Farmers have always been practicing agroforestry, the growing trees on their land; some with no doubt have knowledgeable ideas that growing trees play crucial roles in the soil and water conservation. Agroforestry is a general name for land-use systems in which woody perennials (trees, shrubs, etc.) are grown in association with herbaceous plants (crops, pastures) with or without livestock in a spatial arrangement, a rotation or both, and in which there are both ecological and economic interactions between the tree and non-tree components of the system.

The main components of agroforestry systems are trees and shrubs, crops, pastures, and livestock, together with the environmental factors of climate, soils, and landforms. Other components are bees, fish, etc.

Perennial woody tree species are important components of the agroforestry system that play an important role in soil conservation. Trees retain soil, primarily through surface litter cover and understorey vegetation, an increase of organic matter content, nitrogen fixation, improvement of physical properties, and enhanced efficiency of nutrient use.

Role Of Agroforestry In Soil Conservation

Soil conservation refers to the maintenance of soil fertility and control of erosion. Soil fertility maintenance in agroforestry systems is achieved through the addition of organic matter, basically through leaves that fall from trees and covering the top layer of soil to retain moisture and discourage weeds, while erosion control is achieved through the alleviation of topsoil layer losses. Agroforestry systems have been proven to be efficient in the control of soil erosion.

Agroforestry refers to land-use systems in which trees or shrubs are grown in association with crops or pastures and livestock. From inception, it has contained a strong element of soil management. Well-designed and managed agroforestry systems can control leaching and erosion, maintain soil organic matter and physical properties, and promote nutrient cycling. By these means, agroforestry can make a major impact on sustainable land use

Soil conservation was formerly likened with erosion control. This belief is still found in many places; it leads to planning measures and projects in which erosion is thought of in terms of loss of soil material, and its control is treated in isolation from other aspects of agricultural improvement. It is now known that the major adverse effect of erosion is the lowering of soil fertility through the removal of organic matter and nutrients in sediments that have been eroded.

The second trend is the recognition of forms of soil degradation other than erosion; the various kinds of physical, chemical, and biological degradation are sometimes grouped as a decline in soil fertility. It is now recognized that there can be serious soil-degradation problems even in areas where erosion is not a problem and that it is part of the task of soil conservation to address this, which leads to the view that the primary objective of soil conservation is the maintenance of soil fertility. To achieve this, control of erosion is necessary, but by no means sufficient, condition. Equally important is the maintenance of the physical, chemical, and biological properties, including nutrient status, which together leads to soil fertility.

In summary, Soil conservation involves the maintenance of soil fertility which requires:

  • Control of erosion
  • Maintenance of organic matter
  • Maintenance of soil physical properties
  • Maintenance of nutrients
  • Avoidance of toxicities.

 Activities That Enhances Soil Conservation

 The presence of woody perennials in agroforestry systems may affect several biophysical and biochemical processes that determine the health of the soil substrate. The less disputed of the effects of trees on soil include Amelioration of erosion, primarily through surface litter cover and understory vegetation;

Maintenance or increase of organic matter and diversity, through continuous degeneration of roots and decomposition of litter; nitrogen fixation; enhancement of physical properties such as soil structure, porosity, and moisture retention due to the extensive root system and the canopy cover; and enhanced efficiency of nutrient use because the tree-root system can intercept, absorb and recycle nutrients in the soil that would otherwise be lost through leaching.

Important components of agroforestry systems are perennial woody tree species. They reduce nutrient losses from the productive system through efficient nutrient cycling. The addition of nutrients through litter decomposition, dead root biomass, and N2 fixation increases the significance of tree species in the improvement of soil nutrient status.  Agroforestry systems are recommended by researchers and planners for increasing agricultural production and also to improve soil nutrient status.

#1. Addition of litter

The litter contains a considerable amount of nutrients necessary for plant growth and acts as a major component of nutrient cycling in agroforestry systems. Production of litter and decomposition returns organic matter and nutrients from aerial parts of the plants to the soil surface to improve s fertility of the soil. Leaves, twigs, barks, reproductive and amorphous materials of tree species and crop residue are the major components of litter production in agroforestry systems.

#2. Nutrient Addition through Root Biomass

Nutrients released from the fine root biomass are another important pathway of the improvement of soil nutrient health. Trees allocate a large portion of gross primary production belowground for the production and maintenance of roots and mycorrhizae. In such conditions, soil organic carbon is increased and soil properties are improved.

#3. Biological Nitrogen Fixing

Increasing the use of nitrogen fertilizers in intensive agriculture decreases its use efficiency and causes environmental problems. Many of the marginal farmers cannot afford to buy nitrogen fertilizers and therefore yield of the annual crop suffer. The atmospheric biological nitrogen fixation has nutrient cycling potential in agroforestry systems and plays an important role in resolving these problems.

In the case of legume, most of the nitrogen-fixing tree species is from Mimosoideae and Caesalpinioideae, with fewer in Papilionoideae subfamilies. Within these sub-families, 98, 60, and 30% of the tested mimosoids, papilionoids and caesalpiniods, respectively showed potential to fix atmospheric nitrogen. The non-legume families, such as Betulaceae, Casuarinaceae, Chrysobalanaceae, Coriariaceae, Eleagnaceae, Myricaceae, Rhamnaceae, Rosaceae, Ulmaceae, and Zmiaceae also showed potential for N2 fixation. There were more than 650 tree species are known to fix nitrogen.

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