Phosphorous (P) is a plant essential macronutrient which is vitally important for plant establishment and growth. Plants access phosphorous from the soil solution in its soluble form, phosphate. A lack of phosphate could result in poor rooting, slow establishment and delayed spring growth.
Macronutrient contributing to 3-6% of a a plant's nutritional makeup
Promotes successful crop establishment and early spring growth
Deficiency symptoms include poor rooting and purple discolouration of older leaves
Phosphate run-off is the most likely loss pathway
Best applied during early stages of growth, close to the seed if establishing a crop
Phosphorous containing fertilisers include Superphosphate (16%), Di-ammonium phosphate (20%), GRP (12%) and Granuphos (11.5%)
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, phosphorous 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.
This is referred to as “soil P build-up”. Once this soil P level has been achieved, the objective is to maintain the soil P level by replacing the P removed in animal products or in hay or silage. This is called “soil P maintenance”. Where the soil P level is greater than that required, there is an increased risk of P loss to the environment. In such cases, P application should be avoided so that the soil P level will decline to the optimum level, so that this risk is reduced. It is advised that wherever possible, animal manures should be recycled to minimise chemical fertiliser needs.
Phosphorus fertiliser advice for grazing is based on the following 6 steps:
Determine maximum P allowance, and confirm that the P fertiliser requirement of the holding is compliant with SI 31 of 2014.
P fertiliser rates advised for build-up to soil P index 3 are presented below. Note that application of build-up rates may be required over a number of years for P-deficient soils to move from Index 1 to 2 and from index 2 to 3. At very extensive stocking rates, or in areas governed by specific legislation such as national heritage areas (NHAs) or special areas of conservation (SACs), it may be appropriate to fertilise soils to a level such that index 2 is maintained.
|Soil P index||Mineral soils|
|4||Do not apply chemical P|
Each year, P is removed in either animal produce (meat, milk) or as silage or hay. This P must be replaced by applying “maintenance rates” of P as recycled P from concentrate feeds used, recycled animal manures or chemical fertiliser P. Soil P maintenance rates advised for grazed pasture should be equal to the amount of P that leaves the holding as product. In general, 1 kg of P is required to replace the P removed in either 1000 litres of milk or 100kg of liveweight gain.
|Grassland stocking rate|
(kg/ha Org N)
Rates of P fertiliser based on concentrate feed usage level of zero.
To calculate the available P contained in slurry and farmyard manure, multiply the quantity of each material that is applied to grassland by the P content of that material.
|Organic fertiliser type||Total P contained in 1 tonne³ (kg/t)|
|Dungstead manure (cattle)||0.9|
|Slurry (layers 30% dry matter)||2.9|
|Layers (55%) dry matter||5.5|
Concentrate feed must be considered as a source of P for grassland since most of the P consumed will be recycled in animal excreta to the soil. A deduction of 5kg P should be made from the total P advice for each tonne of concentrate feed that is used on the farm. A deduction for concentrate feed used can be made across the entire grassland area as follows:
|Grassland Phosphorus Index stocking rate (kg/ha/year)||Index 1||Index 2||Index 3||Index 4|
|Available Phosphorus (kg/ha)|
|Grassland stocking rate greater than 170 kg/ha/year|
Phosphorous 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, phosphorous typically travels around one millimetre and so is very reliant on roots retrieving it.
On applying phosphorous to the soil surface, up to 75% can become fixed or ‘locked up’ within the same season and so a large proportion of the phosphorous within the soil is unavailable for plant uptake.
While this is partly due to phosphorous being highly reactive within the soil, the problem can be worsened where the soil pH is too high or too low. In acidic conditions, phosphorous becomes less available due to strong bonding with iron and aluminum and in alkaline conditions, phosphorous becomes less available due to reactions with calcium and magnesium ions.
Phosphorous 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 phosphorous 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
To help prevent phosphorous deficiency, soil testing should be completed at least every 3-5 years.
Maximum grass yields are typically reached in soils which contain 5.1 - 8.0mg/litre phosphorous while maximum cereal yields are typically reached in soils which contain 6.1 - 10.0mg/litre phosphorous– otherwise known as index 3. Where soil phosphorous 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 phosphorous 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.
Most phosphorous 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 phosphorous is very immobile in the soil, where there is a history of phosphorous application, the concentration of phosphorous tends to be highest in top few centimetres.
When soil particles move - either due to soil erosion or run-off - there is potential for phosphorous to be lost from the system.
Phosphorous run-off from soil into water courses can cause eutrophication – algal blooms – which are very damaging to aquatic ecosystems.
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.
Avail 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.
GRP 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.
The timing of phosphorous applications is dependent on the phosphate index. Generally, phosphate is most effective when applied during early stages of growth.
Placing phosphorous close the seed has been shown to improve crop performance, particularly for crops like potatoes which have a weak root system.
Where the phosphorus index is at target, enough phosphorous should be applied to meet the demands of the growing crop – otherwise known as maintenance. Where the phosphorus index is below target, maintenance levels of phosphorus should be applied, plus additional phosphorus to help build soil reserves closer to the optimum level.
Farmers should also take into account crop offtakes - the amount of phosphorous which is expected to be removed in crop material (E.g. straw, grain, silage) - when considering phosphorous requirements.
|Granuphos||11.5%P + 7.5%Mg + 32.2%Ca||The world’s most reactive soft rock phosphate in granular form, which provides sustained release phosphate|
|Ground Rock Phosphate (GRP)||12%P||The world’s most reactive soft rock phosphate in powder form, which provides sustained release phosphate|
|Superphosphate||16%P||A water soluble, granular phosphate|
|Avail||1-16%||Phosphorous containing grades with an Avail coating to prevent phosphate fixation or ‘lock-up’|