Mineralizable-N in the soil under various leys and its effect on the yields of following wheat

Abstract

Soil samples taken in the autumn after ploughing ryegrass, clover, and ryegrass/clover leys were used to measure the mineral-N (ammonium-N + nitrate-N) in the fresh soil (mineral-N_fresh), the increase in mineral-N on incubating the fresh soils (Δmineral-N_fresh), and the increase in mineral-N on incubating the re-wetted air-dry soils (Δmineral -N_air-dry). Mineral-N_fresh and Δ mineral-N_air-dry were measured on further soil samples taken the following spring. Values of Δmineral-N_air-dry, not only correlated best with grain yields and N uptakes by wheat without fertilizer-N, but also with yield responses and fertilizer-N recovered from fertilizer-N applied to the winter wheat.Treatment of the ley altered measurements on samples taken in the autumn but not those taken the following spring.Soil samples taken in the autumn 1960 from under three-year grass leys were used to measure mineral-N_fresh, Δ mineral-N_fresh and Δ mineral-N_air-dry Spring wheat was grown in 1961 followed by spring barley in 1962. Further soil samples were taken in spring 1962 after cultivations were complete and before the barley was sown or fertilizers applied.A mineral-N_air-dry was the best measurement to use on soils from under grass leys. Values depended on grass species, and were increased by N applied to the ley. Differences had largely disappeared 18 months later. A mineral-Nalr.dry was positively correlated with grain yields of spring wheat grown both with and without fertilizer-N, and with the yield response or the nitrogen recovered from, a dressing of 56 lb N/acre.With fertilizer-N yields of winter wheat after the mixed leys tended to the same maximum value independently ofmineralizable-N in the soil. After grass leys maximum yields of spring wheat given fertilizer-N increased with increasing mineralizable-N in the soil.