Sugarbeet Yield and Quality as Affected by Nitrogen Level

Thii study was conducted, under several climatic and soil conditions, to determine the effect of N level on beet yield and quality and to further develop and me both soil and tissue test methods for predicting N fertilizer needs for efficient refined sucrose production. Previous studies indicate that N fertilizer needs for =Wm= sucrose production may be predicted by con. sidering yieldpotential and all N sources. SuFarbeets (Beta vulgaris L.) were grown under field conditions at N fertilizer levels varying from 0 to 448 kg N/ha on six sites throughout southern Idaho to de. termine root yield, sucrose percentages, sucrose yield, purity index, and plant N uptake in relation to the residual NO•N, mineralizable N, fertilizer N, and petiole NOr N. These experiments demonstrated that the N fer• tither needs of sugarbeets can be determined by relating the root yield potential to the measured residual NOrN plus a measured or estimated mineralizable N level for an area. Optimum N level from all available soil and fertilizer sources has been found to vary between 5 to 6 kg/metric ton of beet roots produced. Using data from the current experiment and a previous study, N fertilizer could be predicted within 56 kg N/ha of that needed for maximum sucrose yield in 85% of the sites using measured NOrN and mineralizable N levels, 67% using measured NO•N and average mineralizable N levels, and only 12.5% using recommendations by fieldmen. Linear cow+ relations were found between the total available N, total plant N uptake, other plant N variables, and root quality factors, like percentage sucrose and imErti7 index. These relationships confirm previous fin and will be useful for predicting root quality, optimum arrest date, and for verifying recommended fertilization prat. tiers. The use of the proposed soil and tissue test will improve root quality and sucrose production, as well as production rigs:1mq, that will economically benefit the consumer, producer, and manufacturer. Additicomi index words: N test, Petiole analysis, N uptake. N ITROGEN has the greatest influence of all the mineral elements on root quality and sucrose production of sugarbeets (Beta vulgarts L.). Sugarbeets grown with inadequate N generally have a high sucrose percentage and low impurities, but root and sucrose production are limited. Too much N increases root impurities while reducing sucrose percentage and, consequently, limits refined sucrose production (7). Optimum amounts of soil and fertilizer N are desirable for adequate top and root growth, while maintaining sufficiently high sucrose percentage and purity for profitable sucrose extraction and yield. • Soils vary widely in their ability to supply N for plant growth. This N-supplying potential varies with soil type, past fertilization. and cropping history, as well as rainfall received and the irrigation water applied that affects the extent of N loss by leaching from soils (6, 13). 'Contribution from the Western Reeon, ARS-USDA; Univ. of Idaho College of Agriculture Research and Extension Center cooperating. Received 25 Apr. 1975. *Soil scientists and agricultural engineer, respectively, Snake River Conservation Research Center, Kimberly, It) 83341. Most N fertilizer recommendations are based on past fertilization and cropping histories. Although some of these recommendations are reliable, many have been found to be excessive in southern Idaho (6). There is need for using both soil and tissue testing procedures for accurate fertilizer recommendations for maximum sucrose production and profits. Methoils have been developed for predicting N fertilizer needs for sugarbeets based on the amount of NO3-N in the root zone (8, 11). However, mineralizable N has been found to be a major supplier of N for plant growth and to vary widely from one area to another (6, 13). For a N fertilizer prediction procedure based on a soil test to be applicable over a wide area with many soil types and management conditions, an estimate or measurement of mineralizable N is also needed. Recently, methods have been proposed (3) for more accurate recommendations that consider both the mineralizable N and NO 8-N. The objective of these experiments, under several climatic and soil conditions, was to further develop and refine these methods for predicting N fertilizer needs for maximum refined sucrose production. THEORY AND BASIC RELATIONS Previous studies have shown that for maximum sucrose yields, the Nrequirement is 5.5 0.5 kg/metric ton of beet roots (3, 6). The upper limit of 6 kg N/metric ton of fresh beet roots was used in this study because farm managers generally apply more irrigation water than needed for maximum production, causing N loss below the root zone. At this rate, the potential yield, Y (metric ton/ha), for a sugarbeet field, if limited by N, will be: Y = Nr/6, NT/6 < Yz [la] or Y/YI, = NTAYso NT t 6Y5 [lb] Where Ys is the expected maximum yield under a given management level and climatic zone when N is not limiting (obtained from individual farm records), NT (kg/ha) is the total net N available to the crop, determined as follows: NT = EiN t a.N. a..N. N, where E, = efficiency of applied N fertilizer (NO. crop extractable NOrN NOEN in the soil depth sampled N. = soil NO.-N in the soil depth sampled crop extractable mineralizable N a„, X field mineralizable N in soil depth sampled