|
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 |
