March 11, 2021

There is still time to test fields scheduled for silage production this year if they have not been tested in the last four years. Every year, many soil samples from grassland are still below target for phosphate, potash or magnesium. Land primarily used for silage production can quickly become depleted, especially of potash, if slurry and manures are not regularly returned to those fields. Where three or four cuts are taken in a season, potash removal can be as high as 300 to 350 kg/ha, reducing soil reserves to well below target index 2, unless a combination of manure and fertilizer potash is applied. Potash is required to maintain the osmotic pressure in the plant cells. Without this, photosynthesis will be restricted, reducing the accumulation of dry matter in the plant. Symptoms of potash deficiency are rarely seen but the impact of a lack of potash will be apparent in disappointing grass yields.

The uptake of potash along with most other nutrients is restricted in acidic soils, so it is worth testing your soil’s pH now whilst you still have time to correct it. Areas of acidity can be rectified quickly by using granular lime at up to 500 kg/ha.

All grass leys need to regenerate their root systems in early spring so fertilizer programs should start with at least a combination of nitrogen and phosphate. Fields where good quantities of slurry and manures have been applied will be at less risk. It is important to check the nutrient content of manures in order to use them efficiently. The AHDB grass and forage recommendations booklet 3 gives estimated nutrient content figures, but as manures vary in content it is advisable to test the manures produced on farm for accurate figures. This enables a more effective grassland nutrition plan to be produced, improving nutrient use efficiency and profitability as well as reducing the carbon footprint of the farm.

It is important to apply the correct amount of nitrogen for each cut. As this depends on the combination of manures and fertiliser used, it is essential to understand the nitrogen content of your manures before you start.

Nitrogen and sulphur are key requirements for growth but to maximise yield the plant needs adequate supplies of magnesium too. Magnesium is a vital component of chlorophyll, which the plant needs for photosynthesis, and so its availability should not be overlooked on silage fields, where a soil index of 1 is needed. If manures are not available to rectify any deficiencies, build in an application of 100 kg of MgO one year in four. On grazing blocks, the magnesium content of grass must be maintained to reduce the risk of hypomagnesaemia. Grass samples can be tested during the season to ensure adequate magnesium levels.

All fertilizer planning decisions should be based on a knowledge of the soil’s nutrient status and of the manures available for application across the farm. Monitoring the mineral content of grass during the season will confirm whether adequate phosphate, potash and magnesium are present in the forage. There is still time to update fertilizer plans for this year to improve nutrient use efficiency and grass yields to improve farm margins.

Contact your agronomist, advisor or soil sampling provider or call NRM direct to arrange for soil samples to be tested for pH, phosphate, potash and magnesium, and for tissue testing during the season.