Many farmers receive soil test results from labs reporting in parts per million. Other labs report in pounds per acre. Until Gary Steinhardt, Purdue University Extension soil specialist, led the charge to revamp soils judging rules, Indiana FFA and 4-H soils judgers were given soil test results for phosphorus and potassium in pounds per acre. During the revision, this shifted to parts per million.
Then when people with Illinois FFA worked with Steinhardt to adopt a version of Indiana’s land judging rules in 2019, they requested phosphorus and potassium soil test results be reported on soils judging pit cards in pounds per acre.
Pounds per acre or parts per million — does it matter? It matters to Jim Camberato, Purdue Extension soil fertility specialist.
“The extractants used in soil testing labs only extract a portion of phosphorus actually in the soil,” he explains. “Soil tests deal in concentrations. Labs determine the concentration of phosphorus extracted in a solution of soil. Concentrations are best reported in parts per million.”
For fertilizer recommendations, if your lab reports in parts per million but your chart uses pounds per acre, simply multiply by two, Camberato says. For practical purposes, soil test values of 20 ppm and 40 pounds per acre are the same.
“It matters when you start tracking how much phosphorus a crop removes and how much you add in fertilizer, and making the math match,” he says. “Remember that the soil test doesn’t account for all phosphorus tied up in the soil.”