Optimizing phosphorus intake, a review of knowledge

1 - What is the role of phosphorus in plants?

After nitrogen, phosphorus (P) is the second most important element for crop yield and quality.

It is essential

• for photosynthesis
• for cell growth
• for plant respiration
• for the formation of seeds, pollen, etc.

Phosphorus creates extremely strong bonds with other atoms. It is in these bonds that living creatures store their energy in the form of ATP (Adenosine Triphosphate). We can therefore say that phosphorus is at the heart of the energy metabolism of all living creatures.

2 - How does a plant absorb phosphorus?

To be absorbed by a plant, phosphorus must be in a soluble or ionic form, HPO₄^– or HPO₄^2-. But this form is unstable, and phosphorus is strongly attracted by the cations in soil. Unfortunately, when it binds to these cations, it becomes insoluble and can no longer be assimilated by plants.

This effect is linked to the pH of the soil.

In soils with an alkaline pH (pH>6.4), phosphorus will bind to calcium ions. In these soils, calcium and phosphorus are then hardly available, because they are bound together in the form of tricalcium phosphate, which is insoluble and therefore cannot be assimilated.

In more acidic soils, phosphorus binds with iron, manganese and aluminum. Red soils take their color from the iron oxide present. Anyone who cultivates these soils knows the challenges associated with plant availability of P. The triple positive charge of iron combining with that of phosphorus, the resulting iron phosphate is both insoluble and very difficult to break down.

Some microorganisms are able to convert insoluble P in the soil into forms available to plants through the excretion of enzymes or organic acids.

This is notably the case of some rhizobacteria, but also of the mycorrhizal fungi.

3 - What happens in the case of phosphorus deficiency?

The addition of phosphorus at sowing favors vigor at start-up and stimulates the growth of the root system, which will explore the phosphorus reserves in the soil more rapidly.

In the case of phosphorus deficiency, a plant slows down or stops growing, which can reduce the yield by 5-20%.

An extreme lack of phosphorus will lead to the death of the plant. That is why giving plants an effective phosphorus supply via the roots is so important.

Phosphorus has limited mobility in the soil and deficiencies can occur even when soils are rich in P if

• the root systems are poorly developed
• if plants are exposed to biotic or abiotic stresses that limit their nutrient uptake

4 - Sources of phosphorus

There are two external sources of phosphorus to cover the needs of crops:

5 - Optimize phosphorus intake?

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Soaring fertilizer prices, supply difficulties… many agricultural systems depend on phosphorus inputs, and their shortages could impact food security.

It is also important to know that phosphorus is far from being used totally by the crops during input.

Part of it will often be held back and rapidly blocked in the soil.

As seen above, phosphorus will bind with mineral elements such as calcium in alkaline soils (pH>7.0) or with iron, manganese and aluminum in acidic soils (pH<6.4) and take on a solid form that cannot be assimilated by plants.

According to CSIRO (Commonwealth Scientific and Industrial Research Organization – the Australian governmental organization for research) in the 6 weeks following its application, more than 75% of the soluble phosphate is lost. Billions of dollars of applied phosphate are now locked up in agricultural soils.

As long as investment is made in fertilizing the soil,
one can ensure that the plants will assimilate the phosphorus

Solutions exist for optimizing phosphorus intake: Some microorganisms (rhizobacteria, mycorrhizae) are true links between the soil and the plant. They have the ability to stimulate root density so that the plant absorbs more elements, but also to solubilize phosphorus by breaking its links with other atoms, making it bioavailable.

Microorganisms do not replace phosphorus inputs, which remain necessary to balance crop exports, but these solutions optimize the resources already present or allow the most to be made of phosphate fertilization.

Microorganisms are therefore true allies to crops, weaving complex relationships with plants.

Lallemand collaborates with numerous universities and research institutes around the world to study the role of microorganisms, to measure their interest in agronomy and to select those which will be the tools of successful and resilient agriculture.