Long-term fall in phosphate use raises concerns

The development of soil-release agents and protected forms of phosphate means growers can meet crop needs without having to apply large quantities of TSP, SSP or rock phosphate.

Disincentivised by higher prices for phosphate (P) fertiliser, Britain’s growers have gradually reduced application rates to the extent that many soils now contain worryingly low levels of this essential macronutrient.

Data from the British Survey of Fertiliser Practice (BSFP) revels how, at the national level, phosphate use has fallen by 68% since 1983 and by 62% for potash (K) (see Fig. 1).

This implies that growers have relied on soil reserves to meet crop needs, but the extent of this drawdown means that the soil nutrient balance – the difference between what is applied and what is taken off by the crop – has fallen to a historical low.

According to DEFRA data, the overall phosphorus balance of UK soils in 2023 was a surplus of 2.9 kg/ha, only a small increase of 0.1 kg/ha on 2022 when the balance reached its lowest level since the UK annual time series began in 2000. The dominance of arable agriculture in England explains most of the downward trend. In 2023, the phosphorous balance for soils in England was just 1 kg/ha, a small increase from the 0 kg/ha in 2022 but still only a mere 9.5% of the level in 2000 (see Fig. 2).

This aggregated data, however, hides the wide variations seen in soil analysis. Of the roughly 30,000 samples collected nationally by Agrii and RHIZA in 2022, more than a fifth (21%) fell below index 2 for phosphate, while more than a quarter (26%) were below index 2 for potassium. Crops grown on these soils would require large applications of both nutrients (see Table 1).

Conversely, 16.1% of phosphate samples analysed were at an index 4 or higher, while 24% of potassium samples were at an index 3 or higher. In both cases, growers could continue to save on the seasonal application, a so-called ‘P & K holiday’.

Avoid the ‘hunger gap’

For Ben Wainwright, Agrii fertiliser product manager, the downward trend in phosphate use is concerning. Growers are only likely to see a visible deficiency if it is severe. There may be a shortfall, only seen in harvested yield, which goes unaddressed.

“For it to be visibly apparent, such as in the form of purple stems or leaves, the deficiency has to be severe,“ Mr Wainwright says.

“In these situations, an application high in P2O5, such as Triple Super Phosphate, is the best course of action, although it is best applied preventatively rather than as a cure.

“The optimal approach would be to apply a product with a protective coating such as Agrii Protected Phosphate (APP). This is a coating on TSP or DAP granules that reduces lock-up in the soil by calcium (Ca), aluminium (Al), iron (Fe) and magnesium (Mg) and increases the quantity of phosphate that is biologically available to the crop. Trials indicate that application rates can be cut by up to 20% depending on the soil index,“ he adds.

Advisers often talk about the ‘hunger gap’, which is the difference between optimum availability and visible deficiencies, but it is hard to detect and is often masked by other, often related conditions.

Release P reserves

For those growers with good soil reserves, typically Index 2 or better, an application of Agrii-Start Release as a spray to the soil surface at either the pre- or post-emergence timing up to growth stage 30, will increase the quantity of soil available phosphate 60-days after application.

“Agrii-Start Release is a unique soil phosphorous activator for all soils with less than 30% organic matter. It can be used on high or low pH soils and increases the availability of other nutrients, including zinc, manganese, boron and copper.“

Widely trialled across most crops and soil types, Agrii-Start Release has consistently given a positive response (see Table 2).

“The yield gains in winter wheat and spring barley have been impressive. Across a range of soil indices, its application has consistently delivered a positive return on investment,“ Mr Wainwright says.

To make efficient use of all sources of crop nutrition first requires an understanding of what is already available in the soil if unnecessary applications are to be avoided. The introduction of the CSAM1 nutrient management action as part of the Sustainable Farming Incentive (SFI) has helped to increase the area of soils analysed annually.

“The CSAM1 action offers a payment of £6/ha to encourage growers to test soils. This is roughly equivalent to a broad-spectrum soil test so can cover the expense while delivering the benefit of a wider analysis beyond the requirements of CASM1,“ Mr Wainwright says.

“Good data is the basis of an informed decision. Knowing your exact growing conditions aids your ability to tailor applications and optimise production,“ he adds.

For any questions about this article, please email Ben at ben.wainwright@agrii.co.uk