Synopsis

In most tropical soils, the growth and production of crops is limited by the availability of one or several nutrients, such as nitrogen in sandy savanna soils and phosphorus in most acid soils (Ewel, 1986 von Uexkull, 1986). Especially in humid climates, nutrient deficiency is often associated with soil acidity and aluminium toxicity, which may reduce the root development of plants and further aggravate the problems of insufficient nutrient supply from the soil. In dry climates, nutrient...

Estimating Nitrogen Fixation in Field Settings

It can be argued that 15N-isotope dilution using labelled fertilizers has no role to play in estimation of nitrogen fixation by trees, for the following reasons. A wide variability in rooting patterns can be found even within a single provenance of a tree species, as soil chemical and physical properties exert a very strong influence on root distributions. As mentioned before it is difficult to ensure that 15N is incorporated to depth and unlikely that non-fixing plants can be identified that...

Role of soil organic matter in soil fertility

The organic matter content of a soil influences a wide range of soil properties and processes and, despite the fact that plants do not require organic matter as such for their growth and development, it is considered one of the most important components of soil fertility (Gregorich et al., 1994). This is especially so in agriculture with low external inputs. In soils dominated by low-activity clays such as kaolinite, the cation exchange capacity, and thus the ability of the soil to retain...

Nutrient efficiency

As a consequence of low initial soil fertility and insufficient access to mineral and organic fertilizers to correct nutrient deficiencies and compensate for nutrient exports and losses, smallholder agriculture is widely practised under nutrient-limited conditions in the tropics. To be a viable land-use form under such conditions, agroforestry systems have to make efficient use of the nutrients available in the soil and of nutrient additions in mineral and organic fertilizers so that they are...

Prediction of nutrient release from biomass

As noted before, the dynamics of nutrient release from decomposing biomass under agroforestry conditions and the factors influencing it are the object of intensive studies. The principal objective of this research is to identify parameters that allow quantitative prediction of the time course of nutrient release from biomass as influenced by attributes of the biomass itself (the resource quality) and the physicochemical environment in which it is decomposing. As a rule, the decomposition of...

Sample collection and analysis

Rainwater, throughfall and stemflow are usually very dilute solutions that are highly susceptible to contamination both in the field, from leaves, insects, birds, mineral fertilizer, etc., and in the laboratory. Thus, collectors should be washed very carefully, and frequent collection of the samples is advisable to reduce the risk of sample loss by contamination and to prevent chemical alterations due to microbial action, affecting especially phosphorus and ammonium. Stemflow samples often...

Methods for Soil Sulphur J Lehmann

Arable soils of the humid tropics often have low total sulphur contents because of low contents of parent materials, strong weathering and high leaching losses. In comparison with other macronutrients, sulphur received little attention as a plant nutrient in tropical crop production in the past and few results have been published (Kang et al., 1981 Acquaye and Kang, 1987 Motavalli et al., 1993). Sulphur deficiency in tropical agroecosystems has recently increased, however, due to the more...

Sequential fractionation methods

A central goal of some agroforestry practices is to increase nutrient efficiency by keeping applied nutrients in available form or to make recalcitrant nutrients available (Sanchez, 1995). Application of organic Fig. 5.3. Flow chart of the sequential phosphorus extraction method (after Tiessen and Moir, 1993). Pi, inorganic phosphorus Po, organic phosphorus conc., concentrated. Table 5.1. Schematic comparison of plant availability and characteristics of soil phosphorus pools obtained by...

Effects of land use on soil organic matter

Because of the importance of organic matter for soil fertility, the effects of land-use change, management practices and fallowing on the organic matter content of tropical soils have often been investigated. Numerous studies in the tropics have demonstrated loss of soil organic matter and organic nutrients following the conversion of forest or savanna ecosystems into either annual (Nye and Greenland, 1960 Pieri, 1989 de Ridder and van Keulen, 1990 du Preez and du Toit, 1995) or perennial...

Leaching of nutrients from organic sources

Of particular relevance for agroforestry is the efficient management of nutrients in organic materials, including biomass and manure, for increased crop uptake and reduced leaching losses (see also Chapter 6). Nutrient release from organic sources is generally more difficult to predict than from mineral fertilizers and so developing practices to counteract leaching is particularly important. Nutrients are often released from organic sources at a time when there is little crop uptake and...

Radioisotopes

Adsorption experiments can also be conducted with 32P to determine isotopically exchangeable phosphorus (Fardeau et al., 1996 Frossard and Sinaj, 1997). Whereas green manure did not affect phosphorus extracted by Bray's solutions or resin or phosphorus adsorption on an Oxisol in Brazil, it increased the amount of isotopically exchangeable phosphorus (LeMare et al., 1987). Exchange processes between organic and inorganic soil phosphorus pools have also been studied with the radioisotopes 32P and...

Aluminium measurement in soil

To relate problems of aluminium toxicity and induced nutrient deficiencies in plants to soil properties, aluminium may either be measured in the dry soil or in the soil solution. The former approach is more compatible with standard soil sampling and laboratory practices and is, therefore, more common. Exchangeable acidity is usually measured by leaching a soil sample with 1 M KCl and quantification of the extracted acidity (aluminium and protons) by titration (Hendershot et al., 1993b)....

Soil organisms and their functional domains

Soil organisms have evolved in an environment that imposes three major requirements (i) to move in a compact environment with a loosely connected porosity (ii) to feed on low-quality resources and (iii) to adapt to the occasional drying or flooding of the porous space (Lavelle, 1997). A continuum of adaptive strategies based on size is observed in soils, from microorganisms to macroinvertebrates (Swift et al., 1979). Soil invertebrates have been divided into micro-, meso- and macrofauna...

Methods for Potassium Calcium and Magnesium in Soil

The cations potassium, calcium and magnesium occur in several forms in the soil that differ in their availability to plants. The most readily plant-available fraction is that in the soil solution, followed by the exchangeable fraction, which replenishes the soil solution if nutrients are removed by either plant uptake or leaching. Potassium fixed in clay interlayers becomes available at a time scale from hours to weeks. The least available forms are various primary and secondary soil minerals,...

Hierarchy of Agricultural Systems as a Background to the Understanding of Farmers Constraints

Figure 2.2 represents a spatial hierarchy of agricultural systems within which farmers' decisions are made. The highest level in the hierarchy, supraregional systems, occupies the largest land area and can transcend national boundaries. Macroeconomic processes, as well as certain geological processes, are best understood at this level. The lowest level (soil systems) covers the smallest spatial unit and is the level at which specific biological processes such as nutrient uptake may be...

Trees and soil fertility

Whatever the reasons farmers have for planting or protecting trees in a specific case, they nearly always fulfil several functions simultaneously. Trees may have been planted on a hillslope to produce timber or fruits, but they may also protect the soil from being eroded. Trees planted or retained for fodder are often nitrogen-fixing and may improve nitrogen availability in the soil. Similarly, trees that have been allowed to regenerate in a riparian zone because of environmental regulation,...

Nutrient capture from the subsoil nutrient pumping

Lateral Root Nitrate

Nutrient capture by trees from the subsoil can include nutrients released by weathering of primary minerals and also nutrients leached from the topsoil that are then recycled by the trees. Capture of newly weathered nutrients is restricted to relatively young soils where weatherable minerals still occur within the reach of tree root systems, including colluvial or alluvial soils with irregular nutrient distribution with soil depth. Despite the prominence of nutrient pumping by trees as a...

Synchrony and synlocation of nutrient release with plant uptake

When biomass decomposes on or in the soil, the nutrients may either remain in the soil in mineral form, be incorporated in the soil biomass and soil organic matter immobilization , be taken up by plants, or be lost from the system through leaching or in gaseous form. The relative importance of these different pathways depends on the respective nutrient, the decomposing material, and the biotic and abiotic conditions under which the decomposition process takes place. It has been hypothesized...