Regular use of superphosphate fertiliser increases soil fertility and pasture production to a level which is able, in this case, to support 12 stock units over a year. A stock unit is equivalent to one ewe and lamb. If fertiliser is no longer used, productivity and stock rate gradually drop to the level supported by the natural fertility of the soil. The dotted line shows the level of productivity if the same rate of superphosphate application continues.
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Te Ara - The Encyclopedia of New Zealand
Source: Ravensdown
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