Environmental Engineering Reference
In-Depth Information
P movement down the profile on sandy soils at Crowborough Farm. This was highly
expected because according to Nyamapfene (1991), such sandy soils have very low P
sorption capacity. Muchovej and Rechgil (1995) noted that the P sorption capacity of
sandy soils with low clay content, Fe/Al oxides and organic matter, was limited. Despite
being less mobile compared to other nutrients, Withers & Sharpley (1995) quantified that
the P movement in the soil becomes significant once 25 % of the P sorption capacity of
the soil is saturated. According to Reemtsma et al. (2000), the movement of P down the
profile was also enhanced by the anaerobic conditions that favour the reduction of iron
(III) to iron (II) and the release of phosphates and Fe
2+
in solution.
4.5
Metals
Results with respect to metals concentrations in the soil are presented in Table 10.7. With
respect to K, the metal essential to plant growth, and the levels measured were
comparable at all sites. The statistical test for a significant difference showed that there
was no significant difference between the control and the sprinkler-irrigated sites.
However, potassium levels in the furrow-irrigated site were significantly higher than that
of the other sites at all depth profiles. Muchovej & Rechgil (1995) observed that after 40
years of K application, no K accumulated in the top 75 cm of a sandy soil due to
leaching. This explains the low K levels in the sprinkler-irrigated site, which receives the
highest hydraulic load. The relatively high K concentrations at the control site could be
attributed to a number of reasons. Firstly, granitic soils have high concentrations of K and
no K deficiencies have been reported on sandy soils (Nyamangara et al. 2000,
Nyamapfene 1991). Thus the high native K from the weathering of K-rich felspars in
granites masked the effects of wastewater-derived K. Secondly, compared to other basic
cations such as Mg and Ca, K is required by plants in high amounts (Landon 1991), thus
it could also be argued that the K added through wastewater irrigation (30 mg/l) and crop
uptake have reached an equilibrium. The high K in the topsoil (0-30 cm) was partly due
to adsorption of K on the cation exchange sites. For all the sites, there was a decrease in
K concentration at the 30-60 cm depth and a rise in concentration at 60-90 cm depth. This
showed that K was leached down the soil profile as supported by Muchovej & Rechgil
(1995), who observed that leaching in sandy soils could be as high as 90 % of the K
input.
A major characteristic behavior of trace metals is their ability to accumulate in the
topsoil. Contrary to this, there was no distinct evidence of accumulation. In general, the
levels of trace metals in all the sites were comparable. This showed that the native
concentrations in the soil were relatively high. In general, high Cd concentrations were
obtained in all the sites including the control. Henning et al. (2000) even reported higher
concentrations in the control compared to sludge-amended soils. They argued that such
results were due to non-homogeneity of the sandy soils. In the topsoil (0-30 cm) depth
profile, the highest concentration was obtained at the control site and the lowest was
observed at the sprinkler-irrigated site. According to Pennsylvania State University
(1985), Cd uptake by plants can be very high especially in acid soils of high Cd
concentration and has the potential to accumulate in the plant without showing
phytotoxicity effects.
