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Why di-ammonium phosphate is not the best choice of phosphate fertiliser

By: Jez Wardman

Phosphorus is considered the second most important nutrient after nitrogen in terms of its influence on plant growth and development. Whilst phosphate availability in soil is influenced by many factors including pH, however, the actual form of phosphate can be one of the greatest influences on availability.


Why di-ammonium phosphate is not the best choice of phosphate fertiliser
Why di-ammonium phosphate is not the best choice of phosphate fertiliser

A traditional approach to phosphate management has involved the application of 'straight' products such as di-ammonium phosphate (DAP), mono-ammonium phosphate (MAP) or triple superphosphate (TSP) or alternatively using PK fertilisers such as 0-20-30.  However, this may not be the best choice because not all phosphates are the same and this can have quite an influence on how they behave in the soil.

When it comes to applying phosphate then surely it’s simple – phosphate is phosphate right??

There are actually different forms of phosphate (P) that could be in a solid fertiliser product and they all behave slightly differently so it is useful to know the different ways they act.

Ortho-phosphate (Ortho-P) is found in DAP, TSP, MAP as well as most NPK compounds. Ortho-P is immediately available to the crop which means that the crop can utilise the P straight away. This is great if the crop has the ability to take all that P up in one go but the likelihood is that the biomass isn’t enough at that early spring timing to utilise all the P. Unfortunately if the P isn’t used it can become unavailable to the plant (fixed) because it becomes attached to iron or aluminium ions in the soil or precipitates out with calcium ions. This also happens quite quickly meaning that up to 40% of the Ortho-P you applied isn’t available to the plant only 2 weeks after you applied it.

Poly-phosphate (Poly-P) is our next form of P, this form takes a short while to become plant-available as it has to break down into Ortho-P first. This means it is better able to penetrate the soil and is protected initially from being fixed. Therefore products with Poly-P are able to supply P over a longer period of time without any losses.

Di-calcium phosphate (DCP) is the final form of phosphate. DCP is not prone to fixation in soils which means it can provide P to crops over a longer period.  DCP availability is triggered by weak acids produced by plant roots which breakdown the DCP into a plant-available form of P. Citric acid is exuded by the plant’s roots when they are ‘looking’ for nutrients, therefore DCP only becomes available when the plants are actively exuding acids, meaning it can stay in the soil, without fixing, until required.

A mixture of different forms of phosphate ensures season-long supply.

Typically as crops begin to develop they rely on soil available phosphorus for continued growth. Phosphate availability in the soil is influenced by many factors including pH, other nutrients such as aluminium, iron and calcium, soil moisture and temperature. It is important therefore to ensure that phosphate is available to avoid limiting growth during the period of rapid with shoots and roots development.

This season-long availability is best achieved by combining different forms of phosphate: Ortho-P for immediate availability, DCP to allow plants to actively trigger P availability when needed and Poly-P to protect the P from being fixed to supply P over a longer time.  This system using different forms of P for extended phosphate availability is known as P-Extend and is common to all YaraMila compound fertilisers.

YaraMila fertilisers all contain mixtures of different types of phosphate which means they are able to supply phosphate throughout the growing season, ensuring the crop is never lacking in this key nutrient.

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