The Fertility Turnover of Pastures
Pastures take up very large amounts of plant nutrients from the soil. The herbage is then consumed by stock and the nutrients contained are in great part returned to the soil in the form of dung and urine. There is in this way a cycle of fertility from soil to pasture herbage to livestock – to urine and dung – and back again to the soil and a return to herbage. This cycle is not a perfect one. The nutrients are continually being depleted in various ways. A proportion of them is retained in animal products, such as milk. P. D. Sears (J. Brit. Grassl. Soc., Vol. 5, 1950, pp. 267–280) estimates that 600 lb gallons of milk contain the equivalents of the following fertilisers:
160 lb sulphate of ammonia (nitrogen).
60 lb of superphosphate (phosphorus).
15 lb of potassium chloride (potassium).
25 lb of agricultural limestone (calcium).
These nutrients are also taken up into the flesh and bone of the grazing animals, and this means that their sale causes further depletion. The largest losses, however, of precious minerals may occur within the farm. Transfer in dung and urine from grazed areas to sheds, races, stock camps, gateways, hedges, and trees, leaching generally and especially under urine spots and fixation by the soil, all remove from circulation a serious proportion of the plant nutrients.
Expressed as equivalents of well-known fertilisers, limestone, and trace elements, the annual turnover of nutrients of first-class pasture is approximately as follows:
|Nitrogen as sulphate of ammonia||20–25 cwt/acre.|
|Phosphorus as superphosphate||4 cwt/acre.|
|Potassium as potassium chloride||5–6 cwt/acre.|
|Sulphur as superphosphate||300 lb/acre.|
|Magnesium as Epsom salts||250 lb/acre.|
|Calcium as superphosphate||400 lb/acre.|
|or Calcium as 80% pure limestone||250 lb/acre.|
|Copper as bluestone||½ lb/acre.|
|Molybdenum as sodium molybdate||1/3 oz/acre.|
Nitrogen may suffer the largest losses, followed by calcium, potassium, sulphur, phosphorus, and magnesium.
Under New Zealand conditions adequate amounts of nitrogen are supplied to pastures by clovers, particularly white clovers. The root nodule bacteria of these legumes have the ability to convert atmospheric nitrogen into organic forms which become available to the associated grasses. For this reason, the main purpose of applying fertilisers, lime, and trace elements to the pastures is to stimulate clover growth so that, in turn, grasses may benefit from the nitrogen supplied by these species. Good clover growth means good grasses will grow well.
Clovers, particularly white clovers, need a continuous good supply of phosphorus, potassium, sulphur, magnesium, and several trace elements. In addition, soils must not be too acid. Where soils cannot release these needed elements right through the year at an adequate rate to maintain vigorous clover growth, or where soils are too acid, the supply of nitrogen to grasses will diminish, pasture production will fall, and low-fertility demanding species of grasses will replace high-fertility demanding grasses. This would be the situation on almost all soils in New Zealand if no fertilisers or lime were used. Fertiliser, lime, and trace elements, therefore, are needed to supplement the natural supply of these nutrients from the soil. They are also used to make good any losses which occur under pasture farming as already discussed.
The supplying power of soils for individual nutrients varies considerably. That for phosphorus is inadequate in nearly all soils of New Zealand, that for sulphur and molybdenum is inadequate in a large proportion of soils. The supplying power of potassium depends not only on soil type but to a great degree on the amount of potassium lost on individual farms and paddocks in past years. It also depends on the level of pasture production. Thus paddocks frequently hayed or cropped in the past are usually more potassium-deficient than others. On farms where pasture production is high, there is a need for a rapid release of potassium from the soil and so potassium deficiency is more likely to occur there than on farms with low-quality pastures. Moreover, losses of potassium by transfer and under urine spots are thought to be higher on highly productive farms than on low-producing farms, and higher on dairy than on sheep farms.