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Choosing the correct phosphorus fertiliser

The main way of correcting phosphorus deficiency in soils is to add a phosphorus-containing fertiliser. However, your current situation will determine the actual form you should opt for. This post is solely focused on these phosphorus-containing fertilisers. We will be publishing additional blog posts about the symptoms and challenges of phosphorus deficiency, and also on how to release phosphorus already bound in the soil.

There are a number of different sources of phosphorus found in fertilisers. The chemists at fertiliser companies have chosen the specific ones used in a formulation based on 1. performance on the plant, 2. stability in the blend and 3. cost.

Here is a non-exhaustive list of common phosphorus-containing ingredients used in fertiliser:

  • Rock phosphates – ‘Beneficiated’ (cleaned up) rock phosphate is permitted for use as an organic fertiliser. This is before it is converted to conventional fertilisers by a series of chemical reactions. Whilst organic, beneficiated rock phosphate is not readily soluble, so is rarely used in conventional agriculture.
  • Superphosphate and Triple superphosphate- These are two forms of mono-calcium phosphate. They are both commonly found in granular arable fertilisers. They suit this role as unlike the phosphates listed below they do not dissolve straight away, instead only a gram or two will dissolve for every litre of water they are exposed to. Superphosphate is created by reacting rock phosphates with sulphuric acid, and triple-superphosphate is created by reacting rock phosphate with phosphoric acid.
  • Monoammonium phosphate (MAP) and diammonium phosphate (DAP) – both used in liquid NPK blends (compound fertiliser) and also sold as a straight granule product. They are created by reacting phosphoric acid (sourced from rock phosphate with ammonia from the Haber-Bosch process).
  • Potassium phosphate – mainly used in horticulture either on its own as a PK fertiliser, or blended into soluble fertilisers (both liquid and granular). Useful in situations where growers want healthy flowers, fruits and roots, but without stimulating excessive vegetative growth, and so preferable to MAP/DAP as that contains nitrogen. Can also be used to adjust the pH of foliar solutions and the effects on the crop. Alkaline forms will close plant stomata in periods of stress, whilst acidic potassium phosphate will open stomata and promote transpiration when this is desirable.
  • Potassium pyrophosphate – mostly horticultural but also in some arable foliar formulations. Similar use to mono potassium phosphate, but the phosphate releases over a period of days and thus controls its release and limits lock up.
  • Phosphoric acid – It might surprise you to find out that this hazardous acid is found in both conventional and organic fertilisers! In fact some arable foliar fertilisers based on phosphoric acid can have a pH of 1.0, and many organic fertilisers are also not far off this level too. 

It is not just humans that need to be cautious when it comes to phosphoric acid fertilisers, as the plant can also be harmed by improper use. Any acidic fertiliser can strip calcium out of the leaf in the same way acidic rain drains a plant of essential nutrients.

Phosphoric acid is found in organic fertilisers because it is permitted for use to break down biological material in organic fertilisers. This is the reason you may see a fish fertiliser with very high phosphorus levels. It is not the biological material that is especially high in phosphorus, it is the acid used to extract and solubilise it.

  • Dicalcium phosphate and tricalcium phosphate: found in animal feed and in controlled-release fertiliser granules. However, these are not normally plant available. In controlled release fertiliser granules they are used as a binding agent (along with ferric phosphate. The only way to release dicalcium phosphate and ferric phosphate in the soil is to use Eutrema’s Phosphorus Liberator (more on this in the next post).

Controlled-release fertilisers are a relatively expensive product, so are usually only used in horticultural, not agricultural crops.

Understanding the diverse array of phosphorus fertilisers is pivotal for optimising soil health and crop productivity. Stay tuned for further insights on managing phosphorus deficiency and enhancing soil fertility in our forthcoming posts.

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