Environmental Engineering Reference
In-Depth Information
which will be released by the weathering of minerals and the microbial decomposition of
humus. Figure 18.15 illustrates the
Figure 18.15 The distribution of cations in relation to soil
colloids, and their relative availability to plants.
Source: After Briggs (1977).
concept of nutrient availability with respect to cation nutrients (e.g. potassium, calcium).
A final important feature of nutrient availability in soils is that of ion antagonism . By
this mechanism one nutrient element exercises an influence on another so that the latter
may not be absorbed by the plant. For example, calcium and iron are antagonistic, as are
calcium and potassium. There may be ample iron or potassium in a soil for normal plant
growth, but if the content of calcium is high the iron and potassium will become
unavailable for plant growth. This condition of iron or potassium deficiency shows itself
by a yellowing of the plant leaves ( chlorosis ). The plant root is unable to absorb much
iron and potassium physiologically in the presence of large numbers of calcium ions.
CONCLUSION
The properties of soils are important for determining soil fertility. The health and welfare
of the human race depend upon the ability of soils to provide a sustainable yield of good-
quality food. The capacity of soils to do so reflects their physical, chemical and biological
properties.
Physical properties depend upon soil texture or particle size, and soil structure or
aggregation. These two properties determine the ability of the soil to retain moisture for
plant growth, to allow the drainage of excess water and to permit rainwater to infiltrate
into the soil. Water in soil is a key element in soil fertility, and the ability of the soil to
retain sufficient water for plants, but not enough to exclude oxygen from larger pores, is
dependent on texture and structure.
The chemical aspects of fertility are greatly influenced by the colloidal properties of
soil. The cation exchange capacity, percentage base saturation and soil pH are all
interrelated. The ability of the colloids to hold cations against leaching losses is an
important aspect of chemical fertility.
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