SUBSOIL NITRATE AND MAIZE ROOT DISTRIBUTION IN TWO IMPORTANT MAIZE SOILS IN THAILAND

Abstract

Early attempts to apply N models and decision aids for maize were not successful when predictions far exceeded field-determined N requirements. One hypothesis for the overprediction was that residual nitrate could be accumulating in the dry off-season and that maize rooted sufficiently deeply to use this nitrate. However, there was a lack of information on the content of subsoil nitrate and the distribution of maize roots with depth. Field experiments were conducted in sites of the Lop Buri (Lb) and Pak Chong (Pc) soil series, which are two of the most representative soils of the maize-producing region of Thailand. Subsoil nitrate-N (NO3−-N) was measured before and after maize was grown; NO3−-N status was assessed after heavy rains occurred and maize root distributions were measured. Subsoil NO3−-N levels at the two sites increased with increasing N rate. Before maize was planted maximum levels of NO3−-N were found at the 20- to 40-cm and 0- to 20-cm depths of Lb and Pc soils, respectively. Subsoil NO3−-N declined at both sites after maize was more than 40 days old. Nitrate-N status was not significantly different between different depths; however, the maximum NO3−-N levels in Lb and Pc soils were found at 40- to 60- and 20- to 40-cm depths, respectively. At 40 days after emergence, maize roots were more voluminous in the 49- to 62- and 43- to 50-cm depths for the Lb and Pc sites, respectively; roots clearly grew into the soil zones where NO3−-N was high. It is probable that N availability from subsoil NO3−-N diminished the fertilizer N requirement of maize. These results support the hypothesis that failure to include subsoil NO3−-N concentrations in the decision aids led to the overprediction of N fertilizer needs, which had been predicted solely from surface NO3−-N levels. In retrospect, a comparison of soil taxonomic classifications would have suggested the possible importance of including subsoil nitrate in the decision aids.