REVISED UNIVERSITY OF IDAHO FERTILITY GUIDELINES FOR SUGARBEETS
Jeff Stark, Christi Falen, and Terry Tindall
The Cooperative Fertilizer Evaluation Program (CFEP) has been in operation since the 1993 growing season. During the period from 1993 to 1997, 46 on-farm fertilizer research trials for sugarbeets were conducted. There were 8 trials in western Idaho, 22 trials in Magic Valley, and 8 trials in southeastern Idaho. These trials involved the cooperative efforts of growers, sugar, and fertilizer company fieldmen, consultants, and University of Idaho research and extension personnel. In addition, a number of recent studies conducted by University of Idaho researchers have also provided data on sugarbeet nutrient management.
As a result of these combined efforts, we now have a greatly expanded database on sugarbeet nutrient management for Idaho, particularly in terms of on-farm testing of fertilizer responses. About three-fourths of the projects (37 trials) evaluated sugarbeet response to nitrogen, while the remainder evaluated phosphorus and potassium responses. Because of the large number of tests evaluating nitrogen, there is a high degree of confidence in the nitrogen guidelines. However, there may be future refinements made to the phosphorus and potassium recommendations as additional studies are conducted.
During 1997, information produced from these trials was analyzed and summarized. The results were recently used to develop revised University of Idaho recommendations for N, P, and K fertilization of sugarbeets. The nitrogen fertilizer recommendations (Table 1) reflect appropriate fertilizer rates for well managed, commercial sugarbeet fields and are based on typical levels of nitrogen use efficiency, nitrogen mineralization, and soil nitrogen variability observed in growers fields. The recommended N rates are based on yield goal, soil test inorganic N in the top two feet, and previous crop. Soil test N is calculated by adding the soil test value (ppm) of nitrate-N (NO3-N) for the first foot of soil to that for ammonium-N (NH4-N), and then adding the resulting value to the sum of NO3-N plus NH4-N for the second foot. Following grain, the N recommendation is increased by 15 lb N/acre per ton of straw residue returned to the soil up to a total of 50 lb N/acre. Following row crops such as beans or potatoes, no adjustment in the N recommendation is required. The N fertilizer rates that produced the highest estimated recoverable sugar yields in the various CFEP trials were highly correlated (r=0.91) with the recommended N rates from the revised fertilizer guide (Figure 1).
Split N applications often increase nitrogen use efficiency, sugarbeet tonnage, and sugar production. Research conducted at the Kimberly R&E Center during 1992-94 showed that split N fertilization generally increased estimated recoverable sugar and net economic return/acre compared to applying all N preplant. However, growers need to avoid applying significant amounts of N late in the growing season, which can stimulate top growth at the expense of sugar production.
Phosphorus and potassium recommendations are presented in Table 2 and Table 3. Recommended phosphorus rates are based on soil test P values, obtained from sodium bicarbonate extraction, and percent free lime in the soil. The lime adjustment is made to account for the reduced P availability resulting from P tie-up by free lime. Potassium recommendations are based on soil test potassium concentrations. Additional CFEP trials will be conducted during 1998 and 1999 to increase the database on sugarbeet response to phosphorus and potassium.
The cooperative efforts of the Idaho sugarbeet growers, fertilizer company fieldmen, crop consultants, and the University of Idaho coordinated through the CFEP program, have contributed greatly to the nutrient management database for sugarbeets. The revised recommendations have been shown to be highly effective in providing appropriate recommendations for maximizing sugarbeet yield, recoverable sugars, and net economic return to growers.