Planning spring phosphate applications

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
Planning spring phosphate applications

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 grass phosphate uptake 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 P requirement should be applied in early spring and have the lion’s share of it applied by April. A fresh P 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 P.

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. 

Grazing maintenance rates for phosphorus on dairy farms is 14 kg/ha or 11 units/acre at a stocking rate 170 kg/ha of organic nitrogen, and 19 kg/ha where the stocking rate is above 170. If a recent soil test indicates that your soil P index is 2, then an extra 10 kg/ha (8 units/acre) of P is required for build-up. If the soil test shows a P index of 1, then an extra 20 kg/ha (16 units/are) of P is required over and above the maintenance dressing.

On drystock farms, the grazing maintenance rate for P is 10kg/ha (8 units/acre) for a stocking rate of up to 170 kg/ha of organic N. Phosphorus build-up rates for index 1 and 2 soils are 20 kg/ha and 10 kg/ha respectively. These build-up rates should be applied in addition to the maintenance rate.

Soil test results are invaluable for optimising phosphate applications.

More information on grassland nutrition

Recommended grassland fertilisers

The following compound fertilisers are recommended for grassland and supply combinations of nitrogen, phosphate, potash and sulphur (NPK+S) in appropriate ratios for grassland and in the case of the booster products with added selenium for animal health.