Asia |
|
|
|
Plants as a nutrient source There is a limit to the capacity of plant residues to supply nutrients, as what is not there in the first place cannot be recycled. The original source for phosphorus, potassium, and the secondary and micronutrients is rock weathering. If the soil parent material is low in these elements then, however closed the soil-plant system may be, it cannot become richer without external inputs. Nitrogen on the other hand originates from atmospheric fixation and can be increased in situ by biological means. Plant litter, either naturally occurring or resulting from a farm practice such as mulching or leaving crop residues on the soil surface, will contribute to the replenishment of soil nutrients. As a nutrient source the humus resulting from litter breakdown has the following favourable characteristics (Young 1987): Different plant residues (i.e. from different parts of the plant as well as from different plants) will decay at different rates and vary in their chemical components. From a biological soil management perspective, there are differences in the "quality" of different plant residues (Swift et al 1979). Litter of high quality (high in nutrients, low in lignin and polyphenols) decays and releases nutrients rapidly; that of low quality (low in nutrients, high lignin and/or polyphenols) decays slowly. Woody residues (stems, branches, twigs and coarse roots) are of low quality, but so are some herbaceous products including straw. The significance of litter quality for agriculture is that it opens up the possibilities of using residues from different plants for varying purposes. High quality residues, because they decay rapidly, could be used to provide a short-term release of nutrients, their application timed to meet peaks in crop requirements. Low quality residues when applied as a mulch will remain as a protective cover for much longer while giving extended release of nutrients, protected against leaching until mineralised. This has been recognised in the context of agroforestry where leaves of different trees and shrubs vary widely in their quality and rates of decomposition. For instance the leaves of Leucaena leucocephala decay within a few weeks, those of Cassia simea, at an intermediate rate, whilst Gmeligna arborea, Acacia mangium and many Eucalyptus species are relatively slow decaying (Young 1989). Knowledge of differences in litter quality offers the scope for using combinations of plant materials to provide for both a rapid release of nutrients and a slower regular release over a longer period. There are four management alternatives for using litter to supply nutrients: placement on the surface, burial in the soil, composting, or use as fodder with the nutrients returned via the manure. Buried litter decomposes faster than surface litter (Wilson et al 1986) but surface placement is desirable for erosion control. Burial, composting or use as fodder and/or livestock bedding may be more desirable for cereal crop residues, which are high in lignin, than for the generally high litter quality of tree leaves. The decay of dead roots, the below ground equivalent of litter, is also a source of plant nutrients. Little data is available on the nutrient content of root residues or on rates of addition of residues to the soil, but given the amount of root biomass in proportion to the total plant the contribution from this source can not be ignored (Ingram 1990). Biological nitrogen fixation takes place in the soil through non-symbiotic and symbiotic means. Non-symbiotic fixation is that carried out by free-living soil organisms. It can be of substantial importance relative to the modest requirements of natural ecosystems, but is small in relation to the greater demands of agricultural systems. Symbiotic fixation occurs through the association of plant roots with nitrogen-fixing bacteria. Many legumes are associated with Rhizobium, while a few non-leguminous species are associated with Frankia (Young 1989). Nitrogen fixation by herbaceous legumes has long been a recognised agricultural practice (either as a productive crop, e.g. pulses, groundnuts), a green manure crop (e.g. Stylosanthes spp, Centrosema pubescens, including grass-legume leys), or a cover crop in perennial plantations (eg. Pueraria phaseoloides). In improved cropping systems it is usual to recommend that legumes be grown in rotation with non-legume crops. In many traditional cropping systems in Asia and the Pacific cereals and rootcrops may be intercropped with one or more legume crops such as beans, pigeon peas and groundnuts. Such traditional systems, along with intercropping of annual crops with nitrogen-fixing multi-purpose trees and shrubs, have in recent years become the focus of much research attention as a means of introducing organic nitrogen to the soil, for the benefit of the non nitrogen-fixing crops. With regard to crop production in the tropics, the conventional wisdom is that high yields, particularly of cereal crops, depend on inputs of commercial nitrogen fertilisers, low soil nitrogen levels being a major factor in low yields under low input systems. On the basis of work undertaken by ICRAF and others it appears possible to identify trees and shrubs with a nitrogen-fixing capability (when grown in agroforestry systems) of 50-100 kg N/ha/yr (Young 1989). This opens up the possibilities for developing low external input farming systems using on-farm biological means to raise soil nitrogen levels thereby increasing crop production. In addition to making available additional supplies of nutrients, there is scope for improving the efficiency of nutrient cycling in agricultural systems by improving the uptake capacity of plants. Under low nutrient conditions in nature, most plants are infected by one or other type of root-inhabiting mycorrhiza. The very fine and extensive mycelial network put out by these fungi improves the efficiency of nutrient and water uptake by greatly increasing the plants effective absorbing surface within the soil (Swift and Sanchez 1984). Like nitrogen, low phosphorus levels limit the agricultural productivity of tropical soils. It appears that mycorrhiza not only improves the efficiency with which plants take up available phosphorus, but with other organisms they can make rock phosphate soluble and transfer it to the host plant (Kugler 1986). Research into mycorrhiza suggests that most of the important crop plants in both commercial and subsistence agriculture in the tropics have the capacity to form associations with appropriate mycorrhizal fungi. In low nutrient status soils crops may derive very considerable benefits from such associations (Swift and Sanchez 1984). Mycorrhizal infection is favoured by minimum tillage and low inputs of fertiliser and pesticides and so lends itself most readily to agriculture under low-input constraints. As a plant's ability to fix nitrogen may be improved by inoculation with the appropriate Rhizobium, in the future it may become possible to improve the plants uptake of phosphorus and other soil nutrients by inoculation with the appropriate mycorrhiza. The established feeder root system of trees and shrubs is believed to exploit a greater volume and depth of soil for soil nutrients than those of annual or pasture crops (Swift and Sanchez 1984, Ingram 1990). In particular tree roots are believed to be able to capture nutrients freshly released by weathering in the deeper soil layers (below 2m) and to transfer them to the above ground parts of the plant. Such nutrients can then be made available to annual crops by utilising the natural tree litter and prunings (leaves and fine stems) as a mulch. Agroforestry combinations appear to have considerable potential for enhancing the sustainability of agricultural soils in terms of nutrient status. The identification and use of multi-purpose trees and shrubs that can carry both nitrogen-fixing bacteria and mycorrhizal fungi infections therefore offer opportunities for the development of sustainable low external input farming systems. |