Effects of phosphate rock powder application on alfalfa in calcareous soil

Phosphate rock powder is a type of insoluble phosphorus (P) fertilizer, and is generally considered unsuitable for application in calcareous soil. Alfalfa is highly capable of utilizing insoluble phosphorus in soil. Three grades of phosphate rock powder (PRⅠ, PRⅡ and PRⅢ) were selected as phosphorus fertilizers, while a soluble phosphorus fertilizer (calcium superphosphate) was used as a control (CK). We conducted a cultivation experiment on three alfalfa varieties (Algonquin (M1), Longdong (M2), and Gold empress (M3) in calcareous soil to explore the effects of phosphate rock powder application on alfalfa. The growth, nutrient quality, P uptake, utilization efficiency in two cuts of the different alfalfa varieties, and soil phosphorus content were assessed. The results showed that M1 and M3 alfalfa under PRⅠ, and M2 alfalfa under PRⅢ, achieved higher heights than CK in the second cut harvest. The dry weight of M1 in the two cuts under PRⅠ was highest out of all the treatments, with a total dry weight of 8.61g (46.9% higher than that of CK). The dry weight of M2 in the two cuts and the total dry weight were also higher than that of CK, but PR application did not significantly improve the dry weight of M3. In addition, PRⅠ and PRⅢ increased the crude protein content of M1 in the second cut. The crude ether extract of M1 and M2 under PRⅢ were all higher than that of CK, and the crude ether extract of M1 and M3 were higher than that of M2. The calcium content of alfalfa shoots was highest in PRⅡM3 for the first cut (2.18%), but this was not significant when compared with the calcium content of the CKM3 shoots (P > 0.05). In the second cut, the calcium content was highest in PRⅠM1 and PRⅢM2 (1.56% for both). PRⅠ and PRⅢ significantly increased the phosphorus uptake efficiency of M1 and M2 in the second cut, while PRⅠ increased the phosphorus utilization efficiency of M1 and M2 in the first cut. PRⅡ increased the phosphorus utilization efficiency of all three varieties in the second cut when compared with that of CK. Soil total phosphorus content increased significantly under all PR treatments except for PRⅡM1, and those of PRⅠ and PRⅢ were higher than that of PRⅡ for all three varieties. The soil available phosphorus content was increased under all PR treatments when compared with no phosphorus application, although they remained lower than in the CK condition. Therefore, the effects of phosphate rock powder application were similar or superior to those under calcium superphosphate usage in terms of alfalfa growth, nutrient quality, and P uptake and utilization in calcareous soil. However, the effects differed based on the phosphate rock powder grade and alfalfa variety used. The M1 and M2 varieties appeared to benefit most from phosphate rock powder application, and PRI appeared to be the superior fertilizer of the PRs studied.