Fig. 5. (A) Predicted apparent N recovery response surface of Kentucky bluegrass turf fertilized with varying rates of N and K. = 35.771 + 0.106K – 3.332 x 10^–4 NK. R² = 0.750. (B) Predicted N use efficiency response surface of Kentucky bluegrass turf fertilized with varying rates of N and K. = 4.126 + 0.012N + 0.047K – 1.816 x 10^–4 NK. R² = 0.860.