Agronomy Advice

Planning spring phosphate applications for grass

By: Philip Cosgrave

Phosphorus is a key nutrient for grass, and its role in energy supply, root growth and tillering makes its availability crucial for grass growth in the spring. Although the plant's requirement for phosphate is small compared to nitrogen, its availability is essential.

Planning spring phosphate applications for grass
Planning spring phosphate applications for grass

Do you apply just nitrogen in early spring? The best strategy is in fact to also apply early phosphate for both grazing and silage. Phosphate availability is reduced at low temperatures in spring and In saturated fields phosphate availability will be further reduced. These conditions increase the solubility of soil iron and aluminium which in turn affect the availability of soil phosphate.  Phosphate uptake by grass in April and May can reach 0.6 kg per day. At this rate of uptake the release of phosphate from the soil reserve is not sufficient, therefore mineral phosphate is necessary to top-up soil available phosphate to maximise yield and herbage phosphate concentration. 

Apply mineral phosphate in early spring to top-up shortfall of available phosphate from soil reserves

On grazing farms, a portion of your total annual phosphate requirement should be applied in early spring and have the lion’s share of it applied by April. A fresh phosphate application boosts availability at a time when its natural availability is reduced by low soil temperatures in early spring and then by April and May, when grass growth is peaking, there is a very high demand for phosphate.

Typically the phosphate in fertiliser is 100% water soluble; this however creates its own problems. As soon as you apply water soluble phosphorus to a soil, this soluble phosphorus becomes slowly fixed by iron and aluminium.

The phosphate contained in YaraMila NPK’s is a mix of water soluble phosphate and di-calcium phosphate (DCP). This DCP is not fixed by the soil but becomes available as it is triggered by weak acids from grass root exudates. This ideal combination of two phosphate fractions rather than one results in superior availability of phosphate during April and May. 

The physical quality of the fertiliser is another point to consider. For example, if you spread a blended fertiliser such as 27-2.5-5 at a cwt/acre you will have only 25 granules that contain phosphate landing in every m2. This isn’t very many considering there are 200-300 grass plants growing per m2 and phosphate isn’t mobile like nitrogen or potassium is. The equivalent YaraMila compound fertiliser has 250 granules that contain phosphate landing in every m2. This gives the grass receiving the YaraMila CCF an advantage in spring when phosphate is needed most. 

The maintenance requirement for phosphorus on grazed swards is 10 units/acre at stocking rate of 131-170 kg/ha organic N, however if your grazing platform is growing 15 t of dry matter with 80% utilisation, then your maintenance will be closer to 15 units/acre.

Soil phosphorus levels impact greenhouse gas emissions

Until recently the role of soil fertility in mitigating greenhouse Gas emissions has up to now been based on the improving nutrient use efficiency. However, new research seems to show that soil phosphorus levels have a direct effect on soil nitrous oxide (N2O) gas emissions on permanent grassland. N2O is a very potent greenhouse gas, and hence the importance of this research.

It is thought that certain soil microbial populations that are more dominant in low soil phosphorus situations produce more N2O, and with increasing soil phosphorus levels these microbes become less dominant resulting in lower N2O emissions. It is very welcome that this research adds another positive dimension to the existing body of knowledge that supports the key role that soil fertility plays in the future sustainability of grass-based production systems.

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