FERTILISERS, LIME, AND TRACE ELEMENTS
Agricultural Limestone and Molybdenum
These two materials must be considered together. This is because liming increases the availability of molybdenum to plants. And so this trace element can replace lime to a varying degree in many soils, deficient in available molybdenum. The importance of molybdenum as a substitute for lime depends on many factors such as the following:
Where the main beneficial effect of limestone is one of releasing available molybdenum to the plant, the use of molybdenum can replace liming. Where lime is needed to eliminate soil conditions which are detrimental to pasture or crop growth as well as to counteract molybdenum deficiency, lime alone or lime and molybdenum may be used in varying combinations.
In practice, the choice of using molybdenum alone, lime alone, or molybdenum and lime is often determined by economics. On molybdenum-deficient hill country, a moderately vigorous pasture can be obtained by applying molybdenum alone. If thought payable, this pasture may be further improved by using small quantities of lime. On flat, ploughable land of a closely related soil type, however, first-class pasture may be desirable. This may be possible only with the assistance of heavy rates of lime, the use of which would eliminate molybdenum deficiency. Thus on one and the same soil molybdenum, molybdenum and small quantities of lime, or heavy rates of lime without added molybdenum may be used depending on the desirable degree of utilisation of individual paddocks and on the cost of applying lime. New Zealand probably has a greater proportion of soils low in available molybdenum than any other country in the world.
Very small amounts of molybdenum are needed to ensure an adequate supply to pastures and crops. Usually molybdenum deficiency is eliminated by 2½ oz of sodium molybdate applied every three to six years, depending on the soil type.
In New Zealand, lime is applied to reduce the acidity of the soil but very rarely if ever to supply calcium. On strongly acid cultivated land, up to 3 tons of limestone per acre may be needed to obtain the desired reduction in soil acidity. More commonly 1–2 tons of limestone per acre are adequate. On uncultivated land it has been found that as little as 2–6 cwt of lime per acre may produce a worth-while improvement in clover establishment and vigour.
When excessive acidity has been corrected, the amount of lime needed to keep the soil reaction at a satisfactory level has been determined experimentally only at Marton. The results obtained at Marton are being adjusted to other districts, making allowance for factors such as rainfall, soil type, and the size of the initial dressing of lime which has been applied. In districts with 25–30 in. rainfall, it is considered that 1 ton of lime every six to 10 years is adequate maintenance. In districts with a rainfall of 35–45 in., 1 ton per acre every four to five years appears adequate, and in higher rainfall areas 1 ton of lime every three years may be needed to prevent excessive soil acidity.