Recommendations for phosphorus, potassium and magnesium are based on soil test values from these extractants. The Bray-Kurtz P1 test does not extract all of the available phosphorus on soils which contain free calcium carbonate. This test was designed for soils with free calcium carbonate. Mineral soil samples submitted to the MSU Soil Testing Laboratory are extracted using weighed samples. Organic soil samples are measured by volume because such materials usually have much lower densities than mineral soils.
Some laboratories express all elements as parts per million.
The exact conversion varies with bulk density of the soil, but organic soils have a wider variation than mineral soils. This information is for educational purposes only. References to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned. This information becomes public property upon publication and may be printed verbatim with credit to MSU Extension. Reprinting cannot be used to endorse or advertise a commercial product or company.