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Figure 3. A multi-year Purdue study found that the highest alfalfa yields were recorded when fertilizer rates were 50 pounds of P 2 O 5 per acre per year, and 200 pounds or more of K 2 O per acre per year.igure 3. A multi-year Purdue study found that the highest alfalfa yields were recorded when fertilizer rates were 50 pounds of P 2 O 5 per acre per year, and 200 pounds or more of K 2 O per acre per year.
How to calculate a fertilizer ratio. If your soil test report recommends applying 1.5 lb of nitrogen, 0.5 lb of phosphate, and 0.5 lb of potash per 1000 sq ft, you should apply a fertilizer with a ratio of 3-1-1 since you need three times as much nitrogen as phosphate and three times as much nitrogen as potash.