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Analytical Methods

Chemical Analyses: Solids: Organic Matter

All organic substances, by definition, contain carbon. Soil organic matter is a complex and varied mixture of organic substances. Commonly, soil organic matter is defined as the percent humus in the soil. Humus is the unidentifiable residue of plants and animals that becomes fairly resistant to further decay. Reclamationists, however, include other materials that are mainly comprised of carbon as organic matter. Soil organic matter, from this point of view, can be divided into two fractions: (1) the recognizable organic material (straw, woodchips, roots, mulches, insects, etc.) with their partially decomposed remains, and (2) humus. Recognizable organic material eventually decays to the partially decomposed state and eventually to humus. The process of decomposition is carried out by heterotrophic soil organisms that obtain the carbon they need to build their cell constituents from the breakdown of organic materials. Nearly all heterotrophs also obtain their energy to live from the oxidation of the carbon in organic compounds.

Organic matter is very important to the functioning soil system for various reasons. It increases soil porosity, thereby, increasing infiltration and water holding capacity of the soil, providing more water availability for plants and less runoff that may potentially become contaminated. This may be specifically helpful at mine sites where runoff may become acidic and contain high concentrations of heavy metals. The increased porosity also aids in easing tillage of the soil.

The organic fraction of the soil accounts for 50 to 90% of the cation exchange capacity (CEC) of mineral surface soils. The CEC allows important macronutrient cations (K, Ca, Mg) to be held in exchangeable forms, where they can be easily used by plants. Through the CEC, organic matter also provides much of the pH buffering capacity in soils. For more information on pH, click here.

Nitrogen, phosphorous, sulfur, and micronutrients are stored as constituents of soil organic matter, from which they are slowly released by mineralization, aiding in plant growth. In addition, humic acids (a form of organic matter) accelerate soil mineral decomposition releasing essential macro- and micronutrients as exchangeable cations. For more information on macro- and micronutrients and their role in plant growth, click here (link to the macro- and micronutrient section). In addition, organic matter adds erosion resistance to soils. For information on the reclamation activities conducted to minimize hillslope erosion, click here. For information on the reclamation activities conducted to minimize stream bank erosion, click here.

The establishment of cover soil on disturbed surfaces is a common reclamation strategy. Cover soils facilitate the establishment and growth of vegetation. Many times, finding enough cover soil to cover disturbed surfaces can be difficult and costly. When there is not enough soil on-site to satisfy the demand, surface soils may be hauled in from other designated sites. If surface soils are excavated to recover minerals beneath, the surface soils may be stored until reclamation of the area takes place. In these cases, surface soils may be stored for long periods, during which time, the soils may show reduced biological activity, in part due to a loss of mycorrhizae, bacteria, and invertebrates. Stored surface soils also reveal a loss of organic matter and nutrients. Therefore, organic amendments and fertilization of surface soils that have been in storage for several years are necessary to ensure rapid buildup of microbial populations and initiate nutrient cycling.

There are several different types of organic amendments, added for different reasons. Mulches are organic materials applied to the surface (not tilled into the soil) of a disturbance after seeding primarily to reduce erosion. The more common mulches include paper, wood residues, straw, and native hay. Surface mulches reduce wind velocities at the soil surface, shield the soil from raindrop impact, reduce evaporation from the soil surface, trap small soil particles on the site, reduce surface soil temperatures, and help prevent soil crusting. For more information on mulching, click here. Manure, compost, and sewage sludge are other organic amendments generally incorporated into the soil by plowing, chiseling, crimping, or rototilling. These organic amendments benefit the cover soil for the many reasons discussed above, such as increased microbial activity, cation exchange capacity (CEC), porosity and water-holding capacity. All of these characteristics enhance the soil health and provide a medium satisfactory for vegetative growth. For more information on soil organic amendments, click here.

Determination of organic matter content is a routine procedure carried out in soil analysis and testing laboratories throughout the U. S. because of the importance that organic matter plays in aiding plant growth and resisting the effects of erosion. However, there is no satisfactory method for determination of the organic matter content of soil. It can be determined indirectly by measuring the organic C content of the soil and multiplying the result by the ratio of organic matter to organic C normally found in soil. Different researches have suggested different ratios that range from 1.7 to 2.5, with a ratio of 2 appearing to be most universally accepted. The estimation of organic matter content from organic C concentrations is not highly accurate, because the ratio of organic matter to organic C is variable from soil to soil and with depth in the profile. Direct determination of organic matter usually involves destruction of the organic fraction by oxidation or ignition of the soil at high temperature. The loss in weight of the soil is taken as a measure of the organic content. The oxidation method has serious limitations because oxidation is incomplete, and the extent of oxidation varies from one soil to another. For specific methods, see Page et al. (1992).

For more information on organic amendments and organic matter, see Brady and Weil (1999) and Munshower (1994).

 

Chemical Analysis | Physical Properties

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