Turfgrass nutrient demand and soil interpretations
From August 20 Turf iNfo, University of Nebraska:
Plants selectively take up different nutrients. As a nutrient like phosphorus (P) is needed, the plant roots take them up. If it is not needed, then P uptake is slowed or stopped. That is why soil testing approaches that look at the relative amounts of nutrients (i.e. base cation saturation ratios – Ca vs. Mg vs. K) aren’t effective. It’s more important to focus on the amount of nutrient in the soil. Slow and expensive university calibration tests are required to understand how much nutrient is required in the soil to satisfy plant demand for a particular nutrient.
While universities recommended Sufficiency Level of Available Nutrients or SLAN interpretations, their results are specific to the particular experimental site (soils, grass species, cultivar, nitrogen fertility, etc.).
For example, I determined the level of soil test P required to sustain acceptable quality was 7 ppm (Mehlich-3) during my MS. When soil test P levels were below 7 ppm, the grass looked blue and thin. At values greater than 7, the turf quality was acceptable. However, that critical value of 7 was specific to that research green (pure sand, Penn A4, in Madison, WI, and fertilized with 0.2 lbs. N per 1000 ft2 every 14 days). If changed any of those factors and the critical value will change slightly.