The mobility and transformation of soil nitrogen and the relationships between soil and plant nitrogen and yield at different times following application of various nitrogen fertilizers to sweet corn

Abstract

The concentration of NH4-N, NO3-N and their sum (mineral N) were monitored 12 times in 1 or 2 weekly intervals in the soil under a sweet corn crop. The samples were taken on the fertilizer band and to depths of 200, 400 and 600 mm. The NO3-N concentration of the sap expressed from the midrib of the leaf opposite and immediately above the primary cob (sap NO3-N) and that of midrib dry matter (midrib NO3-N) were determined weekly. Under the low rainfall and optimized irrigation of this trial the concentration of mineral N in soil to the depth of 400 mm or more was a good predictor of yield response to application of N at 10 of the 12 sampling times. The concentration of either NH4-N or NO3-N in the soil to any depth and the concentration of mineral N in the surface 200 mm correlated with the yield at only a few times of sampling. The concentration of mineral N in the top 200 mm of soil 1 or 2 weeks after top-dressing of N was highly correlated to yield. The concentration of sap NOS-N and midrib NO3-N decreased continuously until harvest. Both these concentrations were significantly correlated with the rates of basal and top-dressed N in most sampling times. These were also strongly correlated to yield 1 or 2 weeks after N top-dressing. Ammonium sulfate, ammonium nitrate and urea gave similar responses in sap NO3-N and midrib NO3-N and in soil nitrogen after 5 weeks when nitrification of fertilizer NH4-N was complete. These observations indicated that soil mineral N, sap NO3-N and midrib NO3-N all offer potential as techniques to predict the yield response of sweet corn to N application. The sap NO3-N test was simpler, quicker, cheaper and more consistent than other tests.