We would all likely assume that the difference between various phosphate fertilisers are fairly minimal and not worth agonising over. After all, phosphate is phosphate, surely? Not so, according to Natalie Wood, country arable agronomist at Yara.
“There are actually different forms of phosphate (P) that could be in a solid fertiliser product,” says Ms Wood. “All of them behave slightly differently. Knowing this in advance can be extremely useful for your farm, helping you to select a product that will help support the results you expect to see.”
Offering an insight into different forms of phosphate found in fertilisers and the differences one might expect as a result, Ms Wood says: “Ortho-P is found in DAP, TSP, MAP as well as most NPK compounds. Ortho-P is immediately available which means that the crop can utilise the P straight away. This is perfect if the crop has the ability to take all that P up in one go. However, it’s likely that the biomass won’t be enough to utilise all the P at an early spring timing.”
If P is not used it can become unavailable and becomes fixed or precipitates out with ions in the soil, she adds.
“This can all happen quite quickly; up to 40% of applied Ortho-P can be lost to the plant only two weeks after you applied it.”
On the other hand, polyphosphate takes a while to become plant-available as it has to break down into Ortho-P first.
Ms Wood says: “As it is able to penetrate the soil and is initially protected from being fixed, products with Poly-P are able to supply P over a longer period of time with fewer losses.”
Dicalcium Phosphate (DCP) requires citric acid to break down into a plant-available form. The plant exudes this acid from its roots when they are ‘looking’ for nutrients, meaning that DCP only becomes available at that stage. As a result, DCP can stay in the soil, without fixing, until required, Ms Wood adds.
“It’s not a case of one being better – they’re just different. Any products which contain all three forms; Ortho-P, Poly-P and DCP is the surest way to supply phosphate to your crop for longer, making sure it’s never lacking that key nutrient. Whatever you decide, P is vital.”