Potassium in soils under different cropping systems:2. The effects of cropping systems on the retention by the soils of added K not used by crops

SUMMARYThe K balance, the difference between K added as fertilizer or farmyard manure (FYM) and K removed by the crops, was calculated for soils from the Classical and Ley-Arable experiments at Rothamsted and for the Woburn Ley-Arable experiment, for the duration of each experiment. Linear regressions on K balance accounted for 78% of the variation in exchangeable K (Ke) and for 83% in K uptake by ryegrass (KP) in the Classical experiments, for 56 and60% respectively in the Ley-Arable experiments at Rothamsted, and for 39 and 6% in the Woburn Ley-Arable experiment.Regressions of Ke and Kp on K balance suggested that, in the Rothamsted Ley-Arable experiments, rather more than half of the K balance remained extractable by ryegrass from the plots with a rotation of crops, and apparently all of the K balance from those under continuous grass. About one-fifth of the K balance remained extractable by ryegrass from the soils in the Rothamsted Classical experiments and soils given FYM retained K slightly better than other soils. With all soils about half the K extractable by ryegrass was exchangeable to ammonium acetate.The plots with FYM or under continuous grass contain more organic matter than other plots in the same experiments. The following possible effects of increasing the organic matter content of the soils were investigated by calculating the multiple regressions of K, and KB on K balance with either percentage of organic C, total CEC, or organic CEC:(1) loss of K decreased by increasing the water retention and lessening leaching;(2) improved K retention by increasing the total cation exchange capacity (CEC) available for K absorption;(3) improved K retention by a mechanism arising from the different selectivities of clay and organic matter for K relative to Ca.In the Classical experiments, where organic matter usually increases because of FYM additions, effect (2) seems the most probable, perhaps because the K given in the FYM was already absorbed by organic exchange sites. In the Ley–Arable experiments, where the K was given mainly as soluble K fertilizer and the organic matter develops mainly under grass, effects (1) or (3) seemed to operate, probably simultaneously.The Woburn Ley-Arable experiment had no continuous grass plots, the soils differed little in organic matter content and no deductions could be made.