Kentucky Bluegrass Response to Potassium and Nitrogen Fertilization

The response of Kentucky bluegrass (Poa pratensis L.) to potassium (K) fertilization has been inconsistent. The objective of this research was to determine the effects of K fertilization across varying nitrogen (N) rates and clipping management on Kentucky bluegrass clipping yields, quality, tissue K concentrations, apparent N recovery, and N use efficiency. A 2 x 4 x 4 factorial was arranged in a splitplot design and repeated across two years. Main plots were clipping treatments (returned vs. removed) and subplots were N rates (0, 98, 196, and 294 kg ha(-1) yr(-1)) in combination with K rates (0, 81, 162, and 243 kg ha(-1) yr(-1)). There was no positive effect of K on clipping yields and quality even though soil extractable K levels tested low. Higher K rates, however, increased N recovery and use efficiency for all but the highest N rate. Tissue K response to K fertilization was nonlinear. Yield and quality responses were not correlated to tissue K concentration. Nonexchangeable K levels were high in the native soil, and may have provided an additional source of K for bluegrass. The results suggest that extractable K values alone may not adequately predict available K to Kentucky bluegrass in this sandy loam soil.

Cool-season turfgrass color and growth calibrated to leaf nitrogen

Tissue N analysis a tool available for N management of turfgrass. However, peer-reviewed calibration studies to determine optimum tissue N values are lacking. A field experiment with a mixed cool-season species lawn and a greenhouse experiment with Kentucky bluegrass (Poa pratensis L.) were conducted across 2 yr, each with randomized complete block design. Treatments were N application rates between 0 and 587 kg N ha-1 yr-1. In the field experiment, clipping samples were taken monthly from May to September, dried, ground, and analyzed for total N. Clippings samples were collected one to two mowings after plots were fertilized. Linear plateau models comparing relative clipping yield, Commission Internationale de l' Eclairage hue, and CM1000 index to leaf N concentrations were developed. In the greenhouse experiment, clipping samples were taken every 2 wk from May to October and composited across sample dates for leaf N analysis. Color and clipping yields were related to leaf N concentrations using linear plateau models. These models indicated small marginal improvements in growth or color when leaf N exceeded 30 g kg-1, suggesting that a leaf N test can separate turf with optimum leaf N concentrations from turf with below optimum leaf N concentrations. Plateaus in leaf N concentrations with increasing N fertilizer rates suggest, however, that this test may be unable to identify sites with excess available soil N when turf has been mowed before tissue sampling.

Anion exchange membrane soil nitrate predicts turfgrass color and yield.

Desirable nitrogen (N) management practices for turfgrass supply sufficient N for high quality turf while limiting excess soil N. Previous studies suggested the potential of anion exchange membranes (AEMs) for predicting turfgrass color, quality, or yield. However, these studies suggested a wide range of critical soil nitrate-nitrogen (NO3-N) values across sample dates. A field experiment, in randomized complete block design with treatments consisting of nine N application rates, was conducted on a mixed species cool-season turfgrass lawn across two growing seasons. Every 2 wk from May to October, turfgrass color was assessed with three different reflectance meters, and soil NO3-N was measured with in situ AEMs. Cate-Nelson models were developed comparing relative reflectance value and yield to AEM desorbed soil NO3-N pooled across all sample dates. These models predicted critical AEM soil NO3-N values from 0. 45 to 1.4 micro g cm-2 d-1. Turf had a low probability of further positive response to AEM soil NO3-N greater than these critical values. These results suggest that soil NO3-N critical values from AEMs may be applicable across sample dates and years and may serve to guide N fertilization to limit excess soil NO3-N.

Nitrate leaching from Kentucky bluegrass soil columns predicted with anion exchange membranes

Ideal nitrogen (N) management for turfgrass supplies sufficient N for high-quality turf without increasing N leaching losses. A greenhouse study was conducted during two 27-week periods to determine if in situ anion exchange membranes (AEMs) could predict nitrate (NO3-N) leaching from a Kentucky bluegrass (Poa pratensis) turf grown on intact soil columns. Treatments consisted of 16 rates of N fertilizer application, from 0 to 98 kg N ha-1 mo-1. Percolate water was collected weekly and analysed for NO3-N. Mean flow-weighted NO3-N concentration and cumulative mass in percolate were exponentially related (pseudo-R2=0.995 and 0.994, respectively) to AEM desorbed soil NO3-N, with a percolate concentration below 10 mg NO3-N L-1 corresponding to an AEM soil NO3-N value of 2.9 micro g cm-2 d-1. Apparent N recovery by turf ranged from 28 to 40% of applied N, with a maximum corresponding to 4.7 micro g cm-2 d-1 AEM soil NO3-N. Turf colour, growth, and chlorophyll index increased with increasing AEM soil NO3-N, but these increases occurred at the expense of increases in NO3-N leaching losses. These results suggest that AEMs might serve as a tool for predicting NO3-N leaching losses from turf.