Nitrogen fertilization applied as sidedressing in corn can be optimized by considering additional soil characteristics as complementary indicators of soil N availability. The objectives of this study were to evaluate the potential use, critical level and best sampling time of the soil nitrate (NO3--N) concentration to predict soil N availability in corn, as well as to verify if the determination of the soil ammonium (NH4+-N) concentration, in addition to NO3--N concentration, enhances the efficiency of N availability evaluation. A field experiment was conducted in two growing seasons (2002/03 and 2003/04) in Eldorado do Sul, state of Rio Grande do Sul, Brazil. Different levels of soil N availability were simulated by the application of five N rates (0, 50, 100, 200, and 300 kg ha-1) 20 % at sowing and the rest sidedressed at V3. The experiment was structured in a randomized block design with four replications. NO3--N, NH4+-N and soil mineral N (NO3--N + NH4+-N) concentrations in the 0-30 cm soil layer at V3, V6, V10 and at silking, as well as corn grain yield were evaluated in both growing seasons. In general, NO3--N soil concentrations were sensitive to the applied N rates, mainly in V6 when the best relationship between NO3--N and corn grain yield was found. The critical level of soil NO3--N concentration, above which a response to N application is unlikely, was estimated at 20 mg kg-1 in the studied soil, at growth stage V6. The determination of soil NH4+-N concentration, in addition to soil NO3--N concentration, improved the prediction of N availability. A better relationship of soil mineral N (NH4+-N + NO3--N) than of NO3--N concentration with corn grain yield was observed. These results indicate that the development of kits for instant determination of soil mineral N concentration is necessary. NO3--N and NH4+-N as complementary indicators of soil N availability for corn appear promising and studies should be developed under different climate and soil conditions to adequate their use to the management of sidedressed N in corn.
Zea mays; N efficiency use; soil nitrate tests; grain yield; plant growth stages
