Phosphorus source and rate for Canola in the Southern Coastal Plain

Canola quality oilseed rape (Brassica napus L.) is a new crop for the Southern Coastal Plain. Little is known in this area about the P rate needed in relation to Mehlich-1 extractable soil P. Additionally, an ''active'' P rock is available and previous studies indicate that rape is able to use P rock more readily than some other plants due to dissolution near the roots where organic acids are exuded. Field studies were conducted to determine canola responses to P rate at ''low'' (0 to 15 ppm) Mehlich-1 soil test levels using P rock, concentrated superphosphate, and a mixture of the sources. Yield responses were attained in all five site-years of the study with 95% of maximum yield reached with application of 33 lb P2O5/acre on a Greenville soil and 100 lb on a Tifton soil. A Mehlich-1 extractable soil P test of 8 ppm was determined at 95% of maximum yield for the Greenville soil and 29 ppm for the Tifton soil. This indicates that Mehlich-1 was not extracting as much available P from the Greenville soil, which has a high P fixation capacity, as from the Tifton soil. The P rock source supplied P to plants and increased yield equal to concentrated superphosphate even though the initial soil pHs of the sites were only moderately acidic (pH = 5.0 to 6.1). Likewise, the critical P soil test levels were similar for the two sources of P even though soil P levels were greater following harvest when P rock was applied than when concentrated superphosphate (CSP) was applied. The Mehlich-1 extractant probably was dissolving P in the rock that may not have been available to plants.