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

Dep. of Plant, Soil, and Environmental Sciences, Montana State Univ., Bozeman, MT 59717

^* Corresponding author (acxyjb@ montana. edu)

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 intraspeciflc [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 <30 µg P g^–1 soil, but the response was curvilinear above soil test values of 30 µg P g^–1 soil. However, soils with lower soil test levels did not yield as much, even at the highest P rates, as those soils testing above 30 µg P g^–1 soil and with no additional P added. The preplant soil test P concentrations as well as the residual (postexperiment) bicarbonate-extractable P soil test concentrations increased with increasing rate of fertilizer P. Results of this study suggest advantages of fertilizer strategies that elevate the bicarbonate-extractable P soil test values of agricultural soils having excess CaCO₃ and pH values above 7.8. Only maintenance applications of fertilizer P appear to be necessary when soil test P concentrations are at or above 30 µg P g^–1 of soil and these same soils have relatively high CaCO₃ or CaSO₄ concentrations.

Received for publication October 27, 1995.