Volatilização de N-NH3 na cultura de milho: I. efeito da irrigação e substituição parcial da uréia por sulfato de amônio

A field experiment was carried out on a dark-red latosol (Oxisol) of the Centro de Pesquisas Novartis-Seeds, in Uberlândia, State of Minas Gerais, Brazil. The objective of this study was to evaluate the effects of irrigation, side-dressed application of urea, and urea partially substituted by ammonium sulphate on losses of NH3-N by volatilization in corn crop. The fertilizers were applied at 25 and 36 days after planting. The treatments, arranged in randomized complete block design, were: (i) control, (ii) urea, with irrigation before the two applications, (iii) urea, with irrigation after the two applications, (iv) urea + ammonium sulphate (rate N:S = 2.1:1) at the first application, and only urea at the second application, with irrigation before the applications, and (v) with irrigation after applications. Nine samples of volatilized NH3-N were taken, using semi-open static collectors, which were installed after the first N application, at intervals of 4 to 5 days. With irrigation after N application the NH3-N gaseous losses were 40.6% for urea and 23.0% for urea with partial substitution by ammonium sulphate. With irrigation before N application the NH3-N gaseous losses were 42.8 and 38.6%, respectively. Even though the N losses did not show significant differences among the treatments, the partial substitution of urea by ammonium sulphate was positive when the irrigation was performed after the N application. This treatment also showed the largest stalk diameter, the greatest plant high and the highest leaf-N concentration. At harvest, only the fertilized treatments increased yield compared to the control. The relationship between NH3-N volatilized and corn yield established by an inverse linear regression showed that 19.3 kg-1 of grains were lost per every kilogram of volatilized nitrogen.