November 16, 2021

Assessing Phosphorus Requirements in High PBI Soils

The DGT-P test is provides a new way to assess phosphorus requirements in high PBI soils, such as forest gravels.

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Summit Fertilizers is working with APAL (SA) to build a database of WA cropping responses on high PBI soils to compare the emerging DGT-P soil test with traditional Colwell P approaches. The DGT-P soil test has potential to redefine crop P requirement predictions on phosphorus-fixing soils. Image supplied courtesy of Dr Sean Mason, Agronomy Solutions.

At Bannister, Summit Fertilizers has a thought provoking phosphorus (P) trial that aims to dig deeper into the best soil testing procedures and also which P rates are likely to give the highest economic return to growers with high PBI forest gravel soils.

High PBI Soils

The Phosphorus Buffering Index, or PBI, is a measure of the soil’s ability to bind P.

Soils with a high PBI require more P, because more of what is applied becomes bound to soil particles. PBI is related to the number and type of ‘exchange’ sites in the soil, which ultimately comes down to soil texture and composition.

Clay soils have smaller particles which means a larger surface area, and more sites for P adsorption. Higher levels of certain compounds in the soil, such as iron oxides and aluminium oxides mean that P binds to soil particles more strongly.

Colwell P

For a long time Summit has offered the Colwell P test as a measure of the amount of P available for plant uptake.

With the Colwell P test however, values can be somewhat independent of the soil’s ability to bind phosphorus.

Hence, a Colwell P test should be interpreted in association with a PBI test. This can be problematic however, as these methods have been shown to overestimate available P on certain soil types, including calcareous or acidic soils, or where iron or aluminium are present in high concentrations.

Summit Fertilizers’s partnership with independent soil and plant analysis laboratory APAL, offers a promising new procedure to gauge plant-available soil P.


DGT-P is a more recently released P test offered by APAL. It differs from more conventional soil extraction methods in that it mimics the action of plant roots.

An iron oxide gel disc is placed on a saturated soil sample. The gel acts as a sink, binding forms of P that are able to diffuse through the soil solution and through an additional gel membrane, just like a cell membrane in root uptake (see main image).

The amount of P bound to the gel is then measured.

An advantage of the DGT-P test is that the inherent properties that govern P availability in the soil will determine the test result, so testing for a second correcting factor (such as PBI with Colwell P) is not necessary.

Forest Gravel Research

The Bannister trial site includes P rates from nil up to 50kg/ha in 10kg/ha increments. It is part of GRDC’s NPK nutrition project. Summit has been involved with GRDC for the past five years investigating P responses in WA soils. At the Bannister site, soil was sampled on May 10th down to a final depth of 50cm. Soil analysis was done every 10cm. Colwell P (40 mg/kg), PBI (>200) and DGT-P (11 μg/L) were all measured through Summit’s inSITE soil testing program. The data was back from APAL by May 17th and Scepter wheat was sown two days later at a rate of 85kg/ha. Seeding P fertilizer was banded below the seed and the site received good background nutrition of nitrogen, potassium, sulphur, copper, zinc and manganese.

Figure 1: P rates and NDVI readings taken at Bannister on August 9th 2021.

To-date the trial has produced large differences in plant biomass which should translate to significant differences in yield (figure right). It will also be interesting to see how the different soil test results (Colwell P, PBI and DGT-P) compare in terms of predicting the crops response to applied P.

DGT-P vs the Colwell P Test

Video: Summit Area Manager (Albany West) Mark Ladny evaluates the DGT-P vs Colwell P test on Forest Gravels, Shire of Cranbrook.