Soil acidity and liming
Liming an acid soil is the first step in creating favorable soil conditions for productive plant growth. Crops vary in their ability to tolerate an acidic (low pH) soil. In addition, evidence has shown that soil acidity may influence other crop management problems such as herbicide activity. Soil pH is a good indicator of the need for liming, but a buffer pH measurement is necessary to determine the quantity of soil acidity to be neutralized in order to change the soil pH. The general goal of liming agricultural soils continues to be a soil pH of 6.0 to 7.0.
Raising soil pH requires a quantity of agricultural liming material that is determined by the amount of acidity in the soil and the quality of the liming material. Soil acidity is measured by soil testing; the quality of agricultural liming material is determined by its purity and particle size distribution.
In the context of agricultural problem soils, acid soils are soils in which acidity dominates the problems related to agricultural land use. They are characterized by a pH which is strongly (5.5-4.5) to extremely acid (<4.5), a low cation-exchange capacity and a low base saturation.
Acid soils occur in the tropics and subtropics as well as in moderate climates. Their formation depends on specific conditions of climate, topography, vegetation, parent material and time for soil formation. Acrisols and Ferralsols are most common in old land surfaces in humid tropical climates. Acid sulphate soils occur in the tropics, in low-lying coastal land formerly occupied by mangrove swamps. Podzols are typical soils of the northern coniferous forests but may occur in the tropics, too.
The types of acid soils vary considerably due to different factors in soil formation, especially differences in climate, parent material and vegetation. Acid soils place major difficulties for agricultural use but can be very productive if lime and nutrients are constantly applied and appropriate soil management is practised.
Special management implications are caused by the occurrence of aluminium toxicity. In general soils with pH (H2O) <5 and >60% Al saturation of the cation exchange capacity (CEC) may suffer from Al toxicity. High concentrations of Al affect root growth, uptake and translocation of nutrients (especially immobilization of phosphorus in the roots), cell division, respiration, nitrogen mobilization and glucose phosphorylation of plants. Therefore, in the context of agricultural problem soils, it is necessary to make a distinction between acid soils with Al toxicity and soils without Al toxicity because of the differing management implications for these soils.
Strongly acid soils with aluminium toxicity (pH < 5)
Strong soil acidity considerably effects physical, chemical and biological properties of soils. The low calcium concentrations in the soil solution of acid soils restrict biological activity and soil structure stability. The major production constraint on strongly acid mineral soils is, however, the possible occurrence of high aluminium concentrations in the soil solution which are toxic to plants. Especially many tropical soils which are extremely weathered contain high amounts of exchangeable aluminium due to advanced soil formation processes.
The first effects of aluminium toxicity to be observed are a shortening and thickening of the roots. Roots become brown in colour and branching is reduced. The symptoms of aluminium toxicity are similar to phosphorus or calcium deficiency. If aluminium toxicity further progresses various plant metabolic processes are affected such as uptake and translocation of nutrients (especially immobilization of phosphorus in the roots), cell division, respiration, nitrogen mobilization and glucose phosphorylation of plants.
The tolerable aluminium concentration varies from plant species to plant species. But there are few crops such as tea which may accumulate greater amounts of aluminium without negative effects on plant growth. Tolerance of plants may depend on different factors such as differences in root morphology, ability to increase the pH of the root zone, mechanisms to reduce translocation of aluminium from the roots to the shoots, accumulation of aluminium in the shoots without affecting plant metabolism and mechanisms for not inhibiting nutrient uptake (Ca, Mg, P) despite the presence of aluminium.
Lime application is a common means for increasing soil pH and consequently reducing the amount of aluminium in the soil solution. However, in many tropical areas lime is in short supply. Therefore the breeding of aluminium tolerant crop varieties is of specific importance for tropical agriculture. There are tolerant varieties of various crops available. Especially varieties of upland rice, cassava, mango, cashew, citrus, pineapple and cowpeas may be suitable for arable production on these soils. Also various grasses (Brachiaria decumbens, Paspalum plicatulum, Pueraria lobata, Melinis minutiflora, Hyparrhenia rufa) and legumes (Stylosanthes ssp., Desmodium ssp, Centrosema ssp.) can be productively used on these soils.
Moderately acid soils without aluminium toxicity (pH >5 and <6.5)
In moderately acid soils there are no principal differences with regard to the pH demands of crops if aluminium toxicity is excluded. Even crops which are known to demand more neutral soil conditions such as sugar beet, barley and wheat may give reasonable yields on moderate acid soils if appropriate nutrient and water supply and favourable rooting conditions are ensured. The negative effects of soil acidity on physical and chemical soil conditions can be partly compensated by a high organic matter content. The lower the soil pH, the higher should be the organic matter content of the soil. A high organic matter content helps to achieve favourable tillage conditions, good aggregate stability and soil aeration. Consequently appropriate attention has to be given to increase the organic matter content in moderate acid soils. This can be achieved, for instant, by mulching or no-tillage systems.