Geoscience Reference
In-Depth Information
Soil type
pH
Fertility effects
Very strongly alkaline
> 9·0
Strongly alkaline
8·5-9·0
B toxicity
Moderately alkaline
7·9-8·4
P insoluble
Slightly alkaline
7·1-7·8
Cu, Zn, Co, Fe deficient
Very slightly acid
6.6-7·0
Slightly acid
6·1-6·5
optimum range for availability and plant uptake
Moderately acid
5·6-6·0
bacterial N fixation stops below 5.6; use of lime recommended
Strongly acid
5·1-5·5
Ca, Mg, K, Mo deficient at low pH
Very strongly acid
4·5-5·0
P fixed by Fe, Al, Mn at very low pH
Extremely acid
< 4·5
nutrients according to prevailing pH values. Soils in the
range pH 5·5 to 7·0 are more fertile than those higher or
lower. Adverse effects of extreme acidity (low pH) or
alkalinity (high pH) on plant growth are twofold, as is
illustrated in
Figures 19.10
and
19.11
. Soil pH values at
the extreme ends of the scale increase the solubility of
some metal elements, including heavy metals. High
concentrations of heavy metals give rise to toxicities which
are lethal to 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 like calcium, magnesium,
phosphorus, molybdenum and boron. Such soils may also
contain toxic amounts of ferrous iron, zinc, manganese
and nickel, owing to their increased solubility. Soil micro-
organisms are also most active and beneficial at pH values
in the range 6·0 to 8·0. This explains the importance in
European farming of liming as a means of raising pH to
about 6·0 to 6·5, and so enhancing the availability of
nutrients to plants. Soil alkalinity values in the pH range
8·0 to 8·5 usually indicate the presence of free calcium
carbonate (CaCO
3
) or lime in the soil. This seriously
reduces the availability of phosphorus (P), manganese
(Mn), zinc (Zn) and copper (Cu). Values higher than pH
8·5 indicate the presence of sodium carbonate (Na
2
CO
3
)
and/or high exchangeable sodium which are toxic to most
plants.
BACTERIA AND ACTINOMYCETES
IRON, MANGANESE,
COPPER, COBALT, ZINC
CALCIUM AND MAGNESIUM
POTASSIUM
HYDROGEN
HYDROXYL
PHOSPHORUS
IRON
ALUMINIUM
NITROGEN
TITANIUM, NICKEL, CHROMIUM
3
4
5
6
7
8
9
2
3
4
5
6
7
8
9
pH
pH
and microbial activities.
(toxicities), and H
+
and OH
-
activities.
