In this presentation, I explained how one can determine turfgrass nutrient requirements by estimating turf nutrient use and comparing that to the amount of the nutrient in the soil.
One of the questions during the presentation involved nutrient availability, specifically of P and K. To paraphrase, the question was "How can one know that the P and K measured by the soil test, and compared to the MLSN guideline, are available to the grass?"
One can know the availability by doing a soil test because that is what a soil test is. A soil test is measuring the availability index of an element. When there is a lower amount of an element, there is less of it available to the grass, and when there is a higher amount of an element, there is more of it available. The MLSN guidelines are set at a level that we are confident is more than enough availability to produce good turfgrass.