Page 147 - Soil Degradation, Conservation and Remediation

136 5 Chemical Soil Degradation

fixation process (Slattery et al. 2001). Many Victorian soils are found to be in a pH
range where clover nodulation is reduced. It has a significant impact on the produc-
tivity of pasture and subsequent crop systems that rely on the biological input of
nitrogen. At very low soil pH and under conditions of high aluminum solubility,
aluminum can have a metagenetic effect on Rhizobia. These findings suggest that
the microbial biodiversity might alter over time with changing soil pH. In highly
weathered acidic soil, bacterial growth is inhibited by nutrient toxicities and low
nutrient availability. Acidity changes the soil microbial community and decreases
root and rhizosphere effects. It decreases organic matter decomposition and nutrient
cycling (Siqueira and Morera 1997). Acidity decreases the activity of nitrifying
bacteria which are responsible for the breakdown of organic matter into ammonium
and nitrate for subsequent plant uptake. Fungi generally have a pH optimum below
that of bacteria.

5.3.2.5 Soil Fauna

Soil acidity influences the delicate balance between groups of living organisms in
the soil. Generally, the soil fauna has a significantly reduced capacity to cope with
large changes in soil pH. Most macrofauna including deep-burrowing species
such as worms and termites tend to decrease in abundance in acidic soil conditions
with most activity being confined to the litter layer where the pH is significantly
higher and usually alkaline. Mele and Carter (1999) have shown that a reduced
abundance of earthworms is associated with acidic soil conditions. Some worm
species are able to tolerate acidic soil conditions and play an active role in litter
decomposition, but these worms are also highly affected by the moisture condi-
tions surrounding them. Deep-burrowing and soil-eating worm species are unable to
tolerate low soil pH conditions (Lavelle and Spain 1995).

5.3.3 Management of Acidic Soils

Despite major difficulties for agricultural use, acidic soils can be very productive if
lime and nutrients are applied at proper time and quantity. For management of acidic
soils, the pH need to be adjusted to a desired level by liming, following adequate
irrigation and drainage and selecting suitable crops. Liming is an efficient way of
increasing soil pH and reducing toxicity of iron, aluminum, manganese, and molyb-
denum. Liming also stimulates biological activity in soils and increases cycling
of nitrogen, phosphorus, and sulfur. It increases P availability to plants. It improves
soil structure, porosity, aeration, and water movement in soils. However, over liming
reduces the availability of phosphorus and causes deficiencies of micronutrients
such as manganese, zinc, copper, and molybdenum.