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Abstract
Root growth and nitrate uptake of spinach and kohlrabi were investigated in the field and in solution culture as affected by nitrate supply. Optimum Nmin supply of spinach and kohlrabi was 250 and 150 kg Nmin/ha, resp. Lower fertilization rates decreased shoot yield of both crops, whereas root yield was nearly doubled for spinanch but slightly diminished for kohlrabi. Thus, spinach had about twice as much root surface/g shoot at low than with optimum nitrate supply.
Maximum rate of nitrate uptake (Imax) expressed per cm2 cylindrical root surface determined in solution culture was similar to the average uptake rate (UR) in the field for kohlrabi if optimum nitrate quantity was supplied. For spinach, however, UR was 5 times higher than Imax. Including root hair surface in addition to cylindrical root surface resulted in a close agreement between Imax and UR for spinach. Both crops absorbed nitrate at about maximum rate in the field. The nitrate concentration in soil solution which was necessary to meet the uptake rate of roots in the field is discussed.