The fertilisers most commonly used to supply phosphorus are: (1) Superphosphate containing this element in a water soluble form, (2) “Aerial superphosphate” (90 parts of superphosphate reverted with 10 parts of ground serpentine rock) in which the phosphates are partly water soluble, and (3) Serpentine superphosphate (75 per cent parts of superphosphate reverted with 25 parts of ground serpentine rock) which contains phosphorus mainly in a water insoluble but yet readily available form.
Some lime-reverted superphosphate is also used, particularly in the South Island, where it is desirable to drill in small crop seeds with a phosphatic fertiliser. Superphosphate in contact with turnip, swede, chou moellier, rape, clover, and other small seeds may cause serious loss of germination. Fully reverted and therefore water-insoluble forms do not cause germination injury.
The use of serpentine-reverted superphosphate is peculiar to New Zealand. Serpentine superphosphate has three advantages. It is superior in physical qualities to ordinary superphosphate in that it flows more freely, does not set hard on storage under damp conditions, and does not rot bags. It contains about 5 per cent of magnesium, a considerable proportion of which has been shown to be available to plants. In contrast to superphosphate it stores reasonably well when mixed with soluble potassic fertilisers. Its main disadvantage is a lower phosphorus and sulphur content per unit than that in superphosphate. Both fertilisers are the same price. Aerial superphosphate combines good physical qualities with a higher phosphorus and sulphur content than that in serpentine superphosphate, but it is not satisfactory when mixed with potassium chloride, and it is expected to cause germination damage if sown in contact with small seeds.
The greater part of unimproved land needs phosphatic fertilisers. Exceptions are some of the fertile river flats of both Islands, and some of the soils in the dry parts of Central Otago, and of inland South Canterbury and, possibly, of Marlborough. Therefore it is seldom a question of whether to use or not to use phosphatic fertilisers. The question is rather how much to use initially during the period of pasture improvement, and how much to use to maintain pasture production at a desirable level.