Sugarbeets (Beta vulgaris L.) have been fertilized with phosphorus (P) in the U.S.A. for the past 30 to 40 years. During this period, numerous studies on P fertilization rates and application methods have been conducted. Much of this information has been summarized (12,14,15, 4). 1Recent soil test data from England and the U.S.A. suggest that the available P levels in many soils are sufficient for maximum root and sucrose production without P fertilization. Detrimental effects of P fertilization at excessive soil P levels have been reported (6, 5) but not all data support these conclusions (12). Increasing fertilizer costs have also made it essential that growers have adequate guidelines upon which to base fertilizer applications. Information has been limited in Idaho for establishing an adequate soil test P level for optimum sugarbeet production. With this as background, we conducted two field experiments evaluating 1) the P fertilizer requirements of sugarbeets at different soil test P levels, and 2) the effects of P fertilization on soils already containing adequate available P levels.
Methods and MaterialsField experiments were conducted in 1972 and 1975 on the Portneuf silt loam (Xerollic Calciorthid) soil. This soil has a weakly cemented hardpan beginning at the 16-to 18-in depth which restricts downward root growth but not water movement. The residual soil P levels (Table 1) in 1972 resulted from a P fertilization experiment conducted on dry beans in 1971, whereas those in 1973 were established by rototilling into the soil various P fertilizer rates in the fall of 1972 after the harvest of a spring wheat crop. The bean straw residue from 1971 and the wheat straw residue from 1972 were plowed down in the spring, prior to the 1972 and 1973 experiments, respectively. The spring applications of N (34-0-0) and P (CSP, 0-45-0) fertilizers were incorporated into the seedbed before planting. The amount of N fertilizer applied was determined by the method outlined by Carter, et al. (3).Soil samples were taken from each residual P treatment in 9-in increments to the 18-in depth before t4 spring fertilizer application. The samples were air-dried and the soil test P level (STPL) was measured by the 'Contribution from the Western Region, Agricultural Research Service, USDA; University of Idaho College of Agriculture Research and Extension Center cooperating.'Soil Scientists, Snake River Conservation Research Center, Kimberly, Idaho SSW 'Numbers in parentheses refer to literature cited.