CRITICAL NITRATE LEVELS OF SUGAR BEETS ESTIMATED FROM ANALYSIS OF PETIOLES AND BALDES, WITH SPECIAL REFERENCE TO YIELDS AND SUCROSE CONCENTRATIONS

Abstract

Sugar beets planted in pots of soil and in the field were fertilized with increasing amts. of N. In the pot expt., a comparison of petioles or blades taken from old, recently "matured," or immature leaves showed that petioles from recently "matured" leaves were preferable for estimating the N status of sugar beet plants. This preference was based primarily on ease of sampling, wide range of nitrate N concn. within the petioles, and plant physiological considerations. Estimations of the nitrate N concn. at which beet growth decreases (critical concn.) within recently "matured" petioles gave values on the dry basis of approx. 1000 ppm. for field beets and 3000 ppm. for beets in the pot expt. Evaluation of the critical nutrient level of a crop must be made at a time when some plants have a deficiency and others have an abundant supply of the plant nutrient under study. Collection of petiole samples prior to visible differences in growth resulted in large differences in nitrate concn. but without any relationship to observed beet wts. When samples were collected late in the season, the petioles were devoid of nitrate even though the beet wts. differed in accordance with earlier N fertilizations. At the beginning of the season, the nitrate concns. of the petioles in the field expt. were highest in the beets with the highest N fertilization; thereafter the petioles decreased in nitrate concns. until at the end of the season there were no large differences in nitrate values. In both pot and field expts., N fertilization at first failed to increase the nitrate concn. of the petioles while the beet wts. increased rapidly. With further increments of N, nitrate and beet wt. increased simultaneously, and finally, increases in nitrate took place without any change in beet wt. The nitrate concns. in both expts. were correlated inversely with sugar concns. of the beet roots.