a role in desertification control, their own production will have to be safeguarded as much as possible! The agroforestry system must view the increased stability of production systems mentioned above, and soil improvement. And this increased productivity of the soil has to be tapped, has to be exploited by the accompanying plants: the herbaceous vegetation of rangeland or crops.
The just treated bottleneck of decreasing surplus value by exploitation plays nevertheless a role. On the one hand hand, the exploitation and export of nutrients collected, kept in circulation and/or stored by the woody plants will make the described mechanisms of the latter less effective. On the other hand, the higher availibility of nutrients and water for woody plants creates such an above and below ground biomass, that they become strong competitors for the herbaceous accompanying plants of the system. And it is the shade from the woody plants that makes them winners in the competition for nutrients and water. As a consequence, man has to control development and production of the woody component of an agroforestry system. In other words, the woody component functions on a sub-optimal level!