ROOT-GROWTH AND NITRATE UTILIZATION OF MAIZE CULTIVARS UNDER FIELD CONDITIONS

In a 2-year field study conducted on a high fertilized Gleyic Luvisol in Stuttgart-Hohenheim significant differences among 10 maize cultivar s were observed in soil nitrate depletion. The different capability of the cultivars to utilize nitrate particularly from the subsoil was po sitively correlated with (a) shoot N uptake at maturity, and (b) root length density (L(V)) in the subsoil layers at silking. ''Critical roo t length densities'' for nitrate uptake were estimated by (a) calculat ing uptake rates per unit root length (U), (b) subsequent calculation of needed nitrate concentration in soil solution (($) over bar C-1) to sustain calculated U according to the Baldwin formula, and (c) reduci ng measured L(V) and proportionate increase of U until needed concentr ation equaled measured concentration. Uptake rate generally increased with soil depth. ''Critical root length densities'' for cultivar Brumm i (high measured root length densities and soil nitrate depletion) at 60-90 cm depth ranged from 7 % (generative growth) to 28 % (vegetative growth) of measured L(V) Measured root length density of each other c ultivar was higher than ''critical root length density'' for Brummi in dicating that the root system of each cultivar examined would have bee n able to ensure N uptake of Brummi. Positive relationships between ro ot length density and nitrate utilization as indicated by correlation analysis therefore could not be explained by model calculations. This might be due to simplifying assumptions made in the model, which are i n contrast to non-ideal uptake conditions in the field, namely irregul ar distribution of roots and nitrate in the soil, limited root/soil co ntact, and differences between root zones in uptake activity. It is co ncluded from the field experiment that growing of cultivars selected f or high N uptake-capacity of the shoots combined with ''high'' root le ngth densities in the subsoil may improve the utilization of a high so il nitrate supply.