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 cultivars were observed in soil nitrate depletion. The different capability of the cultivars to utilize nitrate particularly from the subsoil was positively correlated with (a) shoot N uptake at maturity, and (b) root length density (L(V)) in the subsoil layers at silking. ''Critical root length densities'' for nitrate uptake were estimated by (a) calculating 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) reducing measured L(V) and proportionate increase of U until needed concentration equaled measured concentration. Uptake rate generally increased with soil depth. ''Critical root length densities'' for cultivar Brummi (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 cultivar was higher than ''critical root length density'' for Brummi indicating that the root system of each cultivar examined would have been able to ensure N uptake of Brummi. Positive relationships between root 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 in contrast to non-ideal uptake conditions in the field, namely irregular distribution of roots and nitrate in the soil, limited root/soil contact, and differences between root zones in uptake activity. It is concluded from the field experiment that growing of cultivars selected for high N uptake-capacity of the shoots combined with ''high'' root length densities in the subsoil may improve the utilization of a high soil nitrate supply.