Does it seem like they’re writing in Greek on soil test reports? Do you get confused between “phosphorus” and “P₂O₅”?

Here is a basic primer that takes some mystery out of figuring lime rates and navigating between parts per million and pounds per acre. Matt Clover, Pioneer agronomist and soil fertility specialist, shared these explanations with farmers viewing a virtual webinar:

1:1 water pH. This tells whether a soil is acidic, neutral or basic. Anything under 7 is acidic, 7 is neutral, and above 7 is basic.

Buffer pH. Look here if lime is needed to begin to determine how much to apply. This reading factors in how much liming capacity it will take to raise the pH within a specific soil. For soils with more organic matter, it takes more lime per acre than on a soil with less organic matter.

Liming example. Using tri-state recommendations, if a soil has a pH of 5.5 and an organic matter content of 3.5%, and the target pH is 6.0, the soil lab recommendation will be about 3.5 tons per acre based on buffer pH.

Calcium carbonate equivalent. The recommendation of 3.5 tons per acre assumes you’re applying lime with a 100% CCE rating. That’s not often the case, Clover notes. Your lime supplier can provide information on the calcium carbonate equivalent for his product. Say it’s 84%. Dividing 3.5 tons per acre by 0.84 means you actually must apply 4.2 tons per acre to reach your goal. You may want to do that in more than one application.

Pounds per acre vs. parts per million. Most labs report in ppm for phosphorus and potassium, but a few states, like Illinois, still use pounds per acre in recommendations. From pounds per acre, divide by 2 to get parts per million, Clover says. If you’re in ppm and want pounds per acre, multiply by 2. It’s based on the assumption that an acre furrow per slice of soil 6 inches deep (sampling depth) weighs 2 million pounds.

Elemental vs. oxide phosphorus. Most lab reports are in parts of P₂O₅, the oxide form of phosphorus. If it’s repeated in elemental phosphorus, multiply by 2.29 to get pounds of phosphate.

Elemental vs. oxide potassium. It’s the same concept. This time, to get from K to K₂O, multiply by 1.2.

MAP vs. DAP. Whether retailers offer diammonium phosphate or monoammonium phosphate as the source of phosphorus varies by geography. MAP is fairly acidic, while DAP is a higher pH material. If you have soils that are high in pH, MAP may be a better choice, especially if you’re banding, to reduce chances for nitrate injury to young plants.

Bray P1. This is the standard test for phosphorus used by many labs in the past.

Mehlich 3. Many labs have switched to this test because it is faster and more efficient in the soil lab than Bray P1. If you have past results showing Bray P1 results for phosphorus and your lab now uses Mehlich 3, the lab can provide the conversion so you can make comparisons.   

Critical level. This is the minimum amount of nutrient needed for optimum plant growth. Below this level, most labs label the sample as low for the nutrient. Expect a 60% to 80% chance of a yield response in the first year if a nutrient is low, below critical level.