Page 145 - Soil Degradation, Conservation and Remediation

134 5 Chemical Soil Degradation

5.3.2.1 Nutrient Availability
Soluble and exchangeable ions are the available nutrient forms in soil. Soil pH
regulates the solubility of elements and compounds in soil and governs the avail-
ability of plant nutrients in soil. When soil pH decreases from 6.5, that is, as the soil
becomes more acidic, the solubility of some elements including Fe, Al, Mn, Cu, and
Zn increases and of some others, for example, Ca, Mg, and Mo decreases. On the
other hand, when soil pH rises from 7.0, solubility of Ca, Mg, and Mo increases and
that of Fe, Al, Mn, Cu, and Zn decreases. Aluminum, although highly predominant
in soil, is not an essential element for plants. In strongly acidic soils (pH below 5),
solubility of Al along with Fe and Mn increases to such an extent that they become
toxic to many plants. Moreover, plants may suffer from Ca and Mg deficiency in
acidic soils. At low pH (<5.5), phosphorus is precipitated with Al, Fe, and Mn as
their polyphosphates. At high pH (>8.0), P is precipitated with Ca. At both soil acid-
ity and alkalinity, P availability is reduced to deficiency levels. Availability of P is
usually higher in the pH range of 6.5 and 7.0. Boron deficiency may also occur in
strongly acidic and strongly alkaline soils. Availability of macronutrients (Ca, Mg,
K, P, N, S) and Mo and B is restricted at low pH. On the other hand, availability
of most micronutrients (Fe, Mn, Zn, Cu, Co) increases at low pH. The most satisfac-
tory plant nutrient levels occur at a pH range of 5.5–6.5.

5.3.2.2 Toxicity of Iron, Aluminum, and Manganese

Nutrient toxicity can occur in acidic soils when the pH is 4.8 or lower (Slattery et al.
1999). The two most important toxicities in acidic soils are those of aluminum (Al)
and manganese (Mn) (Slattery et al. 1999). In strongly acidic soils (pH W < 4.3), aluminum
and manganese become more available in the soil solution and are harmful to
plant roots. Aluminum toxicity is the most common plant symptom on acidic soils
and causes root stunting (Slattery et al. 2000). Reduced root growth impedes nutri-
ent and water uptake and results in decreased production. Some plants are more
tolerant than others to high levels of Al in the soil solution. Important productive
plants such as alfalfa, phalaris, canola, and barley are difficult to establish and grow in
acidic soils. Both low pH and toxic aluminum (Yokota and Ojima 1995) irrevers-
ibly affect the establishment of alfalfa. The growing of deep-rooted perennial pas-
tures (such as alfalfa and phalaris) is seen as an answer to slowing the acidification
process. Aluminum may block the uptake of Ca and may precipitate P in plant body.
It may interfere with P metabolism including ATP and DNA functions. Plant roots
become stunted with little branching; root tip and lateral roots become brown. Plants
suffering from aluminum toxicity produce symptoms in leaves that resemble P
deficiency.
Toxicity by oxidized Fe frequently occurs in soils of pH below 4.0. Manganese
−1
toxicity is likely in soils having 200–5,000 mg kg Mn. Manganese concentrations
in the range of 0.2–12 mM have been reported to produce severe growth limitations
in solution culture studies of species such as cotton (Gossypium hirsutum L.)
(Kennedy and Jones 1991), sweet potato (Ipomoea batatas L.) (Mortley 1993),