Introduction to Soil Chemistry

plants 49
Grasses and other similar plants, which may be annual, biannual, or perennial
in their growth habit, do not have woody components, but also add leaves
and stems to the soil each year. These leaves and stems decompose over a
1-year period, adding organic matter to the soil surface. Often these leaves
seem to decompose faster than do tree leaves; however, in all cases the rate
of decomposition will depend largely both the characteristics of the plant
material and local environmental conditions.
All components in organic matter affect its decomposition; however, one,
the carbon:nitrogen ratio (C/N), is particularly important. Soil organic matter
has a carbon:nitrogen ratio in the range of 10:1–12:1. When organic matter
with high C/N ratios, 100:1, for example, is added to soil, microorganisms
decomposing it take nitrogen from the soil solution and analysis of this soil
will result in very low values for inorganic nitrogen. As this organic matter is
decomposed, nitrogen will be released. Organic matter with low C/N ratios
will release nitrogen to the soil solution. Thus, organic matter will have a dramatic
effect on the results of soil analysis. Actively decomposing organic
matter will result in changing analytical results over time.
It might be assumed that there will be different organic matter in soil if
there are different plants growing on it. This is true when the fresh organic
matter and its breakdown products are being investigated. It is particularly
evident with Spodosols and Mollisols. Spodosols have a subsurface spodic
horizon, which results from decomposing acid detritus, leading to leaching of
aluminum and highly decomposed organic matter and often but not necessarily,
iron oxides, to form this horizon. In Mollisols the deposition of both
grass tops and roots each year leads to the development of a thick dark surface
mollic horizon.
Despite these dramatic effects on soil, the organic matter remaining after
the breakdown of plant residues, namely, humus, is generally the same the
world over. The interaction of humus with chemicals, adsorption, cation
exchange, and so on, including those used in analytical procedures, is generally
similar. Thus, often the type of organic matter being added to soil, except as
noted above, is not as important as the amount of decomposed organic matter
already present. However, the components present in humus, specifically,
humic and fulvic acids in humus, vary considerable and thus can change some
of its characteristics. Soil humus will be discussed in more detail below [5].
3.2.2. Roots
It is reasonable to assume that because roots and tops are part of the same
plant, their effects on the soil would be the same. However, this is not the case.
Plant roots, because they are in intimate contact with and are constantly
extracting nutrients and water from and exuding materials into the soil, profoundly
affect its characteristics. This intimate relationship, which includes
physical, microbiological, biochemical, bioorganic, and chemical interaction
between roots and soil, is illustrated in Figure 3.3.