Organic Matter and Soil Quality

Soil, water and air quality are strongly interdependent. Soil quality has different meanings according to the perspective of a person. There is as yet not a welldefined universal methodology to characterize soil quality and to define a clear set of indicators (see Parr et al., 1992; Karlen et al., 1997; Shaxson, 1998; Bouma, 2002; Nortcliff, 2002; Schjønning et al., 2002; Gil-Sotres et al., 2005). For a farmer soil quality may mean sustaining or increasing productivity and sustaining the soil resource for future generations, for consumers it may mean healthful and inexpensive food production, and for an environmentalist it may mean a holistic view of the soil in ecosystems with respect to air and water quality, nutrient cycling, biomass production and biodiversity.

Soil organic matter is a key attribute of soil and environmental quality because it is an important sink and source of main plant and microbial nutrients and more over exerts a profound influence on physical, chemical and biological functions despite its often minor contribution to the total mass of mineral soils. It is commonly recognized that SOM imparts desirable physical condition to soil. Organic matter incorporated into the soil can affect its structure, as denoted by porosity, aggregation, and bulk density, as well as causing an impact as expressed in terms of content and transmission of water, air and heat, and of soil strength. Organic matter to a significant part exerts an influence on chemical properties of soils. During organic matter decomposition, nutrients (particularly N, P, and S) are released into the mineral nutrient pool. Reactive carbon and nitrogen compounds can also be released to the atmosphere (CO₂, N₂O, NO_x) and to surface and ground water (NO₂, NO₃⁻, NH₄⁺, dissolved organic C and N).

Ion Exchange capacity and the retention of metals increase following organic matter additions. Other soil chemical properties such as pH, electrical conductivity, and redox potential are determined greatly by the quality and content of SOM. One of the most fundamental functions of SOM is the provision of metabolic energy which drives soil biological processes. Surface active particles like clay minerals concentrate biological molecules at the solid-liquid interface. They can support the growth of microorganisms adsorbed on these surfaces.