Only What the Turf Needs: Updating the Minimum Levels for Sustainable Nutrition (MLSN) Guidelines.

The Minimum Levels for Sustainable Nutrition (MLSN) project was introduced in 2012 to provide turfgrass managers with baseline nutrient requirements that can be systematically updated as new information becomes available. The Global Soil Survey (GSS) started in 2013 to further refine and validate the MLSN guidelines. To identify the MLSN guideline value (x), we first obtain a vector of soil test extraction values for an element. That vector is drawn from a combined dataset of GSS, PACE Turf, and Asian Turfgrass Center soil test results. The dataset includes soil test results with these properties: 1) turf growing in this soil was performing well at the time of sample collection, 2) soil pH ≥ 5.5, 3) soil pH ≤ 8.5, 4) estimated cation exchange capacity ≤ 60 mmol kg^-1. We then fit a cumulative distribution function to this vector and identify the point at which the probability of a sample (X) drawn from this distribution having a value ≤ the MLSN guideline is 0.1. That is, the MLSN guideline is the value of x where P(X ≤ x) = 0.1. This analysis looks at the distribution of soil test results in soils with good turf, moderate pH, and nutrient levels classified as medium or lower by conventional turfgrass nutrient guidelines. The MLSN guideline identifies a level at which 10% of the samples are less than the guideline, even though the turf was performing well in those soils. It follows that keeping the soil at or above the MLSN guideline is a conservative approach to ensuring nutrients are high enough to support good turf performance. Analysis of the combined dataset as of August 2014 (n = 3721) identifies MLSN values (in units of mg kg^-1, Mehlich 3 extraction) of: K 37, P 21, Ca 331, Mg 47, S 7.