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Predicting In-Season N Requirements of Sugarbeets Based on Soil and Petiole Nitrate¹

Crop quality iii a continuing concern of the sugarbeet (Beta vulgaris L.) industry. A carefully managed N fertilizer program can significantly contribute to improved quality. The objectives of this study were to evaluate sidedressing N for correction of mid or lale season N deficiencies and to evaluate an approach suggested by Giles et al. to predict the occurrence of midseason N deficiency. This method uses the preplant residual soil NO₃- level, preplant N fertilizer rate, and one midseason petiole NO₃-N Concentration in a decay equation to predict the date the petiole NO₃-N level will drop to 1,000 ppm.

Field experiments were conducted at two locations in 1975 and two different locations in 1976 in eastern Colorado. The soil at these sites (Argiustolls or Paleustolls) had an alkaline pH (7.7 to 7.9) and contained a wide range of soil NO₃-N (57 to 232 kg/ha) at planting. Rates of preplant N ranged from 0 to 168 kg/ha and a sidedress N rate of 84 kg/ha was applied at different dates between late May and mid-August. Plant petioles were sampled at approximately 3-week intervals during the season.

Nitrogen deficiency was effectively corrected by sidedressing N through June. Delaying application until after mid-July resulted in yield loss and frequently depressed sucrose concentration. Purity was not affected by sidedressing N at any location. Maximum sugarbeet quality was associated with petiole NO₃-N concentrations of <2,000 ppm on 1 September. The petiole decay equation of Giles et al. to predict NO₃-N concentrations on this date was reasonably successful. The coefficient of determination for calculated vs. actual NO₃- decline in halflife days (t) was r² = 0.63. The average t error was 6 to 7 days. This approach shows promise for evaluating in-season N nutrition sufficiently early to correct incipient deficiencies before serious or possibly any yield loss occurs.

Key Words: Fertilizer N • Sucrose • Purity components • Plant NO₃^– • Soil NO₃^- • Sidedress • Beta vulgaris L.

² Area agronomist, Inter-American Management Corporation, Fort Collins, CO, (formerly graduate research assistant, Dep. of Agronomy, Colorado State Univ.); Western Director, Potash & Phosphate Institute, Davis, CA (formerly professor, Dep. of Agronomy, Colorado State Univ.); and professor, Dep. of Agronomy, Colorado State Univ., Ft. Collins, CO 80523, respectively.

Received for publication June 2, 1980.