I've always liked this paper. It is about N, P, and K applied or withheld to kentucky bluegrass (Poa pratensis) grown in a loam soil and in a USGA sand rootzone. I was reviewing it recently in advance of an upcoming presentation about turfgrass nutrient use. As turfgrass papers go, this one is especially full of useful data. And it is open access, so you can download and read the full paper.

Here are a few highlights from this paper.

• At the start of the experiment, the loam had P and K of 68 and 63 ppm, respectively, by the Mehlich 3 extractant. The sand had P and K of 58 and 39 ppm. 
• "Added P had no significant effect on clipping yield and underground turf biomass in both sites."
• "Despite the low initial soil K levels ... , clipping yield and underground turf biomass showed no significant response to K addition in both sites."
• "Potassium showed no significant main effect on shoot density or foliage colour."
• I suggest having a look at Figure 2 in the paper, which shows how N rate has a large effect on turf shoot density and turf color, but adding more P and K has negligible effects.

Note that the K in the control plot in the sand would have been below the MLSN guideline for K very early in the study, and yet the addition of K fertilizer had no effect, except to decrease the color of the turf at the highest N application rate. This is another indication that the MLSN guideline for K is not an absolute minimum, but it is a level that if one stays above, there is a high level of assurance that turf quality can be optimized, with no risk of K deficiency.