An investigation of analytical procedures for predicting soil nitrogen supply to grass in the field

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Abstract
Procedures based on five methods of soil analysis for predicting potentially mineralisable N, and hence soil N supply, have been evaluated and compared with prediction based on total soil N. The effect of adjusting the analytical results for differences in soil temperature and water status between sites has also been examined. The evaluation was performed by calculating the extent to which the results accounted for variation in the amounts of N in the herbage of perennial ryegrass plots receiving no fertiliser N. The plots were situated at 18 sites throughout the UK: at some sites results were available for more than 1 year, giving a total of 29 site/years. The best prediction, accounting for 65 % of the variation in herbage N yield, was obtained when the amounts of ‚glucose’︁ extracted from the soils by 0.05M Ba(OH)2 were combined with the amounts of NO3‐N extracted by 2M KC1, and the combined values adjusted for soil temperature and water status. A procedure based on the measurement of the NH4‐N released by autoclaving the soils with 0.01M CaCL2 accounted for 51 % of the variation However, procedures based on (a) NH4‐N extracted by acid KMnO4, (b) non‐NO3‐N extracted by 0.01M NaHCO3, and on (c) measurement of the absorbance at 260 nm of NaHCO3 extracts, were no more effective than was analysis for total soil N (less than 36% of the variation accounted for when values were adjusted for soil temperature and water status).