Inputs of Nutrients and Lime for the Maintenance of Fertility of Grassland Soils

Keywords: Phosphorus, Potassium, Calcium, Decreasing inputs, Soil depletion, Young/short-tern grass, Permanent grass, Yields, Nitrogen use efficiency.

To maintain soil fertility and grass production there must be sufficient readily available phosphorus (P) and potassium (K) in soil, which must also be at a satisfactory pH status. To maintain a financially viable farm enterprise the level of fertility of the grassland soil must be related to the amount of grass required to provide adequate, high quality feed for the type and number of animals kept.

The large root system of well established grass, although mostly in the top 20 cm of soil, is very efficient at taking up nutrients. Where arable crops and short-term grass are grown in rotation, the exploitive ability of the grass to extract nutrients from the soil must not be allowed to jeopardise the yields of following arable crops.

The average annual yield of intensively managed permanent grass over a 3/4 year period often exceeds that of similar duration short term grass because of the small yields in the establishment year of the latter. The large crops of grass dry matter remove more P and K and maintaining soil fertility is made more difficult because P and K, especially P, do not readily move down the soil profile. The species composition of permanent grass swards is greatly affected by the interaction of soil pH and nutrient supply.

In the UK, more P and K is used on short term grass than on permanent grass but the overall use of mineral P and K on grassland is less than that on arable crops. There has been an appreciable decrease in the use of P and K on grass in recent years. The extent to which this is due to improved allowances being made for P and K returned to grass in slurry and manures, and during grazing, is not clear.

Omission of P and K fertilisers for a few years should not adversely affect yields unless the soil PK status is low. However, long-term experiments at Rothamsted show that the continuation of a policy of not applying P and K will eventually cause yield losses once available soil P and K reserves have declined below the critical value appropriate for the soil and management system. At low levels of available P and K, nitrogen use on grassland is inefficient and this is both economically and environmentally undesirable.

A E Johnston and P R Poulton, IACR Rothamsted, Harpenden, UK.

C J Dawson, Chris Dawson & Associates, Strensall, York, UK.

M J Crawley, Imperial College at Silwood Park, Ascot, UK

40 Pages, 12 Figures, 16 Tables, 32 Refs.