|Resource Management for Upland Areas in Southeast Asia - An Information Kit (IIRR, 1995, 207 p.)|
|2. Integrated upland systems management|
Agroforestry is an approach to sustainable land-use which is very relevant for the management of upland areas with severe physical and chemical soil constraints.
Agroforestry systems can improve and maintain soil fertility by:
· Increasing nutrient input. The incorporation of trees and shrubs into the farming system can increase the organic matter of the soil and the fixation of nitrogen.
· Recycling nutrients from deep soil layers. Soil nutrients are taken up by tree/shrub roots from deeper soil layers and recycled to the topsoil through litter or lopping (including root residues), forming an almost closed cycle of nutrients.
· Synchronizing nutrient inputs with plant growth. Trees or shrubs can help synchronize nutrient release with crop requirements by controlling the quality, timing and manner of addition of plant residues like prunings and cuttings.
· Preventing erosion. In sloping areas, trees or shrubs act as a physical barrier to reduce nutrient loss from erosion.
The soil-nutrient cycle in the agroforestry system consists of nutrient pools, nutrient flows within the system and nutrient gains and losses.
The most common problems of upland soils
Low nutrient content, particularly deficiencies of nitrogen and
Nutrient losses through soil erosion, leaching and volatilization.
Potential nutrient pools
Key technical considerations for design and maintenance of agroforestry systems
1 Use nitrogen-fixing trees or shrubs to increase gains from symbiotic fixation. Examples of nitrogen-fixing trees are Gliricidia septum, Desmanthus virgatus, Flemingia macrophylla, Leucaena leucocephala, Cajanus cajan, Paraserianthes falcataria, Erythrina spp., Albizzia spp. and Calliandra calothyrsus
2 Select fast-growing and deep-rooted trees or shrubs that can be pruned or lopped more frequently to provide organic matter. Deep-rooted trees can enhance nutrient uptake and act as "nutrient pumps.,, They can store nutrients from below the surface in above-ground biomass. Examples: Acacia spp., Cassia siamea, Gliricidia septum, Desmanthus virgatus, Albizzia lebbek.
3 Plant trees or shrubs (and crops also) along the contour as a barrier to control soil erosion. The most common design is contour planting or hedgerows. Place crop residues, twigs, barks and other materials on the upward side of the hedgerows or spread along the contours to serve as mulch to control surface run -off further.
4 In hedgerow intercropping, the hedgerows can be pruned to a height of 75 cm to 100 cm (I m). Incorporate all cuttings and other available organic materials (e.g., twigs, barks, leaves, fruit residues, etc.) into the soil.
5 Synchronize the timing of tree pruning or lopping with the required nutrients needed by a crop. The potential contribution of plant residues is important, as the nutrients can be supplied to the crops when the nutrients are most needed. The decomposition rate for all organic material is directly proportional to the availability of soil moisture. For example, lopping should be done before the intercrop is planted to allow the residues to be converted from the organic materials to available nutrients.
6 Practice crop rotation for the intercrops. Plant leguminous nitrogen-fixing crops after grain crops to replenish losses from grain harvest. Some legume crops improve nitrogen gains through symbiotic fixation. Example of a crop sequence: mungbeancorn/rice-cowpea.
Practice crop rotation for the intercrops
7 Develop and maintain the upper slopes of farmlands as forested plots. These areas then protect the watershed and serve as a source of fuelwood and other wood products. The overall area planted to trees or shrubs should be about 60 percent.
8 Plant cover crops or green manure crops in fallow areas. Cover crops improve soil fertility through the addition of significant amounts of nutrients such as nitrogen (as much as 200 kg/ha). They also help suppress weeds that might use stored nutrients in the soil. Some recommended cover and green manure crops are: Desmanthus virgatus, Clitoria ternatea (butterfly pea), Centrocema pubescens (centrocema), Phaseolus antropurpureus, Vigna radiata (mungbean), Vigna sinensis (cowpea), Sesbania rostrata (sesbania).