Liming materials can be compared by considering their ability to neutralize soil acidity, fineness, and cost to deliver and spread. The neutralizing value (NV) of a lime indicates its power to overcome acidity.
A neutralizing value of 50 signifies that 100 kg of that material has the same effect on soil acidity as 50 kg of calcium oxide. The fineness of the lime is important because it indicates the rate at which it affects the soil acidity (see surface area p304). It is expressed, where relevant, in terms of the percentage of the sample that will pass through a 100 mesh sieve. Liming materials commonly used in horticulture are listed below with some of their properties. The relationship between the different forms of calcium is shown in Figure 20.5.
Heat » Calcium Water added ^ Calcium Carbonic acid» Calcium Carbonic acid^ Calcium
*as in rain
Calcium carbonate is the most common liming material. Natural soft chalk (or limestone) that is high in calcium carbonate is quarried and ground (NV = 48). It is a cheap liming material, easy to store and safe to handle. A sample in which 40 per cent will pass through a 100 mesh sieve can be used at the standard rate to meet the lime requirement. Coarser samples although cheaper to produce, easier to spread and longer lasting in the soil, require heavier dressings. Shell sands, mainly calcium carbonate, have neutralizing values from 25 to 45, i.e. whilst the purest samples can be used at nearly the same rate as chalk, up to twice as much of a poorer sample is required to have the same effect.
Calcium oxide (also known as quicklime, burnt lime, cob lime or caustic lime) is produced when chalk or limestone are very strongly heated in a lime kiln. Calcium oxide has a higher calcium content than calcium carbonate and, consequently, a higher neutralizing value. Pure calcium oxide is used as the standard to express neutralizing value (100) and the impure forms have lower values (usually 85-90). If used instead of ground limestone, only half the quantity needs to be applied.
In contact with moisture, lumps of calcium oxide slake, i.e. react spontaneously with water to produce a fine white powder, calcium hydroxide, with release of considerable heat. This was an effective way of obtaining a fine lime from the quarried material before there was heavy rolling machinery to grind the coarse lumps. The lime kilns that were used are still a common sight, especially in small ports round the coast (see Figure 20.6) Although rarely used now, calcium oxide has to be used with care because it is a fire risk, 'burns' flesh and scorches plant tissue.
Calcium hydroxide, hydrated or slaked lime, is derived from calcium oxide by the addition of water. The fine white powder formed is popular in horticulture. It has a higher neutralizing value than calcium carbonate and its fineness ensures a rapid effect on the growing medium. Once exposed to the atmosphere it reacts with carbon dioxide to form calcium carbonate.
It should be noted that all forms of processed lime quickly revert to calcium carbonate when added to the soil. Calcium carbonate, which is
insoluble in pure water, gradually dissolves in the weak carbonic acid of the soil solution around the roots (see Figure 20.5).
Magnesian limestone, also known as Dolomitic limestone, is especially useful in the preparation of composts because it both neutralizes acidity and introduces magnesium as a nutrient. Magnesium limestone has a slightly higher neutralizing value (50-55) than calcium limestone, but tends to act more slowly.
Liming materials also provide the essential nutrients calcium and, when present, magnesium to the soil. Bicarbonate is formed from the carbonate in carbonic acid, e.g. rainwater or soil water, around respiring roots to provide a soluble form that can be taken up by plants (see Figure 20.5 ).
Was this article helpful?