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

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 com 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 NO3-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.