Phosphorus in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 30

Result:

•

Part of the applied phosphorus is immediately plant available.

•

Part forms a bank of less available phosphorus (slow release).

•

Part is 'fixed' and is mostly unavailable to crops and pastures.

Part of this 'unavailable' phosphorus may, however, be accessed by 'soil micro-
organisms', particularly mycorrhizae that can then pass it on to crop plants.
Phosphorus uptake by plants is also reduced in compacted soil due to restricted
root growth.
Young organic matter decaying in soil releases plant available phosphorus,
although phosphorus combined in older forms of organic matter is released slowly
over time. On a sustainable farm, there is a continual cycle of use and reuse of
phosphorus, although the phosphorus removed from the farm in plant material
through cropping, grazing or fodder conservation must be replaced by added
fertiliser phosphorus to maintain the balance. The amount of plant available
phosphorus in soil may decrease over time due to pH effects on soil micro-
organisms.

Soil tests
Different states of Australia have different methods of testing for phosphorus. In
Victoria, the normal test for plant available phosphorus is called the Olsen P test
(sample results are shown in Table 3). Results are given in mg/kg (milligrams of
phosphorus per kilogram of soil). In New South Wales and South Australia the
normal test is the Colwell P test. It is similar to the Olsen P test, but values are
higher and are also expressed in mg/kg (milligrams per kilogram). Another
method of testing that is frequently used in New South Wales for available
phosphorus is the Bray Test.

What do the results mean?
Soil tests give a guide to the likely responses by crops or pastures to the application
of phosphorus fertiliser. The plant needs vary depending on the type of soil, with
the optimum phosphorus level lowest on sands, followed by loams, clays and peats,
with calcareous sands having the highest requirement. The type of enterprise will
also affect the amount of fertiliser required (Table 3).
Note . Native tree and shrub seedlings can only tolerate low levels of
phosphorus, and when grown commercially, should only be fertilised with low
levels of P.