When I spoke with Kyle Brown on the Superintendent Radio Network, I mentioned that I had been working on an analysis of the first year data for the Global Soil Survey. That report is now completed and is available for download from the PACE Turf website.

The report includes a summary of the first year results, a map of the sample locations, and a description of just how the data are used in the calculation of a nutrient guideline.

You'll need to read the report to get the full description and see the charts that go step-by-step through the process. Briefly, we take a the soil test results for a particular element. That is a series of numbers. Then we identify a probability distribution that reasonably approximates the test results for that element.

In the Global Soil Survey, all samples are selected from turf that is performing well at the time of sample collection. Because of that, we can infer that the soil was NOT deficient in that element at the time of sampling. So whatever the nutrient level was in the soil, it was enough to produce good turf. All of the samples in the dataset, then, are sufficient in the element, based on the performance of the turf at the time of sample collection.

We combine these two things, the probability distribution that fits the data, and the good-performing turf at the time of sample collection, to identify a level in the soil at which we want to be sure to keep the soil at or above to avoid the risk of deficiency. That is the 0.1 level, the level at which 10% of the samples in the model will fall below. Even though turf can perform well at those levels below the 0.1 level that we select for the guideline, we consider this a way to be conservative and to keep a safety buffer of nutrients above the level at which a real deficiency would occur. Even with this conservative buffer, the guidelines developed using this approach, which involves analysis of good-performing turf from sites all over the world, produces soil nutrient guidelines that are considerably lower than conventional guidelines.