SOIL MINERAL NITROGEN AVAILABILITY TO YOUNG MAIZE PLANTS AS RELATED TO ROOT LENGTH DENSITY DISTRIBUTION AND FERTILIZER APPLICATION METHOD

Minirhizotron observations from 4 experiments with maize in the Wageni ngen Rhizolab showed strong vertical and lateral root density gradient s during the first 9 weeks after emergence. Root length density (Lrv), as determined in core samples 9 weeks after emergence, was positively related (P < 0.01) to the number of roots counted concurrently on min irhizotron walls (n). Lrv/n ratios were 1.13, 1.76, 0.99 and 1.21 cm c m(-1) in the successive experiments. Subsequently, root numbers counte d on previous dates in each experiment, were converted into root lengt h density values and related to thermal time. According to this relati on, the average vertical root extension rates were 0.7 and 1.1 cm d(-1 ) at temperatures of 13 and 16 degrees C, respectively. Corresponding values for the lateral extension rate were 1.0 and 1.6 cm d(-1). Calcu lations indicated that the nitrogen (N) content of a 9 weeks old maize crop could generally not be explained by mass flow only. Transport di stances between roots and mineral N in the soil, may thus have restric ted the availability of N as suggested by preferential uptake of miner al N from soil compartments with a high root length density. The recov ery of N was only slightly improved by fertilizer N positioning close to the plant as compared to broadcast N or placement of N halfway betw een the rows. Recoveries based on the difference method and the isotop ic dilution method, yielded similar values. Dry matter yields were not significantly affected by the application method of N. Apparently, th e root extension rate and the initial availability of N in the soil pr ior to the application of fertilizer-hi, were sufficient to cover shoo t demand under the prevailing circumstances.