Page 14 - Enzymes in Tropical Soils

2 Enzymes In Tropical Soils

For a normal plant growth and development, the need for all nutrient
elements must be satisfied, not only in term of the various kinds of elements but
also in term of the amounts needed. For example, plant macronutrients such as N,
P, and K have been well understood to be needed by plants at high amounts. On
the other hand, plant micronutrients such as Cu, Zn, B, and Mo are needed at
relatively small amounts. In some cases, the amount of nutrient elements is indeed
more important rather than the kind of various nutrient elements. Some soils may
supply enough or abundant nutrient elements while others can not suffice the
amounts needed by plants. Therefore, the growth and development of plants in
these kind of soils are probably disturbed. In this relation, the amount of plant
nutrients in soils becomes a more central issue to understand.
The amounts of plant nutrients in soils available to plants are determined by
two important sources. The first is soil minerals and the second is soil organic
matters. These sources are both important in supplying nutrient elements. In
general, about 90-95% of soil solids of mineral soils consists of soil minerals, while
the rest 5-10% consists of organic matters. Conversely, in organic soils, more than
50% of soil solids consists of organic matters. Both nutrient sources may release
nutrient elements, that may dissolve into soil water, and may then be available to
plants, and a part of it is eventually absorbed by plant roots. The more fertile soils
may release more nutrient elements from soil minerals and/or soil organic matters.
Various chemical and physical processes may influence the release of
nutrient elements through weathering of soil minerals and/or decomposition of
organic matters. The rates of these chemical and physical processes are affected
by various internal or external factors. Among the most important internal factors
are particle size, structure, and physical-chemical composition for minerals, and
physical and chemical composition as well as C/N ratios for organic matters. For
example, minerals with high amounts of oxydizable elements such as Fe and Mn
will weather more easily and will release more nutrient elements. So do organic
matters with high amount of N, that will decompose more easily.
Among the most important external factors are rainfall, soil moisture
content, soil temperature, soil texture and porosity; kinds, population, and
activities of microorganisms and macroorganisms, plant rootings, and soil
enzymatic activities. For example, minerals and organic matters in soils with good
structure will decompose faster. Soils with a good porosity will have a good
aeration, that may enable O 2 to diffuse into the soil system more easily and may
make oxydation of minerals and respiration by soil microorganisms and plant roots
to progress more easily. Together with the more intensive enzyme production, this

Abdul Kadir Salam - 2014