Evolução do N residual e do pH num Podzol da Campina de Faro, por efeito da fertirrega gota-a-gota num pomar de jovens laranjeiras

Abstract

A young ‘Lane Late’ orange orchard grafted on ‘Carrizo’ citrange was planted in 1999, in a Gleyic Podzol, located at Campina de Faro (Algarve), with the application of five N rates, applied through drip fertigation, in increasing levels throughout 3 years (N1 - 10, 15, 20; N2 -20, 30, 40; N3 - 40, 60, 80; N4 - 80, 120, 160; N5 - 160, 240, 320 g N per tree). Annually, in the beginning and in the end of each cultural cycle, soil samples were collected, in the wetted zone, for chemical characterization. The main objectives of the present work were to study the evolution of residual soil N and pH. Residual N in the soil was not significantly affected by N rates, decreasing gradually with the depth, as it was expected in such a sandy soil (96% of sand). Nitrogen was significantly affected by the sampling date, N rate and depth, particularly for nitric N, which decreased at the end of each winter, in consequence of the rainy season. Nitrate levels were significantly greater for the higher N rates, suggesting that such rates were excessive. In the end of the 1st year soil pH increased significantly, probably due to the continuous and localized supply of alkaline irrigation water, rich in HCO3-, Ca2+ and Mg2+. At the end of the 2nd and 3rd years, soil pH maintained identical value as for the initial soil pH, which can be explained by the increasing levels of acidifying fertilizer applied, as well as the acidifying solutions used to clean the irrigation pipes. As for the concentration of the remaining nutrients, although there were significant differences, the values determined in the soil samples were low or very low, not contributing to plant nutrition.