Effect of Phosphorus Soil Test Level on Sorghum‐Sudangrass Response to Phosphorus Fertilizer

Many agricultural soils of the northern Great Plains have excess CaCO3 or CaSO4 and pH levels above 7.8, thereby having high fixation capacity for applied P and resultant poor predictability of crop responses to applied P fertilizer. In addition, initial and residual effects of applied fertilizer P on soil test P values are not well understood. A greenhouse study was conducted with Sordan 79 (sordan), an intraspecific [Sorghum bicolor (L.) Moench] sorghum‐sudangrass hybrid [syn. S. bicolor‐S. sudanense (Piper) Stapf], on three soils with different textures, cation exchange capacities, and CaCO3 contents to determine sordan response to P fertilizer applied to calcareous soils. Soil test P (bicarbonate‐extractable) was adjusted to five initial levels, ranging from 2 to 60 μg P g−1 soil. Fertilizer P was then applied at five levels, ranging from 0 to 40 mg P kg−1 soil. Three successive harvests of the test crop were completed and measurements of dry matter were made. Response surfaces and regression models were developed, describing the relationships among sordan yield, P uptake by the crop, total P applied as fertilizer, and resultant soil test P values. Sordan response to P fertilizer was linear at soil test values of