ABSTRACT
The organic fraction of soils, also called soil organic matter, is derived from the soil biomass, and strictly speaking it consists of both living and dead organic matter. Frequently, the term is used to indicate the dead organic fraction only, and the live fraction is usually ignored. The dead organic matter is formed by chemical and biological decomposition of organic residues. It can be distinguished into (1) organic matter in various degrees of decomposition, but in which the morphology of the plant material is still visible, and (2) completely decomposed material. The first category is often called litter when it lies on the soil surface. In forest and grassland soils, litter is particularly important in the process of nutrient cycling. The second group, the decomposed fraction, consists of numerous organic compounds, but only a few are present in detectable amounts in soils. Some of them are nonhumified, whereas others are humified compounds (Stevenson, 1967, 1982; Flaig, 1971; Tan, 1993, 2003a). The nonhumified compounds have been released by decay of plant, animal, and microbial tissue in their original or in a slightly modified form. They include carbohydrates, amino acids, proteins, lipids, nucleic acids, lignin, pigments, hormones, and a variety of organic acids. The humified compounds are products that have been
synthesized from these nonhumified substances by a process called humification. They consist of a group of complex substances such as humic and fulvic acids. The nonhumic and humic material is collectively called soil humus, though some scientists prefer to use the name humus for designating soil organic matter as a whole (Waksman, 1936). Many suggest distinguishing the whole or the total organic matter in soils into several pools. This concept of pools of organic matter is formulated on the basis of differences in susceptibilities to microbial attack, hence these pools exclude the microbial fraction and are composed only of the dead organic fraction. Three major pools are currently recognized: (1) active, (2) slow, and (3) stable or passive pool (Faustian et al., 1992; Stevenson, 1994; Brady and Weil, 1996). The active pool of soil organic matter is the least resistant fraction and is composed of compounds most readily accessible for food to microorganisms. Under sustainable agriculture, it is the most important fraction for maintaining soil productivity. Stevenson (1994) includes a light fraction in the active (labile) pool, which is defined as “plant residues at varying stages of decomposition.” Its properties and composition are said to be comparable to plant litter. The difference is that the light organic fraction is mixed within the soil, whereas litter is a plant residue on the soil surface as stated earlier. The stable pool is the most resistant organic fraction against microbial decomposition and is characterized by the highest mean residence time (MRT) among the different types of soil organic matter. The mean residence time is a term used to express the age of soil organic matter as determined by C dating. Its use is preferred14 to that of the older term half-lives. The MRT of organic matter in U.S. soils varies from 800 to 3000 years (Stevenson, 1994). The passive fraction is usually composed of the humic matter and is responsible for the chemical activity in soils, e.g., cation exchange capacity, interactions, and water-holding capacity. The slow pool of organic matter is intermediate in nature between the active and passive pools. The amount of organic matter varies from soil to soil. Generally, it is believed that the amount of organic carbon stored in the soil biomass is around 50% of the biomass weight. Based on standard analysis of organic C content, highest organic matter contents,
orgranging from 5 to 10% C , are found in andosols and mollisols, whereas the lowest organic matter content (1 to 3%) is normally noted
in oxisols. Both the climate and vegetation are determining factors in the organic matter content of soils, though Stevenson (1994) believes that the vegetation is secondary to the climate and proposes the following decreasing order of importance of soil formation factors in determining organic matter content in soils:
climate > vegetation > topography = parent material > age
3.1.1 Beneficial Effects of Soil Organic Matter
The soil organic fraction as described above affects the physical, chemical, and biological conditions in soils. Physically, it increases organic matter content, imparts darker colors to soils, and decreases bulk density with an increase in organic carbon content. It improves aggregation of soil particles, resulting in the development of stable soil structures. Chemically, it increases the cation exchange capacity, and the water-holding capacity of soils. Its cation exchange capacity, far exceeds that of clay minerals, and is the reason for the soil’s high buffer capacity. Harmful elements and compounds, such as pesticides toxic to plants and humans, are detoxified by interaction with soil organic matter. Organic matter affects soil fertility by increasing the soil’s nutrient content, especially N and S content. It is the main source of N in soils. It can also have a direct effect by stimulating plant and root growth. A variety of growth-promoting substances are said to be secreted by microorganisms or formed as organic residue decomposes. The fact is that growth and yield of crops have been noted to be better when plants are grown in soils rich in organic matter. However, some doubts exist on this matter, allegedly due to lack of scientific data (Stevenson, 1994). Biologically, soil organic matter is the main source of food and energy for soil organisms. The size of fungal and bacterial population increases and decreases in relation to rising and declining organic matter content, respectively. Earthworms especially are strongly influenced by organic matter content. The population of soil organisms will decline with a decrease in organic matter content. In the absence of soil organisms many, if not all, biochemical reactions will come to a standstill.
