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Potatoes Australia August/September 2016

THE BOTTOM LINE

Conventional testing methods for assessment of soil phosphorus have shown

to overestimate available phosphorus on certain soil types (calcareous, acidic

with high iron or aluminium).

The Diffusive Gradients in Thin-films (DGT) test measures available

phosphorus at more relevant chemical and physical conditions than the

conventional Colwell, Olsen and Bray methods and has been developed to

assess available phosphorus in a wide range of Australian soil types.

The mode of measurement is by diffusion of available phosphorus in the soil

towards a phosphorus sink (an iron oxide gel). It therefore measures both the

initial soil solution phosphorus concentration and also the ability of the soil to

resupply the soil solution pool in response to the removal of phosphorus,

mimicking the action of plant roots better than conventional methods.

DGT critical values have successfully been determined from field trials across

southern Australia for wheat, barley, canola and field pea crops.

Figure 2. Measurement of available phosphorus by DGT. The DGT device is placed

upside down on moist soil (approximately 100 per cent water holding capacity)

for a period of time (typically 20-24 hours).

HOW TO INTERPRET DIFFUSIVE GRADIENTS IN THIN-FILMS

(DGT) VALUES FOR POTATOES

Results from the 15 replicated yield response trials conducted in major South

Australian potato growing regions indicate:

DGT-phosphorus values < 75 µg/L indicate moderate to substantial penalties

would occur if no phosphorus fertiliser were applied.

DGT-phosphorus values between 75-310 µ/L have moderate likelihood of

achieving a small yield increase when phosphorus fertiliser is applied.

DGT values >310 µ/L indicate a very low likelihood of achieving a yield

increase when phosphorus fertiliser is applied.

Figure 1: Relationship between DGT values and potato response (relative yield)