Introduction to Soil Chemistry

48 soil basics iii
Animals also deposit organic matter on soil even when they do not live in
it. Although these deposits are widespread in most cases, they can be concentrated
in watering and feeding areas. In all cases soil near and under an organic
matter deposition will be affected biologically, chemically, and physically.
Before decomposition begins, rain can move both inorganic and organic constituents
into and sometimes through the soil profile. During the decomposition
process additional organic and inorganic compounds and ions will be
produced and leached into the soil.
In addition the physical effects described above, animals can change the
soil’s biological and chemical reactions. For instance, animal paths become
devoid of plants and compacted, thereby decreasing water infiltration and percolation
and oxidation–reduction reactions, particularly when there is continual
use of the paths [1–4].
3.2. PLANTS
Plants have two parts: the tops and the roots. Both are different and have different
effects on soil chemistry and analysis. Because the effects are so different,
each part will be discussed separately. The first thing to note about plants
is that they all can be divided into algae, fungi, mosses, and liverworts and vascular
plants, while the dominant agriculture plants are commonly divided into
grasses and legumes. In addition, these types of plants can be annual, biannual,
and perennial in their lifecycles. Annual plants are particularly interesting in
that both the tops and bottoms die each year and thus add organic matter to
soil from both sources.
3.2.1. Tops
Shrubs and trees have woody stems or trunks and moderate to tall growth
habits and, along with tall growing woody grasses, such as coconut, are
longlived and typically only add leaves to the soil each year. Note that evergreens
and needle bearing trees keep their needles all year long; however,
needles are continuously lost throughout the year, as are leaves from tropical
plants. Organic matter from woody annuals and biennials is added in a similar
fashion. Thus, organic matter from roots, stems, and branches is only occasionally
added to soil after relatively long periods of time. Addition of organic
matter from these types of plants seldom leads to the development of thick O
or A horizons.
Although it often seems that leaves, particularly those of deciduous trees,
do not decompose, it is observed that the layer of leaves on the ground never
becomes thick. In the tropics, where trees grow all year long, the same thing
happens; leaves fall continuously during the year and decompose. Thus, the
leaves of all plants decompose over a year’s period of time, adding organic
matter to the soil.