The Q&A Archives: Does my soil need potassium and phosphorus?

Question: My soil test indicated my garden has average levels of potassium and phosphorus. However, a friend said I should still add some of each element when I plant in spring. Is that true? Sandra Costarella Gresham, WI

Answer: You shouldn't have to add more phosphorus and potassium to the soil, says Lloyd Peterson, soil scientist at the University of Wisconsin, Madison. In the past, when fertilizer was cheap, some commercial growers routinely added phosphorus and potassium as insurance against deficiencies. But since neither element leaches readily, phosphorus and potassium applied this year will be 'banked' in the soil for future years, he explains. Today, fertilizer is costly, and few farmers apply it above what thesoil test recommends. As a home gardener, the only scenario in which you may want to add more phosphorus fertilizer is on soils with low organic matter and pH above 7.0. At that pH, phosphorus, especially rock phosphate, is not readily available. Inthat case, you can apply sulfur to lower the pH. work in one to two inches of manure, or add one to two pounds of superphosphate per 1,000 square feet, says Peterson.

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