The pH values of soil are an excellent single indicator of general soil conditions. The pH value is usually determined by shaking a sample of the soil with distilled water and estimating the acidity of the suspension.
It should be noted that the pH of the suspension is always lower than the pH of the supernatant liquid owing to the surface-adsorbed acidity of the soil colloids in the suspension.
The initial soluble salt contact of soil influences the pH value. The soil is therefore usually leached with distilled water before measuring the pH of the salt-free soil. This procedure is time-consuming and does not represent field situations.
To mask the effect of the initial salt contents of soils on the pH, soil pH is measured in salt solutions of larger concentrations than the concentrations of the native salt contents of individual soil.
Depending on the soil type, 0.01M of either Kcl or CaC12 solution is added to the soil and the pH is measured. The pH values of soil-salt solution suspension are more reliable than that measured in soil water suspension.
The pH values of soil-salt solutions are always about 0.5-1.0 units lower than the pH values of soil-water solutions due to the displacement of extra hydrogen ions H+, from the permanent exchange complex into soil solution by K+ or Ca2+ ions.
Read Also: 7 Main Importance of Soil Acidity
Liming Practice
The productivity of acid soils can be improved tremendously by the application of lime both to neutralize excess hydrogen ions and to reduce the activity of aluminum ions in the soil solution.
That is, liming is precised to increase the pH of acid soils and to eliminate all the deleterious effects of soil acidity as discussed in the preceding sections.
Work at Nsukka. Nigeria by Fori and Okigbo (1972) revealed that there was increased maize yield in limed plots over no lime treatment. Apart from increased crop yield, other benefits of liming are:
- Reduction of soil acidity
- Provision of cationic bases such as Ca and Mg especially if dolomite is used as the liming material
- Increase in solubility of certain plant nutrients such as phosphorus and molybdenum and hence their availability to plants.
- Increased rate of nutrient release from soil organic matter due to increased rate of microbiological activities. The supply of nitrogen, phosphorus, sulphur, and boron comes mainly from soil organic matter.
- Increase in symbiotic nitrogen fixation in the root nodules of legumes through increased activity of nitrogen-fixing bacteria, especially Rhizobium spp.
- Increase rate of nitrification by autotrophic nitrifying bacteria (Nitrosomonas) and (Nitrobacter spp.).
- Precipitation of toxic substances – excess A1, Fe, and Mn – from the soil solution.
- Retardation of certain plant diseases as well as plant pathogens.
Over liming may occur when lime is applied more than required to neutralize the excess A13+ or eliminate Mn toxicity. Over liming may manifest in yield reduction, soil structure deterioration, and decreased availability of P, Bo, Zn, and Mn.
Friesen et.al. (1980) working on two ultisols (one series) an oxic Paleudult and Nkpologu series, an Ustoxic Paleustult found that zinc activity in both soils declined very sharply when soils were limed to pH above 5.0. Another work by Juo and Uzu (1977) on two Nigerian ultisols
(i) Ustoxic Paleustult from Nsukka (Nkpologu Sandy loam) and
(ii) Oxic Paleudult from Benin (Alagba sandy loam) showed severe Mn deficiency on maize growth on the ustoxic Paleustult soil limed to near neutrality even though a little amount of Mn was applied. Following are the definitions of some terms associated with liming practices:
Liming Material
Liming material is any material added to the soil for the purpose of neutralizing or reducing soil acidity.
That is, the liming material which is called lime reduces the activity of hydrogen ion concentration. (H+), in soil solution, most liming materials used are Ca-salts in which the anion is a base (proton acceptor).
Read Also: Soil Reaction: Meaning, Acidity, and Liming
