Environmental Engineering Reference
In-Depth Information
Table 18.8 Soil acidity and alkalinity
Soil type
pH
Very strongly alkaline
> 9·0
Strongly alkaline
8·5-9·0
Moderately alkaline
7·9-8·4
Mildly alkaline
7·1-7·8
Neutral
7·0
Slightly acid
6·1-6·9
Medium acid
5·6-6·0
Strongly acid
5·1-5·5
Very strongly acid
4·5-5·0
Extremely acid
< 4·5
important soil processes, including leaching, podzolization, calcification, salinization and
humification. It is also much influenced by fertilizer use on agricultural soils, as the
continual use of inorganic fertilizers leads to progressive soil acidification. Second, soil
pH has important indirect effects on plant growth. Figure 18.13 illustrates the relative
availabilities of major and minor nutrients according to pH values. Soils in the range pH
5·5 to 7·0 are more fertile than those higher or lower. The adverse effects of extreme
acidity (low pH) or alkalinity (high pH) on plant growth are twofold, as is illustrated in
Figures 18.13 and 18.14. Extreme soil pHs lead to high solubility of particular metal
elements; such high concentrations can easily give rise to toxicities which kill plants.
Second, the influence of soil acidity and soil alkalinity on plant growth is due to indirect
effects on the availability of plant nutrients.
Values of pH below 5·0 usually indicate a deficiency or unavailability of plant
nutrients such as calcium, magnes
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