The new age of soil phosphorus testing is here

Author: Sharon Watt | Date: 06 Nov 2012

The DGT test is a plastic device that uses an iron oxide gel as a P sink, which attracts available P through a membrane. Image Sean Mason.

A new and more accurate method for assessing soil phosphorus (P) availability to crops is to become more widely commercially available in 2013.

Through funding from the Grains Research and Development Corporation (GRDC), the DGT (Diffusive Gradients in Thin films) test has been adapted for use in the grains industry because existing soil testing methods have been shown to be poor predictors of plant available P on certain soil types.

Extensive testing over recent years and trial analysis this year have demonstrated that the DGT test offers considerable improvement in predicting soil available P levels on many cropping soils in south-eastern Australia.

The GRDC is keen to see the DGT test become the standard phosphorus test Australia-wide and it is already calibrated for wheat, barley, peas, chick peas and canola.

GRDC Manager of Commercial Farm Technologies, Paul Meibusch, says that while the DGT test’s adaptation has been almost a decade in the making, its commercialisation has been worth the wait.

“Phosphorus fertiliser can be a substantial input cost for grain growers, so it is critical that they have access to an accurate means of testing P levels in their soils,” Mr Meibusch said.

“Soil testing is fundamental in determining the nutrient status of cropping land, but tests available up until now have not always been reliable, especially on certain soil types, such as calcareous soils and those which are acidic with high iron or aluminium.

“It is likely some growers have been unnecessarily spending money on applying P when it is not needed, and in other cases it is probable growers have not applied enough P and soils have become depleted and crops have subsequently suffered.”

Responsible for the DGT test’s development is Dr Sean Mason of the University of Adelaide who has been working on the project since 2003.Dr Sean Mason, University of Adelaide, holding a test tube in his right hand and tweezers in his left hand.

Dr Mason says the DGT test is a plastic device that uses an iron oxide gel as a P sink, which attracts available P through a membrane.

It is deployed on moist soil (100 per cent water-holding capacity) for about 24 hours, after which the device is washed. The amount of P bound to the gel is removed by washing with a solvent and then measured.

The DGT measurement incorporates the initial soil solution P concentration, as well as the ability of the soil to resupply the soil solution pool in response to the removal of P.

“It is designed to mimic the action of plant roots so is a better method of predicting plant P requirements than methods based on chemical extractions, such as Colwell-P,” Dr Mason said. “It is these DGT deployment conditions and the use of an iron-based gel that sets it apart from other common soil P tests.”

Grain growers and their advisers are encouraged to contact their usual laboratory suppliers as soon as possible to ascertain whether they are supplying DGT-P analysis.

Back Paddock Company has been engaged by the GRDC to help with the initial commercialisation of the DGT-P test. Further information about laboratory set-up for the test, interpretation of DGT-P or locating a laboratory offering DGT-P can be gained by contacting Back Paddock Company.

Back Paddock Company has announced the availability of the DGT test in its SoilMate system for the 2013 winter cropping season and can be contacted via phone on 1800 557 166 or visit

To assist growers with their soil nutrient testing and fertiliser application strategies, the GRDC has published a new Fact Sheet on Phosphorus Management in the southern cropping region. It contains details about the DGT test. The Fact Sheet will be included in the November-December edition of Ground Cover magazine and can also be downloaded via


Caption: The DGT test is a plastic device that uses an iron oxide gel as a P sink, which attracts available P through a membrane. Image: Sean Mason.

Caption: Dr Sean Mason, University of Adelaide, has been responsible for the DGT test’s development.

For interviews:

Paul Meibusch, GRDC
02 6166 4500

Dr Sean Mason, University of Adelaide
08 8303 8107 / 0422 066 635


Sharon Watt, Porter Novelli
0409 675100

GRDC Project Code UA00095

Region South, North, West