To obtain large yields of grass much N fertilizer is usually needed, and when such grass is cut and conserved large amounts of K soon are removed from the soil. Some soils contain so little ‘available’ K that yields are soon limited unless grass is given enough K fertilizer, but other soils supply more K and so need less K fertilizer. Soils differ so greatly in their ability to supply crops with K that it is difficult to generalize about the fertilizer K requirements of grass, especially as the use to which the grass is put greatly affects the soil K ‘balance sheet’. On average, grass in England grows well for not much more than 6 months each year. This implies that, when cattle are maintained on hay or silage for much of the remainder of the year, on average one-third of the grass product each year may have to be cut, removed from the field, and conserved. Much of the N, P and K contained in this grass is not returned to the soil.