Soil nitrogen. VIII.—some factors affectinga correlations between measurements of soil‐N status and crop performance

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Abstract
In 1959, samples of 17 soils from sites of experiments testing N fertilisers with barley were used to measure their N status in the laboratory, and 14 of the soils were used to grow ryegrass in pots. In the fresh soils, nitrate‐N and mineral‐N (ammonium‐N + nitrate‐N) were measured and also the increases on incubating the Iresh or re‐wetted air‐dry soil (Δ nitrate‐Nfresh or air‐dry and Δ mineral‐Nfresh or air‐dry).These measurements and similar ones obtained in 1958 were correlated with yields of barley grown without fertiliser‐N and with the grain response to 0.5 cwt. of N/acre as fertiliser.The laboratory measurements showed that mineralisable‐N increased more by drying soils at 25° than at 15‐18°, and the values determined after storing for 32 weeks before incubating, correlated slightly less well with crop performance than values after 12 weeks of storage. Yields of ryegrass and its content of N were correlated better with nitrate‐N than with mineral‐N in fresh soils.Mineral‐Nfresh and Δ mineral‐Nair‐dry were correlated most consistently with the yield of barley grain from unfertilised soils or its response to fertiliser‐N. With measurements on fresh soils, nitrate‐N correlated better than mineral‐N with crop performance; with air‐dry soils, Δ mineral‐N correlated better than Δ nitrate‐N. The slopes of the lines for the regressions of responses of barley on soil‐N varied between years.