Phosphorus in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 33

Worke d e x a mple
Question : How much plain superphosphate (8.8% P) do I need to raise the
Olsen P level of a dryland pasture on a sandy soil from a low 8 mg/kg to a
moderate level of 18 mg/kg.
From Table 3: a moderate Olsen P level is 18 mg/kg. The current low Olsen P
level is 8 mg/kg. So, the phosphorus required is 18 - 8 = 10 mg/kg.
From Table 4: 100 kg/ha of plain superphosphate adds 8.8 kilograms of
phosphorus per hectare.
From Table 5: for a soil with a bulk density of 1.2 grams/cubic centimetre, it
will require 1.8 kg/ha of phosphorus to raise Olsen P by one unit (1 mg/kg).
So, to raise the Olsen P by 10 units will require 1.8 × 10 = 18 kg/ha of
phosphorus.
Therefore, if 18 kilograms of phosphorus per hectare is required and 8.8 kg per
hectare is supplied by 100 kg of plain superphosphate: 18 × 100 ÷ 8.8 = 205
kilograms of plain superphosphate per hectare.
Note 1 . Soil bulk density figures may be supplied in the soil tests.
Note 2 . A rule of thumb figure is that 1 kg of phosphorus is required per DSE of
production.

Table 5 Figures for calculating fertiliser requirements

Soil bulk density (g/cm 3 ) Figures for calculating

fertiliser requirement

Good 1.2 1.8

Dense 1.4 2.1

Very dense 1.6 2.4

When should phosphorus be applied?
Plants respond best to phosphorus in the early stages of growth and when growing
vigorously (Figure 11). As phosphorus becomes less available to plants over time, it
is best applied when the plant can use it quickly, such as after the autumn break
and at the spring f lush (or both). With slow release fertilisers, such as rock
phosphate, the time of application is less critical, but it would be best applied early
in the season. Rock phosphate is only an option on acid soils unless it has been
treated with appropriate biological additives. Soft rock phosphate and guano
phosphatic fertilisers, however, are much more plant available, and do not
adversely affect soil microbiology as chemically treated phosphate fertilisers do.
Phosphorus applied in fertilisers will be partly available to future crops and
pastures, but much of the applied phosphorus will become 'fixed', and will no
longer be readily available to plants unless soil microbiology can unbind it and
transfer it to the plants. This means that regular annual applications based on soil