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Agroforestry Systems Have the Potential to Enhance Soil Fertility

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Agroforestry systems have the potential to enhance soil fertility due to the diverse interactions between the different components involved such as the trees, the crops and even the livestock .Soil is one of the most important natural resources to suffer as a result of clearance of the vegetation cover. If it is not protected, its productivity declines and it may become difficult to sustain the human and animal population even at its present level. Therefore, protection of this resource is important and an understanding of how this resource is influenced in an Agroforestry system is necessary. For instance, it is generally assumed that trees have deep and spreading roots and hence are capable of exploiting more soil volume and taking up nutrients and water from deeper layer not usually contacted by herbaceous crops. This process of taking up nutrients from deeper soil profiles and eventually depositing at least some portion of them on the surface layers through litter-fall and other mechanisms is referred to as 'nutrient pumping' by trees. It is well known that the development of plants depends on site characters and environmental factors. Many woody species have the largest number of roots and the majority of the fine roots are located in the uppermost fertile portion of the soil profile. Some tree species are shallow rooted. Prosopis chilensis has a shallow and spreading root system whereas P. juliflora, is known to have a very deep root system. Therefore, the trees in an Agroforestry system can help conserve the soil on the farm by reducing soil erosion, increasing soil organic matter, improving soil structure, and assisting in nutrient cycling. Soil salinity and water logging can also be ameliorated by trees which by extension result in fertile soils.

In a broad sense, Agroforestry systems can help improve soil fertility in the following ways:

a) Organic matter and nutrient addition to the soil – This can be contributed by the trees, the crops and even the animals. Tree species contain large quantities of 'living' biomass .About 20 to 25 per cent of the total living biomass of the trees is in roots and there is a constant addition of organic matter to the soil through dead and decaying roots. The major addition of organic matter and other nutrients to the soil from the trees standing on it is through litter fall i.e. dead and falling leaves, twigs, branches, fruits etc. on the other hand, the crops upon death and decomposition, contribute substantial amounts of organic matter into the soil. The animal manure too adds commendable nutrients into the soil thereby enhancing the fertility of the soil. Erythrina abyssica has also been noted to be high in nitrogen when the leaves fall and decompose into the soil.

b) Nitrogen fixation by trees - There is a possibility for improvement of the fertility status of agricultural lands' through additional amounts of nitrogen added to the soil by the tree legume component. Mimosoideae and Fabaceae are well known to fix nitrogen. Therefore, among the various avenues of addition of nitrogen to a soil through natural and biological means, the most significant one brought about by the presence of trees on agricultural lands could be nitrogen fixation by leguminous trees. For example, Leucaena leucocephala grown for forage for 9 months yields about 12,600 kg forage, 3,600 kg protein and 575 kg nitrogen per ha. Another good example here is Gliricidium sepium. Leucaena leucocephala and Gliricidium sepium are also important for supplying organic matter with different characteristics such as greater Carbon-Nitrogen Ration, polyphenol, or lignin content.

c) Nutrient cycling – The Agroforestry system provides a good network of components which make the process of nutrient cycling to flow efficiently with minimal losses into the outside of the system. The nutrient cycling model consists of the soil - plant system that is partitioned into several compartments. The trees’ crown surface forms the boundary of the system where input of bioelements occurs through precipitation. The soil surface is the entry point for inputs into the soil compartment, occurring through fertilizers, rainfall and stem flow. The surface layer may be considered the zone of intensive root activity, with the subsoil constituting the extensive root activity zone. The lower end of the extensive root layer is the boundary 'of the ecosystem to the hydrosphere and lithosphere. Bioelements transported beyond this layer are lost from the ecosystem and appear as output from the system. It has also been noted that Croton macrostachyus helps add Pottasium and Carbon into the soil.

d) Control of surface run-offs – The pasture and the trees in silvopastoral system and agrisilvipastoral systems help reduce the speed of running waters. This helps control and reduces the rate at which soil nutrients are carried away by the moving water. These vegetation cover also help reduce the rate of leaching of soil nutrients into the lower horizons.

The properties that are likely to make a woody perennial suitable for soil fertility maintenance or improvement are;

i) A high rate of production of leafy biomass. ii) A dense network of fine roots, with a capacity for abundant mycorrizal association. iii) The existence of deep roots iv) A high rate of nitrogen fixation v) A high and balanced nutrient content in the foliage; litter of high quality (high in nitrogen, low in lignin and polyphenols
An Agroforestry system with trees with the above qualities will most possible result in fertile soils which will increase the overall output of the farmer in the long run.

From the above discussion, we can see that sound Agroforestry systems have the potential to enhance soil fertility.


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