SOIL NITROGEN

Summary Soils from experiments receiving various treatments in the field were used to investigate the effects of different amounts and forms of organic matter in a clay and a sandy soil on the inter‐correlations between several laboratory methods of measuring available‐N. The correlations between some of these measurements, and the growth and N uptake of ryegrass grown in the glasshouse with and without added fertilizer‐N were also investigated. Seven clay soils and eight sandy soils were used, containing different amounts of organic matter of different kinds, as a result of various field treatments. For the sandy soils, one sample was taken from each plot of the experiment which allowed the effects of field variations on the values determined in the laboratory to be measured. The correlations between methods of measuring available‐N showed that measurements obtained by aerobic incubation, anaerobic incubation, and nitrogen extracted by boiling water, correlated best with one another. These measurements also correlated best with the performance of ryegrass in the glasshouse. N uptake by ryegrass from soils without fertilizer‐N always correlated better with measurements of available‐N than did dry matter produced. With added fertilizer‐N, dry matter correlated better with available‐N than did N uptake at the first cut but worse for the total of three cuts. Fertilizer‐N recovered in the grass at the first cut was significantly and negatively correlated with available‐N. Values of available‐N from the individual samples of the sandy soil showed that those for aerobic incubation had the largest standard error and had the greatest range expressed as a per cent of the mean value, and those for nitrogen extracted by boiling water least error and range. All methods correlated similarly with the performance of ryegrass in the glasshouse. All three methods successfully identified the treatments giving most available‐N.