Soil Phosphorus Availability by DGT


Key Points

  • Present soil testing methods for assessment of available phosphorus (P) have been shown to overestimate available P on certain soil types (calcareous, acidic with high iron or aluminium).

  • The DGT method has been established for assessment of available P in a wide range of Australian soils and measures available P at more relevant chemical and physical soil conditions.

  • A database of DGT results with crop responses across southern Australia reveals a greater accuracy of available P measurement compared to Colwell P with or without Phosphorus Buffering Index (PBI) interpretation.

  • DGT has potential to not only measure the available P status but also to predict P rates required to maximise yields in a deficient scenario.


Overview of DGT

DGT has been developed for the assessment of available P in a wide range of soils. The mode of measurement is by diffusion of available P in the soil toward a P sink (an iron oxide gel) via a membrane which controls movement of P to the sink (figure 1). The gel and protective filter paper are held securely in a plastic piston device. This is deployed on moist soil (100 % water holding capacity) (figure 2) for around 24 hours after which the device is washed, and the amount of P bound to the gel is then measured. Therefore the DGT measurement incorporates the initial soil solution P concentration and also the ability of the soil to resupply the soil solution pool in response to the removal of P, mimicking the action of plant roots better than conventional methods (table 1).


Figure 1: Components of the DGT method including the iron oxide binding gel (orange) underneath a clear diffusive gel. A filter paper is placed on top (not shown).


Table 1: Advantages and disadvantages of P measurement using DGT in comparison to established methods..

Soil Test

- Measures P at soil pH
- More labour intensive

- No chemicals applied
- Slightly more expensive

- More applicable soil moisture
- Sensitive to laboratory variability or contamination

- Value needs no adjustment for other soil characteristics
- Small amount of data for certain crop types.

- Most accurate assessment of available P

Conventional soil P tests
- Cheap
- Measures P at set pH (e.g. 8.5)

- Large database of results
- Chemical applied to soil

- Available commercially
- Large soil dilution

- ASPAC certification available
- Requires PBI measurement to improve interpretation


Figure 2: Measurement of available P by DGT. The DGT device is placed upside down on a moist soil (~100 % water holding capacity) for a period of time (typically 20–24 hours).


How to interpret DGT values—wheat crops

Interpretation is based on field trials across southern Australia (primarily from SA, VIC, NSW with a couple of trials in WA and QLD) over five years (2006–2011) (figure 3). The high correlation coefficients (R2 values) (figures 3 & 4) show that there was a strong relationship between DGT P values and both relative yield and the rate of P required to achieve 90% of relative yield.


Figure 3: Relationship between DGT values and wheat response (relative yield).


The DGT P availability categories (table 2) reflect the soil’s capacity to supply P to the plant rather than the concentration of phosphorus in an extract.


Table 2: Categories of DGT values calculated from curve parameters in figure 3 and expected response of wheat to an application of P.

DGT Range
Colour Code

Very Low
0 – 20

21 – 45

46 – 56

57 – 100

> 100


What rate of P do I need based on DGT values?

If the DGT value for a soil is in the low to marginal range then an application of P is advised, otherwise maintenance P applications are sufficient (figure 4). The rate of P required to boost soil P to an adequate value for wheat crops is given in the table 3.


Figure 4: Relationship between DGT values and the P rate required to produce 90% of relative yield (RL).


Table 3: Required P rates at certain DGT values established from the curve parameters presented in figure 4.

DGT (ug/L)*
P Rate (kg/ha)












*No data obtained below DGT of 13.


DGT interpretation for other crop types

Interpretation is based on field trials across southern Australia (primarily from SA, VIC, NSW with a couple of trials in WA and QLD) over five years.


Table 4:. Classification of DGT values for other crop types established from the relationship of DGT values with response to P for each crop type. Ranges presented indicate how well DGT correlates with response of that crop as indicated by the method of range calculation.


Very Low
0 – 20
0 – 17
0 – 10

21 – 45
18 – 34
11 – 20

46 – 67
35 – 74
21 – 24

68 – 110
75 – 100
25 – 44

> 110
> 44


Further reading and references

Mason S, McNeill A, McLaughlin MJ and Zhang H (2010) Prediction of wheat response to an application of phosphorus under field conditions using diffusive gradients in thin-films (DGT) and extraction methods. Plant and Soil 337: 243–258.


Author: Sean Mason (The University of Adelaide)


The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.

The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.

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