3/31/2010 - By Dr. George Snyder

George H. Snyder and John L. CisarDistinguished Professor, Emeritus and Professor, University of Florida/IFAS

Two years in a row we witnessed the dramatic appearance of numerous brown quarter sized spots in a Poa trivilas overseeding of a Tifdwarf USGA green in Ft. Lauderdale.  In both cases the spotting occurred within days after routine fertilization with ammonium nitrate nitrogen (N) fertilizer, although previous fertilizations with the same N source at the same rate resulted in no damage.   We considered both fertilizer burning and disease as possibilities for the damage.  However, the cause of the spotting was not definitely determined, but it was clearly associated with inadequate potassium (K) fertilization.

The observations were made in a K fertilization study.  The Poa trivialis overseeding received N fertilizations at the rate of approximately one pound per 1000 square feet (5 g m-2) twice monthly, and K fertilizations at approximately 0, ¼, ½, and 1 pounds per 1000 square feet (0, 1.25, 2.5, and 5 g m-2) once per month.  Therefore, on a monthly basis the K to N ratio was 0, 1:8, 1:4, and 1:2, respectively.

The plots were rated a month or less prior to appearance of the damage (Table 1).  In both years the plots receiving no K fertilization rated lower than the others, although they rated near the “acceptable” rating of 6.0.  There were no statistically-significant differences among the three treatments receiving K fertilization in the 2002 study, but in 2003 the treatment receiving the highest rate of K was better than any of the other treatments.

Table 1.  Visual ratings of the Poa trivials turf prior to appearance of the spotting problem(1 – 10 scale, 10 best, 6 minimum acceptability)

K fertilization	Visual Rating		Poa trivialis coverage (%)
lbs/M/month	March 12, 2002	Feb. 11, 2003	April 17, 2002	March 3, 2003
0	5.8	6.3	13.0	19.0
0.25	8.4	7.7	41.0	40.0
0.5	8.4	7.7	45.0	44.0
1.0	8.6	8.7	51.0	55.0
LSD0.05	0.9	0.7	10.8	11.5

The dramatic appearance of the spotting problem was observed on April 12 in 2002 and on March 4 in 2003.  Clearly, it was related to the rate of K fertilization (Fig 2).  In 2002 spotting declined with each successive higher rate of K fertilization.  In 2003 it declined through the ½ pound per thousand square feet rate.

Fig. 2.  Effect of potassium fertilization on visual rating of damage to Poa trivialis

Three to four weeks after the damage was first observed there was a very noticeable reduction in the quantity of Poa trivialis in plots with reduced K fertilization, and especially in the plots that did not receive any K ( Table 1).  The bermudagrass was not as adversely affected, and generally was clearly observable in the absence of the Poa trivialis.

When overseeding with Poa trivialis, superintendents should be aware that insufficient K fertilization can harm the appearance and reduce the stand of this cool season turfgrass.   Applying any rate of K helps, but it appears that a range of 0.5 to 1.0 lbs. of K per 1000 sq. ft. will provide the the best results.  If you are using muriate of potash (0-0-60) as your K source, a lb. of the muriate will provide almost a half pound of  K, while sulfate of potash (0-0-50) will provide about 0.4 lbs of K per lb. of material.