Application of Calcium Fertilizers

Liming is carried out by application of CaCO₃ in limestone, a process that is described in some detail in Troeh and Thompson (98). The neutralizing capacity of the limestone used is measured by comparing it to calcite, which is CaCO₃, with a calcium carbonate equivalent (CCE) of 100%. The fineness of the lime affects its efficiency for liming, and the CCE and fineness and hardness of the lime together give the effective calcium carbonate equivalent or reactivity. Application should occur when the soil is dry or frozen, to avoid damage to the soil by the vehicles carrying the lime. Although soil testing will determine if an application is required, it is often the practice to apply lime a year ahead of a crop in a rotation that has a strong lime requirement (often a legume). An application once every 4 to 8 years is usually effective. Limestone, burned lime (CaO), or slaked lime [Ca(OH)₂] can also be used. Burned lime has a CCE of 179% and slaked lime a CCE of 133%.

The amount of lime required is determined from soil analysis, either by a pH base saturation method or a buffer solution method (98,120). The soil requirement for lime, defined, for example, as the number of tonnes of calcium carbonate required to raise the pH of a hectare of soil 200mm deep to pH 6.5 (120), will depend on the initial pH and also on CEC of the soil. Most soils have a much greater proportion of their cations attached to cation-exchange sites than in solution, meaning that a high proportion of the H⁺ ions present are not measured in a simple pH test. Adding lime to the soil neutralizes the acidity in the soil solution, but the Ca²⁺ ions displace H⁺ ions from the exchange sites, with the potential to make the pH of the soil acidic once more, and this acidity is neutralized by reaction of the H⁺ with the lime. The H⁺ in soil solution is called the active acidity, and the H⁺ held to the exchange sites on soil colloids is called the reserve acidity The greater the CEC, the greater the reserve acidity and the greater the lime requirement (98).

In the pH-base saturation method, the percent base saturation of the soil, the CEC of the soil and the initial pH all have to be measured. To calculate how much lime should be added the percent base saturation at the initial and at the target pH value are read off a graph, and the amount of CaCO₃ to be added is calculated from the difference in percent base saturation at the two pH values multiplied by the CEC (98).

In the buffer solution method, a sample of the soil is mixed with a buffer, and the amount of lime required is read off a table from the value of decrease in buffer pH on adding the soil (120).