Soil nitrogen. VI. —correlations between laboratory measurements of soil mineral‐N and crop yields and responses in pot and field experiments

Abstract Soil samples from sites of field experiments testing N‐fertilisers for barley, potatoes, kale and grass were used for pot experiments with ryegrass and also to measure ‘available’‐N in the laboratory. Mineral‐N in the fresh soil (Mineral‐N fresh ) and the increases when fresh or re‐wetted airdry soils were incubated (Δ Mineral‐N fresh or Δ Mineral‐N air‐dry ) were determined. These were more highly correlated with organic C than with total N content of the soils. Among ‘available’‐N measurements, Mineral‐N fresh and Δ Mineral‐N air‐dry were most correlated with one another. The yields of dry matter and N uptake by ryegrass grown in unfertilised soil, or with two rates of applied N, were significantly correlated with Mineral‐N fresh or Δ Mineral‐N air‐dry at each level of nitrogen, but not with Δ Mineral‐N fresh . The unfertilised yields of barley in field experiments were significantly correlated with Mineral‐N fresh , Δ Mineral‐N air‐dry , and total N; responses by barley to 0.5 cwt. of N/acre were significantly negatively correlated with all three ‘available’‐N measurements and with total N. Unfertilised yields of potatoes were significantly correlated with the three ‘available’‐N measurements and responses to 1.0 cwt. of N/acre were significantly negatively correlated with Δ Mineral‐N air‐dry and total N. Mineral‐N fresh and Δ Mineral‐N air‐dry are suggested as useful measurements for advisory purposes.