Integrated Soil Nutrient Management

The aim of integrated soil nutrient management (INM) is to utilize available organic and inorganic nutrient sources judiciously and efficiently. This involves all aspects of nutrient cycling to achieve tight nutrient balances that synchronize with crop demand for nutrients and nutrient release in soil by the decomposing organic resource while minimizing losses through leaching, runoff, volatilization, and immobilization. Loss of nutrients by immobilization is, however, a temporary loss, which can serve as reservoir for holding nutrients in the soil that would otherwise be lost via leaching or in runoff (Brady and Weil, 1996). Nutrients that are lost to the soil through harvested biomass and other processes must be replaced with nutrients from external sources, or from the currently unavailable reserves in the soil since, for example, there are 10 to 15 times more P in the soil than is usually measured as available P. Declines in soil productivity can be attributed, in part, not only to the quantities of nutrients removed by plants compared to the quantities of nutrients being put back into the system, but also to the unavailability of nutrients in soils due to soil environments such as soil pH, leaching, mineralization, and other processes. Extensive agricultural production with tillage encourages continuous mining of SOM and release of N. Soil organic matter has a C:N ratio of 11:1; thus, sequestration of SOM requires high opportunity cost of N. Therefore, symbiotic N2 fixation through the use of legumes seems the most likely cost-effective means of achieving this N input (Olness et al., 2002). Further, addition of plant nutrients through chemical fertilizers and organic amendments may be necessary to set in motion the restoration of these highly depleted soils. The SOC content cannot be improved without availability of additional quantities of N, P, and K in soils of low inherent fertility (Lal, 2000a; Mokwunye et al., 1996; Renard et al., 1997). An important aspect of INM is the maintenance of SOM, which improves soil structure, increases the soil's water-holding capacity, and improves soil biota populations. Integrated nutrient management practices improve soil biological and physiochemical properties, the buffering capacity and the cation retention capacity of the soil; it is a sink as well as a source of major and minor nutrients. The adoption of INM strategy on low acid clay soils of the tropics is ecologically necessary, economically desirable, and a realizable goal (Franzluebbers et al., 1998).

Traditional farming systems have generally resulted in substantial losses of soil fertility through the loss of SOC, due mainly to organic matter degradation by soil biota (Metting, 1993), and also due to the loss of micro and macro nutrients. With improved land use management and INM, soil biota in Zambian soils can be a sink for atmospheric C (Lal, 2000b). Traditional practices, akin to mining of soil C and N reserves, are major causes for the loss of SOC and SOM from agricultural ecosystems. Thus, balanced nutrient management through sustainable land husbandry practices can play a regulatory role of sequestering C (Franzluebbers et al., 1998).

Soil biota plays an extensive role in the decomposition of organic matter and the production of humus, cycling of nutrient and energy and elemental fixation, soil metabolism, and the production of compounds that cause soil aggregates to form (Metting, 1993; Mendes et al., 1999). Many of the soil biota such as rhizobia and bradyrhizobia are in symbiotic relationships with plants (legumes) (Kuykendall et al., 2000), and also have a beneficial role in nutrient cycling and atmospheric CO2 enrichment (Baldock and Nelson, 2000; Rilling et al., 1999). This affects nitrogen dynamics, and thus, stimulates plant growth (Biswas et al., 2000; Gorissen and Contrufo, 1999; Keeling et al., 1995).

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