Maximise crop response to P applications

Author: Michael Thomson | Date: 18 Sep 2014

Image of Jack Williamson, GRDC Northern Panellist.

By Jack Williamson, GRDC Northern Panel

With grain growers spending a significant portion of their operating budget each year on fertiliser, it is absolutely essential that they obtain a return on investment.

To achieve this, farmers need to have accurate information at hand about the state of the soils prior to the purchase and placement of fertiliser.

Although soil testing is not a new recommendation, its importance has increased in recent years as researchers have improved their understanding of nutrient run-down, particularly in subsoil zones.

Recent research has highlighted that nitrogen applications can be wasted if the levels of other nutrients such as potassium (K), phosphorus (P) and sulfur (S) are not adequate.

The Grains Research and Development Corporation (GRDC) has been supporting a number of trials in the northern zone examining the effect of the deep placement of P. This has allowed researchers to offer the following “rules of thumb” to help farmers interpret P soil tests in terms of likely fertiliser response on northern region vertosols.

Critical P values used to determine likely response or drivers of P availability in northern vertosols

Surface (0 to 10cm)

Subsoil (10 to 30cm)


<20 to 25mg/kg

Likely to get a response to starter P


Likely to get a response to deep P placements, unless BSES P high


Ensure good ground cover to limit erosion risk and to avoid nutrient loss in runoff


Unlikely to see P deficiency in your lifetime



Limited evidence of residual fertiliser accumulation


Limited reserves of slowly available P. Consider replacement of removed P once every five years.


High residual fertiliser load; slowly available to surface roots


Potential to slowly replace Colwell-P reserves and support crop growth in large soil volumes

By Jack Williamson, GRDC Northern Panel

As shown in the table, there are two different types of soil test for P: the bicarbonate extractable P (Colwell-P) test assesses easily available soil P; while the acid extractable P (BSES-P) test assesses slower release soil P reserves and the build-up of fertiliser residues.

These tests indicate nutrient sufficiency and when read in conjunction with the critical nutrient ranges above, the results can be used to support decisions about fertiliser rate, timing and placement.

The importance of starter P is a well-established practice, even though less than 10 per cent of total crop P uptake occurs during the early growth period – without it though the P concentration in the plant tissue will not be high enough to support the formation of vital yield factors such as number of tillers per plant and grain number per head.

It is later in the season and in dry years that P reserves in the topsoil become less important as roots access P from deeper soil layers, which is why soil sampling to at least 30cm is recommended.

Finally, it is important to interpret the Colwell-P soil test results in association with the Phosphorus Buffering Index (PBI), which helps define to the availability of soil P - the higher the PBI, the more difficult it is for a plant to access P.

More information on P nutrition is available in the GRDC Fact Sheet, ‘Soil testing for crop nutrition’, at

Contact Details

For Interviews

Jack Williamson, GRDC Northern Panel, Goondiwindi

07 4671 2265, 0438 907 820 


Michael Thomson, Senior Consultant, Cox Inall Communications

07 4927 0805, 0408 819 666

GRDC Project Code UQ00063, UQ00066, DAN00165, DAN00166

Region North