This is info from Ravensdown (N.Z.)
Physical Description:
Superphosphate is made by reacting finely ground phosphate rock with sulphuric acid to convert the insoluble mineral to a plant available form. Superphosphate contains phosphorus and sulphur in a ratio of 1 to 1.16. When applied at a rate to meet P requirements, the sulphur requirements will also be met in the majority of situations. The colour varies from brown to dark grey with each rock type.
Uses:
Superphosphate is an ideal fertiliser for capital or maintenance applications. It is mainly used as a maintenance fertiliser, providing a pool of soil solution P for pasture. It is also ideal for pastoral development and arable/horticultural situations where immediate responses are required.
Benefits:
Superphosphate also contains Sulphur in the readily available form of sulphate for plants to take up. This makes it ideal for low fertility and cropping situations. The Calcium in Superphosphate can be important in some horticultural and hill country situations where it is uneconomic to use lime.
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For the most part, superphosphate is used in commercial applications to maintain showy flowers in public areas. It does stimulate good roots but must be used very sparingly. Some chemical use veg growers will use superphosphate, and it's personal choice for them. You'd place about a teaspoon full in the planting hole, scruff up the soil, and then plant your seedling. It can also be a topdressing scratched into the soil which will eventually leech down to the roots.
