Environmental Engineering Reference
In-Depth Information
associated with soils' permeability, and represents the capacity of soil to retain its
structure.
Redox conditions and pH are other factors, reflecting the soil environment and are
considered capacity and intensity controlling parameters. These parameters are strongly
related to the different biological and chemical process associated with biological
transformations of constituents in soils. Increased OC concentrations enhance biological
transformations and the oxidation of organic and chemical constituents and lead to
anaerobic conditions and the decrease of pH. These changes result in the reduction of
selected parameters from their original status of immobile precipitates to soluble and thus
mobile forms, which could be leached with the seepage to ground water. Typical
examples are the cases of phosphorous and most metals. Thus decreased pH values are
signaling a decrease in the soil retention capacity. The other factor, which influences
metal mobility in soils is salinity. Increased salinity concentration enhances the release of
metals in solution and their mobility.
Nutrients are important constituents of the soil structure and from an agricultural point
of view are important for crop growth and yields. However, due to the intense
agricultural practices applied worldwide, a substantial amount of nutrients has been
accumulated in soils. Phosphorous is easily accumulated in soils, with a relatively low
fraction available for plant uptake (Pav). The process of immobilization of phosphorous
depends on the following factors:
• Al and Fe oxides contribute to phosphorus fixation in acid soils;
• Calcium compounds control the solubility of phosphorous in calcerous soils;
• Organic matter contributes to phosphorous adsorption.
Most of the applied phosphorous is contained in particulate form, when soil becomes
saturated, significant leaching might occur. Novotny (2003) reports that in the
Netherlands, the annual load of phosphate on maize-growing areas, defined as the
application rate minus the crop intake has been in the order of 150-450 kg/ha, which is
exceeding by far the loading rates determined in this study.
In contrast to phosphorous, nitrogen does not accumulate in soils and is easily
transported to ground water in the form of nitrate. In the case of sludge application rates,
a considerable portion of the nitrogen might be in the form of organic nitrogen or
ammonia, which are prone to biological degradation and usually are retained in the
topsoil layer. Thus, in these cases, immobilization of nitrogen is due to these two
fractions and the factors, which enhance this process, are connected with ammonia
fixation to clay particles or soil organic matter and biological immobilization.
Results reflecting the Crowborough farm soil characteristics after 30 years of
operation are presented in Tables 9.2 and 9.3. General soil characteristics and nutrients at
the different sampling locations and along the depth profiles are presented in Table 9.2.
The pH values show acidification of the soils along the whole depth profile with
respect to the most often used areas (BH3, BH4 and BH5), but BH1 and BH2, which are
not under irrigation, have neutral pH values similar to the control point (BH6). The effect
of long-term irrigation is well pronounced at BH4 and BH5. It results in very high OC,
TP, TN concentrations at the top layer, compared to the control point and the rest of the
boreholes.
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