How to make the most of phosphate fertilisers this autumn

As margins squeeze tighter, many growers are looking at inputs and considering where to make cuts, and phosphate may be one area of focus.

Tim Kerr, head of nutrition at agronomy firm Hutchinsons, shares key strategies for managing phosphate this autumn and suggests if it is possible to get away without applying any.

He recommends starting with the basics: regular soil testing. Where a soil index is above index 2, for most crops it is possible to “take a phosphate holiday”.

See also: Why soil assessment should come before cultivations

If the index is below 2, the index or maintenance system employed in RB209 has long shown the positive economic benefits to applying phosphate, highlights Tim.

“This is still the case at today’s price levels. At index 2, the maintenance system recommends you replace the P removed by the crop; above index 2, for most crops it is possible to take a phosphate holiday,” he says.

Using organic manures to build the levels of exchangeable phosphorus (P) in the soil can be hugely beneficial.

“Expensive phosphate fertilisers are not the only way of adding P to the soil. Organic manures can supply valuable levels,” he says.

“There are useful recycled products that help to bolster the exchangeable phosphate in the soil – the fraction that the roots can convert into utilisable P.”

Organic manures are a great source of P © Tim Scrivener

pH and temperature limitations

However, it is important to be aware of how phosphate behaves in the soil, so growers can use it and manage it as effectively as possible.

“It is widely recognised that phosphate availability in the soil is dependent on factors such as soil pH and temperature.”

Regular pH sampling is important, which should be ideally maintained at about pH 6.5-7 to avoid nutrient lock-up.

“Availability of P can reduce by half if the pH falls from 6.5 to 5.5. An effective liming policy will avoid the pitfall of nutrients being in the soil but not being accessible by the crop in the ground,” he says.

As soil temperatures drop, so does the relative availability of phosphate – as much as 80% of the soil P is rendered unavailable once the soil drops below 10C.

Phosphorus deficiency on a cereal leaf © Hutchinsons

“So it is easy to see that even though a soil test may show an index 2 for phosphate, the amount available can be compromised in certain conditions.

“Phosphate is a limpet-like stubborn nutrient holding on tight to the soil and refusing to move very far at all.”

It relies on the roots searching it out and then it still needs the roots to convert it into soluble P by secreting organic acids, says Tim.

The soil test aims to reflect this, providing a good estimate of what a crop can extract from the soil. However, soil reserves in the rhizosphere can soon be depleted by root uptake.

To increase the efficiency of the phosphate applied, Tim recommends placing phosphate near the seed, as this allows for an increase in the amount of available nutrient without having to build the soil reserves.

“This is a particularly effective way of utilising a nutrient that is both costly and complex to manage – and is especially true as the soil temperature falls through the autumn.

“Using microgranular forms of phosphate further improve the efficiency of the fertiliser, as this increases the area of contact between the soil, the roots and the phosphate.”