Correlation of the Olsen phosphorus soil test: Spring wheat response

Spring wheat producers in central and north central Montana are lacking information to interpret the Olsen phosphorus (P) soil test. This study was conducted to evaluate spring wheat P response and the Olsen P soil test. Field experiments were conducted at 29 cultivated fallow, 5 no‐till fallow, 16 no‐till recrop, and 2 cultivated recrop locations from 1986 to 1995. Phosphorus treatments ranging from 0 to 29 kg P/ha were applied on sites with Olsen P soil tests varying from 4 to 37 mg/kg. Grain yield, protein, and test weight responses were measured. Yields fluctuated from 1082 to 6270 kg/ha and P response from 0 to 1210 kg/ha. A Cate‐Nelson plot of the response data confirmed the current critical level of 16 mg/kg, but P fertilizer recommendations were adjusted. Multiple regression was utilized to develop a highly significant P response equation Y = ‐11.18 + 67.672 P ‐ 1.22 P2 ‐0.98 (P x ST), R2 = 0.55 where Y = grain yield response (kg/ha), P. = P rate (kg/ha), and ST = Olsen P soil test (mg/kg). Grain test weight and protein content were generally unaffected by P except protein was depressed at several environments with high yields and high P soil tests. Growers should use a P starter fertilizer regardless of soil test.