Organic fertilisers are produced in large quantities as by-products of the meat export industry, slaughter houses, and soap manufacture. Typical products are bone meal or bone dust containing water-insoluble phosphorus and some nitrogen; dried blood, a good organic source of nitrogen; and tankage, a powder obtained from cooking, drying, and grinding animal refuse of heterogeneous origin. Tankage contains water-insoluble nitrogen and phosphorus. These organic fertilisers of animal origin are mainly used in commercial market gardens, orchards, and by private gardeners. They may be incorporated in proprietary mixtures, but commonly are applied as so called “Blood and Bone”, a mixture of bone dust and dried blood or tankage and dried blood. Blood and bone contains about 4–7 per cent nitrogen and 10–13 per cent phosphorus. Market gardeners commonly apply 1½-2 tons of blood and bone per acre to leafy vegetables. This heavy rate of application is commonly followed by side dressings of soluble inorganic nitrogenous fertilisers during the growing season. Tomatoes, pumpkins, and cucumber also receive large quantities of organic fertilisers.

Small amounts of organic fertilisers are incorporated into proprietary mixtures which are sold to farmers for the manuring of crops and, to a lesser extent, for topdressing of pastures. The use of these manures in farming is not recommended. A vigorous export trade with organic manures has developed in recent years.

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.

The other trace elements known to be essential for plant growth are manganese, zinc, iron, chlorine, and cobalt for the proper functioning of clover nodules. Cobalt deficiency severe enough to limit the proper functioning of clover nodules, and therefore nitrogen fixation, would most likely be associated with marked cobalt deficiency of sheep and cattle. If it ever existed, it would by now have been corrected. Manganese deficiency in wheat occurs in Canterbury on a few otherwise quite fertile river flats. It occurs also in a few orchards.

Iron deficiency on the other hand is known to occur in young pastures on some of the acid raw peats of the Waikato, and it is suspected on some soils in northern Hawke's Bay. The most suitable materials for supplying iron to these pastures and their rate of application are not yet known.

Copper deficiency in New Zealand is more evident in animals than in pasture or crops. In animals copper deficiency can be caused by an excessively high concentration of molybdenum in pastures with copper concentrations which are known to be adequate for stock health when molybdenum levels are lower. There is no evidence that very high, naturally occurring levels of available molybdenum can cause toxicity symptoms in pasture. The rather large amount of “copperised fertilisers” used by farmers are therefore mainly designed to improve stock health rather than pasture vigour. Nevertheless, there is evidence that on some soils of northern Hawke's Bay and north of Auckland, as well as on raw, acid peatland of the Waikato, copper deficiency of pastures may occur.

Boron deficiency is very widespread in New Zealand but affects mainly brassica root crops such as turnips and swedes. Amongst farm crops, lucerne is also sensitive to boron deficiency but apparently to a lesser degree than swedes and turnips. Pasture responses to boron applications are rare, and are probably confined to clovers. For established lucerne, boron is applied as fertiliser borate at a rate of 10–30 lb/acre mixed with phosphatic fertilisers. With swedes and turnips a similar rate of application of borate is needed for the control of “mottle heart” or “brown heart”, but great care must be taken not to apply borates in contact with seed. Control of the few rare cases of boron deficiency in pasture appears to be obtained by 5 lb of fertiliser borate per acre.

Magnesium deficiency in pasture is almost unknown. On the other hand recent experimental work has shown that losses of magnesium from the topsoil may occur under urine spots and following topdressing with potassic fertilisers. With the widespread use of serpentine superphosphate and aerial superphosphate, appreciable quantities of magnesium are added to soils. It is possible therefore that these fertilisers are preventing the occurrence of magnesium deficiencies. Apple trees and possibly a few other crops of lesser importance are more sensitive to a low supply of available magnesium than pastures.

Sulphur deficiency is very common in New Zealand. Since superphosphate contains about 10 per cent of phosphorus and 11 per cent of sulphur, this fertiliser serves as the main source of sulphur. In addition, small amounts of sulphur, commonly between ½–5 lb/acre and rarely exceeding 7 lb per acre, are added to the soil in rainfall. Many soils in the drier districts of Otago, Canterbury, and Marlborough are only slightly phosphorus deficient but strongly deficient in available sulphur. There is, moreover, the possibility that with regular phosphatic top-dressing the level of available phosphorus increases more rapidly than that of available sulphur because applied phosphorus is mainly retained in the topsoil while applied sulphur may leach quite rapidly out of the topsoil. Following superphosphate topdressing, therefore, the supply of available sulphur may not increase as much as the supply of available phosphorus. To cater for the soils on which sulphur deficiency is more severe than phosphorus deficiency but in which both deficiencies are present, a superphosphate-sulphur mixture is used in increasing quantities. The sulphur used is screened elemental sulphur. It contains a considerable proportion of fines which ignite easily on friction or if near an open flame or spark. The danger of combustion is greatly reduced by dilution with superphosphate or other materials. Hence the proportion of sulphur in a mixture designed for distribution by air is strictly controlled. The safe upper limit is set at 500 lb of screened sulphur per ton of fertiliser material or diluent.

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.

Potassium chloride (often called muriate of potash) is the cheapest fertiliser per unit of potassium and by far the most widely used. Potassium sulphate contains 20 per cent less potassium than the chloride and is used for special purposes only, mainly for small fruit and tobacco leaf production.

The areas recognised as needing potassic fertilisers to maintain reasonably high-producing pastures are growing larger year by year. This spread of potassium-deficient areas may be caused in part by the losses of potassium under pastoral farming and in cash cropping; recent research into soil fertility, however, is leading to better recognition of long-standing potassium deficiencies. The soils most liable to become potassium deficient are those highly leached or in areas with a high annual rainfall. These are the soils with a severe initial phosphorus deficiency as well. Hence, where potassic fertilisers must be used, phosphatic fertilisers are usually also needed. In practice, therefore, potassic fertilisers are mainly applied as potash-phosphate mixtures either as potassic serpentine superphosphate or, less commonly, as potassic superphosphate stabilised by 3–6 per cent of ground limestone. There are, however, differences in the long-term fertility trends of phosphorus and potassium. While regular applications of phosphatic fertilisers result in a gradual rise in the level of available phosphorus, regular applications of potassic fertiliser do not have the same beneficial, long-term effect. On the contrary, the light rates of application of potassic fertilisers commonly used may not completely arrest a downward trend in potassium fertility. For this reason intensively top-dressed areas may gradually become less deficient in phosphorus and more deficient in potassium. The ratio of use of phosphatic fertilisers to potassic fertilisers is therefore expected to become narrower.

The following common crops appear to have a greater need for readily available phosphorus than grass-clover pasture – potatoes, barley, and all brassica species including turnips, swedes, chou moellier, rape.

Autumn-sown wheat does not need a very generous supply of phosphorus, but spring-sown wheat may approach barley in its sensitivity to low levels of available phosphorus. The position of oats in relation to other crops and to pastures is not well known.