Yield and N Percentage of Spring Wheat as Affected by Phosphate Fertilizer Moisture Use, and Available Soil P and N

Yield response of wheat (Triticum aestivum L.) to P fertilizer is quite variable on summerfallowed fields in western Canada. Because yield response to P fertilizer was only partially explained by NaHCO₃-soluble P (available soil P), experiments were conducted in northeastern Saskatchewan to evaluate N and moisture effects on yield response and protein percentage of wheat. Over a 4-year period, 18 N and P fertilizer trials with spring wheat were seeded on six soil types of the Black (Udic Haploboroll), Dark Gray (Mollic Cryoboralf), and Gray Wooded (Typic Cryoboralf) soil areas in northeastern Saskatchewan. Before seeding, soil samples were taken for N and P soil analyses and moisture determination. Maximum and minimum air temperatures and rainfall were recorded daily at each site in order to make estimates of soil moisture by a budget system (rainfall minus evapotranspiration) throughout the growing season. Grain yields ranged from 1297 kg/ha (control) on a soil having a NaHCO₃,-soluble concentration of 8.4 µg P/g to 4,973 kg/ha where 11 kg N and 22 kg P/ha were applied on a soil testing 17.9 µg P/g. Wheat yield responses were negatively correlated with sodium NaHCO₃-soluble P, exchangeable ammonium-N and positively correlated with nitrate-N in soil at planting. Negative correlations occurred between wheat yield response and soil moisture deficit (rainfall minus evapotranspiration) at both heading to soft dough and from soft dough to ripe growth periods; wheat yield response was positively correlated with moisture deficit from jointing to heading (mean R² = 77.6%). Yield response at the highest rate of 29 kg P/ha was not significantly affected by moisture deficit. Yields of the control had a significant quadratic relationship to NaHCO₃-soluble P (R² = 64.3%). Nitrogen percentage of the controls was increased by high nitrate-N in soils. With but one exception, high moisture use and low moisture deficit at later growth stages were associated with lower N percentage of grain (mean R² = 91.4%). Most of the variability of wheat yield response to P fertilizer was accounted for by soil N, P, and moisture. Management practices that will increase the amount of N in the soil and reduce the amount of water available in the late stages of crop development will produce grain with the highest protein percentage on summerfallow fields in northeastern Saskatchewan.