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 Раk Chong (Pc) soil series, which are two of the most representative soils of the maize-producing region of Thailand. Subsoil nitrate-N (N03 – 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 N03 – 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 N03 – 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 N03~-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 N03~-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. (Soil Science 2007;172:861-875)
CCURATE N fertilizer recommendations for maize production are important for maximizing productivity and profit while minimizing environmental impact of fertilizer use (Fageria et al., 1997). Nitrate is often the dominant source of N for maize because it generally occurs in higher concentrations than ammonium N, and it is free to move to the roots by mass flow. Attanandana et al. (2004) found that maize yield with and without N was not significantly different in some soil series. In some maize growing soils of Thailand, there is the trend to have high subsoil nitrate contents because maize has been rotated with legumes and also because some farms were overfertilized with N, which can increase the residual nitrate. On-farm demonstrations have generally shown good estimation of N fertilization requirements, but there are also situations where overestimates or underestimates of N fertilizer requirements occur (Attanandana et al., 2004). Preliminary attempts to apply N models to predict N requirements for maize, such as Decision Support System for Agrotechnology Transfer (DSSAT-N) and the Nutrient Management Support System (NuMaSS) decision aid, were not successful when predictions far exceeded field-determined N requirements (R. S. Yost, personal communication, 2004). One hypothesis for the overprediction was that the decision aids predicted N requirement based only on the surface nitrate content, whereas the crop actually benefitted from residual nitrate, much of which was in the subsoil or at least deeper than the 0- to 20-cm layer.