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,...

Monetary and opportunity costs of adoption

The generic soil fertility and agroforestry practices that are discussed in the other chapters of this book include improved fallows (tree-crop rotations), tree-crop associations (such as hedgerow intercropping and shaded perennial crops), contour hedgerows and boundary plantings. Each of these options entails various monetary and non-monetary costs for farmers. Therefore where mct sum of monetary costs in year t, and oct sum of opportunity costs in year t. The monetary or out-of-pocket costs...

Susceptibility of different nutrients to leaching

The leaching risk for a nutrient increases with its mobility in the soil. Among nutrient anions, nitrate is particularly easily leached because it shows negligible interaction with the negatively charged matrix of most topsoils and is, therefore, very mobile in the soil (see Section 5.2). Nitrification rates are variable in tropical soils, but can be sufficiently high to make nitrate the dominating form of mineral nitrogen even in acid soils (Robertson, 1989 Schroth et al., 1999a). As a...

Nutrient capture from the subsoil nutrient pumping

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...