Soil Phosphorus Availability

Most plants take up the bulk of their phosphorus requirement early in their life, in the seedling stage of annuals and early regrowth of perennials. While phosphorus is not mobile in soils, it is one of the more mobile nutrients in plants.

Phosphorus (P) is one of the nutrients, along with nitrogen (N) and potassium (K) required in large quantities by plants.

Most plants take up the bulk of their phosphorus requirement early in their life, in the seedling stage of annuals and early regrowth of perennials. While phosphorus is not mobile in soils, it is one of the more mobile nutrients in plants.

Phosphorus is required for cell division at growing points, and is particularly important in stimulating root development. Consequently, the best responses to phosphorus fertilizer are obtained if it is applied early, e.g. banded with or near the seed at planting in annual crops, and at the start of the main growing season in perennial crops and pastures.

Phosphorus is most available for uptake by plants in the pH range 6.5 - 7.5. At pH below 5.5, slowly soluble oxides of iron, aluminium and manganese form, reducing phosphorus availability, while at pH above 7.0, slowly soluble calcium phosphate is formed.

Phosphorus in soil is quite complicated as many soils can “tie up” large amounts of phosphorous making it unavailable to plants. Therefore, the ability to measure the availability of P in soil will assist in determining fertiliser requirements.

Current soil tests measuring phosphorous include Colwell P or Olsen P and generally require a Phosphorus Buffering Index (PBI) measurement to improve interpretation. PBI measures ‘P-sorption’ and involves mixing a quantity of soil in solution with a range of P amounts for a set period. The amount of P remaining in solution measures the soil’s ability to fix phosphorus. The index is adjusted for pH.

A new method of measuring soil phosphorous, Diffusive Gradients in Thin Films Phosphorus (DGT-P) is currently being introduced to commercial laboratories.

Nutrient Advantage has added this method to the range of analyses effective from the beginning of February 2013.

DGT-P is different to standard measures of soil phosphorus as it reflects the capacity of the soil to supply P in the soil rather than to the concentration of phosphorus in an extract. The DGT-P method aims to “mimic” the action of plant roots by only measuring the phosphorus in the soil that is accessible to the plant.

DGT-P was originally developed at Lancaster University in the UK. The analysis uses diffusion of available phosphorus in the soil toward a phosphorus sampler or “sink” (iron oxide gel) via a membrane which controls movement of phosphorus to the sink.  The gel and filter paper held within the sampler is then placed on moist soil for around 24 hours after which the amount of P bound to the gel is then measured.

Before offering DGT-P, Nutrient Advantage has completed extensive analytical testing with the assistance of Dr Sean Mason from the University of Adelaide who developed and tested the DGT technique, using Australian soils and different crop varieties.

For further details about DGT-P please see Facts sheet and other information found on Sean Mason’s website www.soilquality.org.au - data interpretation protocols have been developed (see below).

                

At present, DGT-P is more labour intensive and the consumables are more costly, making the analysis more expensive than conventional soil P tests.  Please note that DGT-P is not currently ASPAC Certified but work is currently underway in this regard. 

To date interpretation of DGT values have primarily been related to wheat crops based on the work of Dr Mason. There is also some data relating to barley and beans. Currently Nutrient Advantage is not offering fertiliser recommendations based on DGT Phosphorous test results through Nutrient Advantage Advice.

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