Nitrogen for Pasture: On all but the most fertile soils, the supply of nitrogen to pasture is regulated by the presence and thrift of the right type of clovers. But, in spite of vigorous clover growth, nitrogen deficiency may occur in young pastures and at certain times of the year. In young pastures sown on land of low nitrogen fertility, grasses commonly become nitrogen deficient in spite of vigorously growing clovers. Usually this type of nitrogen deficiency is only of one to a few years' duration. As the nitrogen fixed by the clovers is gradually passed on to the companion grasses, the deficiency corrects itself. In the meantime there is a period of clover dominance during which grasses may just survive. Nitrogenous fertilisers are seldom used to correct this temporary imbalance of clovers to grasses. In fact there is not enough experimental evidence to say whether nitrogenous fertilisers under these conditions are of benefit and, if they are, what would be the best technique of using them. Again, nearly all pastures suffer from seasonal deficiencies of nitrogen. The greatest shortage of this element occurs in late winter - early spring. At this time of the year nitrogenous fertilisers are used occasionally to stimulate out-of-season feed. Quick-acting mineral sources of nitrogen, such as sulphate of ammonia and lime-stabilised ammonium nitrate (both fertilisers contain about 20.5 per cent), are most popular, but ammonium sulphate-nitrate (26 per cent N.) is also being sold in small amounts.
On the whole, therefore, the use of nitrogenous fertilisers on grazed pastures is limited to the stimulation of extra growth from late autumn to early spring. Nitrogenous fertilisers are very effective, however, in raising the yield of paddocks shut up for grass seed.
Nitrogen for Crops: The occurrence of nitrogen deficiency in crops depends in some degree on farm management. Crops following medium to good pasture seldom respond to nitrogenous fertilisers. Crops grown following poor pasture sometimes respond to nitrogen, but surprisingly often they do not. Crops grown after a succession of other crops usually respond to the application of nitrogenous fertilisers, except on the most fertile soils. Within this general situation, there is evidence that nitrogen deficiency in crops is less common today than 20 or 30 years ago. Better quality pastures, leading to a greater supply of available soil nitrogen following cultivation, are thought to be the reason for this trend. Crops vary greatly in their demand for nitrogen. Maize appears most sensitive to low levels of available nitrogen, followed by potatoes. Greenfeed cereals are also rather sensitive but not as much as Italian ryegrass. Spring-sown chou moellier and other leafy fodder crops appear to be intermediate in their requirements. Wheat and barley demand moderate nitrogen fertility. At high levels of nitrogen supply, however, lodging and increased susceptibility to disease is common. Swedes and turnips sown in spring also demand only moderate nitrogen fertility, but turnips sown as a catch crop in autumn appear to need a high level of nitrogen fertility.
Crop rotations are usually designed to avoid the need for fertiliser nitrogen. Crops demanding a fairly high level of nitrogen are commonly grown following pasture, and crops demanding less nitrogen fertility are grown after other crops. Thus a typical rotation on Canterbury cropping land might be pasture, potatoes, wheat, barley, pasture. Although some nitrogen would be applied to potatoes, usually 1 cwt of sulphate of ammonia per acre, this is done not so much because a nitrogen response is probable but rather as an insurance appropriate for such a valuable crop.