Plant

Phosphorus is essential for the general health and vigour of all plants. It is required for plant respiration and photosynthesis as well as cell division and growth.

Phosphorus is a key component of Adenosine tri-phosphate (ATP) – a compound which combines with sugars to generate energy for all living things. This energy is crucial for establishing a strong root network and to promote early shoot growth in young plants, which is why phosphorous is often applied around the time of crop establishment.

Like nitrogen, phosphorus is also a component of several plant proteins including DNA and RNA – the molecules which carries the cell’s genetic code.

In legumes, phosphorus is important for the development and function of nitrogen fixing root nodules called rhizobia which recover nitrogen from the atmosphere.

Soil

Phosphorus is often described as the ‘lazy nutrient’ due to its severe lack of mobility in the soil. Unlike other nutrients which can move several millimetres once dissolved into the soil solution, phosphorus typically travels around one millimetre and so is very reliant on roots retrieving it.

On applying phosphorus to the soil surface, up to 75% can become fixed or ‘locked up’ within the same season and so a large proportion of the phosphorus within the soil is unavailable for plant uptake.

While this is partly due to phosphorus being highly reactive within the soil, the problem can be worsened where the soil pH is too high or too low. In acidic conditions, phosphorus becomes less available due to strong bonding with iron and aluminum and in alkaline conditions, phosphorus becomes less available due to reactions with calcium and magnesium ions.

Phosphorus which is ‘locked up’ contributes to the soil phosphorous reserves and should become available at a later date, providing the soil conditions allow.

Plants deficient in phosphorus appear dark green in colour with purple pigmentation. Since phosphorus is mobile in the plant, the bottom leaves are always affected first and show the earliest signs of deficiency

Where is the risk of deficiency highest?

• Heavy soils
• Acidic or high pH soils
• Where there is limited root structure

Testing

To help prevent phosphorus deficiency, soil testing should be completed at least every 3-5 years.

Maximum cereal and grass yields are typically reached in soils which contain 16-25mg/litre phosphorus – otherwise known as index 2.  Where soil phosphorus levels are below this threshold, farmers should work to build soil reserves towards the target index, which can take several years.

While leaf analysis can be used to give an indication of plant phosphorus levels at a point in time, it is not useful in predicting the nutritional requirement of a crop over an entire season and so should be used in conjunction with soil analysis rather than in isolation.

Run-off/soil erosion

Most phosphorus is removed from the soil by plant uptake, but a small amount – around 5kg/ha on average – can be lost through soil erosion and run-off into water courses.

Because phosphorus is very immobile in the soil, where there is a history of phosphorus application, the concentration of phosphorus tends to be highest in top few centimetres.

When soil particles move - either due to soil erosion or run-off - there is potential for phosphorus to be lost from the system.

Phosphorus run-off from soil into water courses can cause eutrophication – algal blooms – which are very damaging to aquatic ecosystems. However, the majority of phosphorus in UK waters originates from industrial sources and sewage outlets.

 This risk of run-off and soil erosion is heightened where there is lack of ground cover, poor soil structure, on sloping ground and where there is strong winds or heavy rainfall.

Right Product

OEP can help prevent phosphate lock up, meaning more phosphate is taken up by the plant and is less vulnerable to run-off and erosion losses. This can be particularly useful on soils with a sub-optimal pH.

GAFSA BioPHOS is an ideal phosphate fertiliser for low P, acidic soils in areas of high rainfall where a steady release of P is required to maintain productive pasture, crops or trees.

Right Time

The timing of phosphorus applications is dependent on the phosphate index. Generally, phosphate is most effective when applied during early stages of growth.

Right Place

Placing phosphorus close the seed has been shown to improve crop performance, particularly for crops like potatoes which have a weak root system.

Right Rate

Where the phosphate index is at target, enough phosphorus should be applied to meet the demands of the growing crop – otherwise known as maintenance. Where the phosphate index is below target, maintenance levels of phosphate should be applied, plus additional phosphate to help build soil reserves closer to the optimum level.

Farmers should also take into account crop offtakes -  the amount of phosphorus which is expected to be removed in crop material (E.g. straw, grain, silage) - when considering phosphorus requirements.