Phosphate in the spotlight

2 November 2020

Eurofins Agro is publishing a series of articles on important nutrients for horticultural crops. Our first article was about nitrogen. This time we are looking at phosphate. 

Phosphorus (P) is one of the most important plant nutrients. In the early stages of a crop, it is an important element in root development. As the season progresses, a phosphate deficiency often causes dark coloration on the leaves, stunted growth, and delayed fruit ripening. In greenhouse horticulture, analyzing phosphorus in the soil and in nutrient solutions can help to optimize fertilization.

Phosphorus in the plant

The element phosphorus has the following functions in the plant:

  1. Phosphate plays an important role in supplying the plant with energy. It is a component of the energy carriers adenosine triphosphate (ATP) and sugar phosphates. These energy carriers are transported throughout the plant as packets of energy. Wherever maintenance or growth processes are taking place, these substances provide the energy the plant needs.
  2. Phosphate plays an important role in biological membranes, keeping out certain unwanted substances and drawing in beneficial ones. 
  3. Phosphate is a structural component of the nucleic acids DNA and RNA, which play a major role in a plant’s genetic makeup.
  4. Phosphate is stored in seeds as phytin. Phytin is crucial to the plant during germination: it forms the first phosphate reserves, which the young plant can tap into to build up its root system and energy. The plant cannot yet take up enough of this nutrient from the soil at this early stage because its root system is not yet well developed.

Phosphorus in the soil

Phosphorus occurs in the soil in various forms:

  1. Dissolved in soil moisture. This is the phosphate that is directly available for uptake by the plant. In the laboratory, Eurofins Agro measures directly available phosphate in greenhouse soil with 1:2 water extraction. Phosforus can also be measured in a calcium chloride (CaCl2) extract. This measured quantity of phosphate is known as the plant available quantity. 
  2.  As precipitation:
    - below a pH(KCl) of 5.0, phosphate primarily precipitates with iron and aluminum. 
    - above a pH(KCl) of 6.0, phosphate primarily precipitates with calcium.
    Phosphates that precipitate with iron and aluminum are quite firmly fixed in the soil and are therefore poorly available to the plant. Phosphate that precipitates with calcium first enters the soil in a so-called labile fraction. This means that this phosphate can also return to a soluble state. However, this solubilization process is very slow. This quantity of phosphate is determined in the analysis via the P-Al method, also known as the potential available quantity of phosphate. 
  3. As a stable fraction. This solid form of phosphate can only become soluble by transitioning to the labile phase. However, the transition from the stable to the labile phase takes even longer than from the labile phase to the solubilized phase. This process can take years. 

The transition from the one phosphate phase to the other in the soil is determined by chemical balances. These are highly dependent on the acidity (pH) of the soil. The organic matter content of the soil also has an effect on this. The higher the organic matter content, the more phosphate can be fixed.

Applying phosphorus fertilizer

Plants take up phosphorus in the form of H2PO4-. Phosphate can be applied as artificial or organic fertilizer. Biostimulants can also be used to improve phosphorus uptake. 

An artificial fertilizer adds phosphate to the soil, for example in the form of the water-soluble fertilizers monopotassium phosphate or monoammonium phosphate. Phosphorus can also be applied as polyphosphate. In this form, the phosphorus is packaged in a molecule, sometimes combined with a cation such as potassium.

If the conditions are such that the plant cannot absorb sufficient phosphorus, it can be helpful to use biostimulants. These often contain probiotic rhizobacteria and fungi (mycorrhizae). These microorganisms stimulate root formation and break down organic matter. The fungi work with the plant and increase the surface area of the roots, allowing the plant to take up more phosphate. In addition, these products contain fulvate, which also occurs in humus and makes phosphate easier to absorb. 

Analyzing phosphorus for horticulture

In greenhouse soil, we measure the reserves of phosphorus (P-Al) in the soil and the plant available phosphorus. The soil reserves are measured in samples taken before the crop is planted.

In potting soil, coir substrate and organic mixtures, Eurofins Agro measures phosforus  in the 1:1.5 extract. The pH and EC are not measured in the filtrate but in a mixture of potting soil and demineralized water.

In other samples, such as water, nutrient solutions and artificial fertilizer, Eurofins Agro measures phosphorus directly in the sample or in an extract. That is then given as P or the oxide P2O5.  Phosforus total, P-total,  is given in the crop analysis reports (dry matter method), while P is given in the plant sap analysis reports.

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