The potassium requirement of crops grown in rotation

A 5‐year trial measured the effect of varying rates of potassium fertiliser applied in combination with two rates of nitrogen to a rotation: potatoes, kale, barley, grass cut for conservation and wheat. Residual values of potassium applied during the rotation were measured on a crop of spring wheat planted in the sixth year. Crops grown in the rotation responded increasingly to potassium as follows: kale, barley, grass, wheat, potatoes. The higher rate of potassium applied (706 lb/acre a K in 5 years) was justified by the total value of crops harvested; this return was derived mainly from potatoes and wheat. The higher rate of potassium applied was insufficient to maintain the soil in K balance (applications less removal in crops) when the higher rate of nitrogen was used. Soil analysis indicated no residues from the lower rate of K applied but the higher rate raised available K levels appreciably. The wheat crop grown after 5 years under the rotation showed residues from the lower rate of K applied equivalent to 55 lb/acre fresh K but no additional residue from the higher rate.