Laboratory methods of measuring soil nitrogen status and correlation of measurements with crop responses

Abstract

Soil samples from three field site experiments testing nitrogen (N)‐fertilizers for wheat, corn and cotton were used to establish whether laboratory measurements of available N in soils were correlated with yields of unfertilized crops and with crop responses to added fertilizer‐N in pot and field experiments and to assess the value of such measurements for advisory purposes, Available N in soils was measured by (a) biological methods: mineral N content of fresh soil (Min‐Nf), increase in mineral N in fresh soil incubated for 21 days at 25'C (AMin‐Nf), increase in mineral N of rewetted air‐dry soil and incubated as above (?MinNad), and (b) chemical methods: N estimation by boiling water, N estimation by boiling water and potassium sulphate solution and estimation of N extracted by cold, dilute barium hydroxide solution. Available N estimated by the biological methods was correlated with organic carbon (C) and total N content of the soils. Among available N measurements, Min‐Nf and AMin‐Nad were slightly correlated. The yields of dry matter and N uptake by ryegrass grown in unfertilized soil or with two rates of N applied were correlated with Min‐Nf and AMin‐Nad. Grain yields from unfertilized plots or responses to fertilizer N, in the field experiments, were also correlated with Min‐Nf and AMin‐Nad. It was concluded that Min‐Nf and AMin‐Nad can be used to predict available N for advisory purposes.