A comparison of several nitrogen- availability indices on four Natal maize-producing soils

Four methods used in obtaining nitrogen-availability indices were compared using soil and plant samples taken from maize trials conducted on four widely differing Natal soils. NO 3 − N was determined after extraction of the soil with NH 4 HCO 3 , Ca(OH) 2 and KCl, using a specific ion electrode, phenoldisulphonic acid and brucine respectively. In addition, NO 3 -N and NH 4 + -N were determined after steam distillation of KCl extracts. The measurements so obtained were correlated with the rates of nitrogen application and leaf-N content at flowering. The four NO 3 − -N determinations were all closely correlated with each other and with the level of N applied. The overall correlation across soils between N applied and increase in NO 3 − -N exceeded 0,85. Inclusion of NH 4 + - N did not materially improve the relationship between soil test and N application rate. NO 3 − -N was also well correlated with leaf-N content. The levels of NO 3 − -N measured in the absence of fertilizer N relate well to previously measured N responses on these soils.