Fresh Thinking on Phosphate in the Spring


Fresh phosphate in the early winter or spring can increase wheat yields by as much as 1t/ha even where soil P indices are more than adequate. But it must be in the most available form.

This is the finding of several years of Agrii research on different soil types across the country as part of the Best of British Wheat Initiative to identify and overcome factors standing in the way of modern, high output crop performance.

“Conventional triple super phosphate (TSP) applications at our AgriiFocus Technology Centre trials in Wiltshire give us no yield benefit at soil indices of 2 or more,” notes senior agronomist, Andrew Richards. “This is just as RB209 suggests.

“However, when we employ the specialist coating, P-Reserve to protect TSP from lock-up we see just how valuable extra phosphate can be. It’s all a matter of mineral availability; something that the official guidance still tells us nothing about.

“Yet we know phosphate is very immobile in the soil. What’s more, its availability to crops can be seriously restricted by a whole range of factors – including soil texture, stone content, pH and organic matter; crop rooting; and the weather.

“We also know that wheat has a particular demand for phosphate to support root and shoot development and grain fill. So we clearly need to switch our fertilisation thinking from the traditional approach of maintaining soil indices to supporting crop requirements. Especially where we’re looking to move-up a gear in crop performance.”

For early-October sown wheat, AgriiFocus trials at the same rates of total nitrogen show diammonium phosphate (DAP) to be a better source of available P in the spring than TSP.

They also show coating TSP with P-Reserve can boost an 11.5 t/ha crop by over 0.5 t/ha to significantly outperform DAP.

An even greater benefit – up to 1t/ha – was recorded from phosphate availability-enhanced TSP in early-November crops at the Technology Centre. In this case a single late November application proved more valuable than either a single spring application or a 50:50 split.

“Crop availability is at least as important as soil index in phosphate fertilisation,” stressed Agrii trials manager, Dr Syed Shah responsible for the AgriiFocus research. “And later-sown crops with smaller root systems in colder ground are especially responsive to early applications of available P.

“Even at soil indices of 3 we still see responses to fresh available phosphate. We also see responses in trials on both neutral (pH 7) and acid (pH 6) ground. So it’s not just an issue of calcareous soils with their particular nutrient availability issues.

“Our work shows the value of applying 100 -150 kg/ha of available phosphate to wheat in the early winter or spring even if soil indices suggest a maintenance dressing of only 60-80kg/ha is required,” he said.

“This is not surprising since, even under the most favourable conditions only around 25% of the phosphate applied is taken up by wheat in the season in which it is applied. In most cases this is very much lower due to antagonism from other nutrients like calcium in high pH soils and iron and aluminium in low pH soils, as well as nitrogen across all soil types.


“For spring applications, my preference would be DAP for the greater availability we’ve seen under the most challenging high pH conditions. Where TSP is being used – as it has to be in closed period for N and almost always is in variable rate P and K application regimes – it should definitely be treated to ensure sufficient crop availability.”


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