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 superpbosphate 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. Research QuestionCanola is a promising crop for farmers in the Southern Coastal Plain as it can be doublecropped with “summer” crops such as corn, cotton, peanut, and soybean. Soils of the Southern Coastal Plain are relatively infertile and there is little information available for optimum fertilization of the crop under those conditions. Our study was designed to determine the best P fertilization of canola on soils when the soil test for P was “low.” Literature SummaryStudies in other areas have indicated a marked response in seed yield of canola and other rapeseed cultivars to fertilizer P when soil reserves are low. The availability of P for plant uptake is greatly affected by soil pH, with the conclusion from previous studies indicating that phosphate rock (PR) is available to most crops only at low soil pH. Other studies indicate that rapeseed can use the P in PR more readily than can many other crops, possibly due to the ability of its roots to acidify the soil. Additionally, P from a PR mined in North Carolina is reportedly more available to plants than most other PR sources. Our previous research indicated that the Greenville soil fixes P to a much greater extent than the Tifton soil. Study DescriptionFive field experiments were conducted over 3 yr (1991–1994) at Tifton and Plains, GA. The initial soil P tests were 6 to 7 ppm (low). Experiments were conducted for all 3 yr on a Tifton soil and for 2 yr on the Greenville soil. Phosphorus sources: PR, Concentrated (triple) superphosphate (CSP), and a combination with 50% of the P from each source. Phosphorus rates: 0, 20, 40, 80, and 160 lb P₂O₅/acre per year. Phosphorus was broadcast and mixed with the soil to a depth of approximately 4 in. just prior to seeding canola cv. Bingo in October. Plant height, leaf P, plant P, and winter survival were determined during the growing season. Seed yield and test weight were determined from combine harvests in May. Soil samples, taken following harvest, were analyzed for P by the Mehlich-1 procedure and those values were related to seed yield. Applied QuestionsHow much P is required for canola when the soil test is “low”? Canola seed yield was increased by application of P in all site-years of the study. Near maximum yield were reached when 100 lb P₂O₅/acre was applied on the Tifton soil and 33 lb was applied on the Greenville soil (Fig. 1). The Mehlich-1 soil P required to reach near maximum yields was 29 ppm for the Tifton soil and 8 ppm for the Greenville soil (Fig. 2). The differences are believed due to the differences in P fixation capacities of the soils. These data suggest that P recommendations for these soils should differ from the 80 lb P₂O₅/acre currently recommended in Georgia for “low,” in order to insure near maximum yield on the Tifton and associated soils and to avoid over application on the Greenville and associated soils. Fig. 1Relative seed yield of canola in the first year experiments at a site as affected by rate of P applied. Data are a percentage of the highest yielding rate in a particular site-year. Fig. 2Relative seed yield of canola plotted against Mehlich-1 soil P test following harvest. Data are a percentage of the highest yielding P rate for a particular site-year. Is phosphate rock a suitable source of P for canola in the Coastal Plain? Canola seed yields were equal whether P was supplied by North Carolina PR or by CSP even though the soil pH was as high as 6.1. However, some erroneously high soil test P values were obtained when sampling soils following PR application.