Beth Guertal's excellent webcast on "Fact, Myth, and Legend in Soil Fertility" (watch on Turfnet here) starts with facts and myths about nitrogen. One of the facts is this one -- N fertilizer rates are not based on the amount of N in the soil.

The reason for this is simple. Grass roots take up nitrogen in the nitrate and ammonium forms. Turfgrass soils have relatively low concentrations of nitrogen, compared to the amount the grass will use. This chart shows the soil inorganic nitrogen (N in the ammonium and nitrate forms) from the Global Soil Survey samples as of August 2014, with each dot representing a single soil producing good performing turf. The red line is the median of those data (7.85 ppm) and the green line is the amount of N there would need to be in the soil to supply an annual rate of 20 g N m^-2 (4 lbs N/1000 ft²).

The soils producing turf have much less N in them, at any one time, than the grass will use over the upcoming months. Using the 6.7 conversion factor (explained here and here) for 1 g of an element applied to the surface, converted to ppm in the top 10 cm of the soil, there would have to be 134 ppm (20 * 6.7) of nitrogen in the soil to supply the amount the grass would use. That much N would be toxic, and that much N could cause uncontrolled growth if it didn't kill the grass -- that is not the way turf gets fertilized.

Because the amount of N in the soil is so much less than the amount the grass will use, soil tests for nitrogen are not useful as a predictor of fertilizer nitrogen.

Elements for which a soil test can predict fertilizer requirements will have levels of the element in the soil, compared to the amount the grass will use, looking completely different than shown above for nitrogen. Take a look at the same soils (data here), but this time with the Mehlich 3 K (potassium) plotted.

The green line (67 ppm) shows the estimated amount of K that the grass would use over the time that 20 g N m^-2 were supplied. And the red line (70 ppm) shows the median Mehlich 3 K of the Global Soil Survey samples. The green line is the amount the grass may use, and all the black dots to the left of that will require fertilizer K. But unlike the chart for N, there are also some soils with K well to the right of the green line. In those soils, the amount of K in the soil is more than the amount the grass will use, and fertilizer K would not be required.

Because the distribution of K in soils includes levels of K that are both less than the grass will use, and more than the grass can use, one can use soil tests to determine K fertilizer recommendations.

For more about this, see:

• the Global Soil Survey
• using temperature to estimate N use
• calculating turf nutrient requirements with a + b - c