Environmental risk of chemical agriculture - Environmental Deterioration and Contamination: Problems and their Management - page 97

the aeration of the topsoil, while the anaerobic proportion decreases drastically as
related to the soil parts well supplied with oxygen and this process, as a consequence
of the activity of the aerobic decomposing microflora, tends to increase the rate of
soil organic matter decomposition. In the course of organic matter decomposition,
large amounts of CO 2 —a greenhouse gas—are formed. The acidifying effect of over-
fertilization intensifies leaching and—by providing large amounts of plant available
nitrogen and phosphorus for microbial populations under such conditions in which
easily decomposable carbon sources (plant residues, organic manure) are unavailable—
also supports humus decomposition. The above mentioned processes (acidification,
leaching of nutrient elements, loss of organic matter) result in a joint negative impact
on the physical (structure, aggregate stability) as well as hydrophysical soil properties
(moisture regime) and may weaken the resistance of soil to erosion. The unilateral
loading of macroelements results in a relative microelement deficiency (it has been
well known since Liebig (2013) that available mineral nutrients should be present in
adequate quantity and ratio for the harmonic growth of plants; crop yield is determined
by the nutrient element that is in minimum as related to other elements or is present in
a lower amount than required by the plant). Because of soil acidification, the ratio of
microfungi will increase over bacteria within the microbe population, and as is known,
numerous potentially phytopathogenic and phytotoxin producing species will occur
among these.
Pesticides and their residues in soil, as chemical stressors, exert additional effects
on physico-chemical and biochemical soil properties. Upon their administration, agro-
chemicals remain in the environment for shorter or longer periods. The longer their
half-life (DT 50 ) in soil, the larger the risk due to their adverse effects through food chain
interactions on non-target populations. This is why persistent substances—such as
persistent organic pollutants (e.g., DDT)—are undesirable components in agricultural
practices. Yet short-lived toxic substances applied on soil, the so-called soil disin-
fectants, are also of particular concern, as these compounds are mostly nonspecific
neurotoxicants. Soil disinfection is particularly doubtful from the ecotoxicological
point of view as these agents may eradicate entire soil-borne ecosystems in the treated
areas.
The above mentioned processes lead to deterioration in species diversity of microbe
populations responsible for the multifunctionality of soil, thereby decreasing the
adaptability of the microbe population. This is a key issue not only in maintaining
the ecological equilibrium, but also in respect of the repeated regeneration of soil
fertility.
3.1 Environmental impacts of fertilizers
Over-fertilization is mainly the attribute of industrialized agricultural production, with
its environmental health risks (Table 4.1). Environmental contamination due to fertil-
ization may occur not only on the fertilized agricultural field, but also on the fertilizer
production or storing site, and improper spreading and application may also be a
considerable source of pollution.
The environmental health risk of extreme potassium dosage is not considerable.
The element is present in soil and in a relatively high concentration in humans/animals.
Its acidifying effect is minimal, and the redundant potassium is discharged with urine.
Yet health effects may occur due to extensive potassium exposure. As a negative