Page 144 - Soil Degradation, Conservation and Remediation

5.3 Acidification 133

soil is left significantly more acidic. Removal of produce by burning, for example,
burning of stubble, does not change the acid/alkali balance of the soil, but gives a
redistribution, leaving alkali at the soil surface as ash. If the ash then gets washed
away, as might occur by rains following a fire, this would leave the soil more acidic.

5.3.1.4 Buildup of Soil Organic Matter

While increasing organic matter has many benefits, including improvement of
soil structure, it also increases soil acidity. The acidification caused by a buildup in
organic matter is not permanent and can be reversed if the organic matter breaks
down. However, there will be a permanent change in the acid status of the soil if the
topsoil containing the organic matter is eroded or removed.

5.3.1.5 Acid Rain
Watmough and Dillon (2003) reported that depletion of nutrient-base cations
(Ca, Mg, K, and others) in soils occurs due to acid deposition. Acidification and
depletion of nutrient cations due to acid deposition have been reported for several
forested regions in North America. Continuing regional inputs of nitrogen and
sulfur are of concern because of leaching of base cations, increased availability of
soil Al, and the accumulation and ultimate transmission of acidity from forest soils
to streams (Fenn et al. 2006). Acid deposition is known as acid rain. It occurs when
emissions of acidic materials such as sulfur dioxide (SO 2 ), ammonia (NH 3 ), nitro-
gen oxides (NO x ), and acidic particles from the combustion of fossil fuels and
other industrial processes undergo complex chemical reactions in the atmosphere
and fall to the earth as wet deposition (rain, snow, cloud, fog) or dry deposition (dry
particles, gas). The effect of acid rain on the soil depends greatly on the ability of
soils to neutralize the acid. Calcareous soils are most effective in neutralization
of acidity in acid rains and prevent lowering of soil pH. Anthropogenic soil acidi-
2−
fication due to acid rain is connected to high soil solution concentrations of SO 4
−
and NO 3 (Matzner and Davis 1996).

5.3.2 Effects of Soil Acidity

Acidic soils may have some or all of the following problems: reduction in the
amount of nutrients being recycled by soil microorganisms (e.g., nitrogen supply may
be reduced); phosphorus in the soil may become less available to plants; induced
deficiencies of calcium, magnesium, and molybdenum; aluminum and manganese
toxicity to plants and microorganisms; and uptake of heavy metals by plants.