PACE Turf - Turfgrass Information Center

Estimating Morgan P from Mehlich-3 P

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This histogram illustrates an attempt to estimate the equivalent Morgan P MLSN value using soil pH and Mehlich-3 (M3) P, Al and Ca according to the Cornell University conversion equation. The same PACE Turf soil data that was used to determine the initial MLSN M3 guidelines was converted to Morgan extract equivalents. Unfortunately, a large portion of the M3 soil database P values resulted in negative Morgan P estimates. The negative values were assigned a Morgan P value of 2 as required by the Cornell conversion equation. There is not a simple conversion from a M3 P test result to Morgan P and visa versa. The MLSN Morgan P guideline value will have to be determined independently. Until that time, the M3 P guideline coupled with the Phosphorus Saturation Index (PSI) can be used to help monitor soil P levels for good turf performance and to avoid negative environmental impact that might occur with over-application of phosphorus. Maintain the PSI between 0.1 and 0.23 and soil M3 P above 21 ppm.

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