## Is the annual N rate a useful measure?

##### 20 April 2015

The annual N rate is useful for comparison of the same site from year to year, and I use it for that purpose, but when it comes to making decisions about N addition, I'll use a monthly, weekly, or daily assessment. The annual N is the sum of those additions, and it is whatever it comes out to be.

I like the temperature-based growth potential (GP) of PACE Turf as a way to estimate the N use of a particular species at a particular site on a daily, weekly, or monthly scale.

My interest in this topic is related to seeing the same grass grown in quite different conditions. I've recently seen Miniverde bermudagrass in Thailand, Tennessee, and New Delhi. Comparing annual N rates for Miniverde in those locations is nothing more than a curiosity. To get a reasonable estimate of what might work, one needs to consider the "growing season" at each location, and the GP does just that.

I looked up the climatological normal temperatures for Bangkok, New Delhi, and Knoxville, and then calculated the monthly GP and plotted for each location.

Those three locations are quite different, but one can grow the same grass in each of them. How does the annual N estimate work out for these three cities? Using a monthly maximum N of 4 g m-2 (see this guide for more details on how this is done), I get an annual N use estimate of 10 g N m-2 in Knoxville, 31 in Delhi, and 43 in Bangkok. That's about 2 lbs N/1000 ft2 in Knoxville, 6.2 in Delhi, and 8.6 in Bangkok.

Similar differences occur with cool-season grasses too. Imagine the annual N use of creeping bentgrass in London, Minneapolis, and Sydney.

Using a monthly maximum N of 3 g m-2 for creeping bentgrass, these annual totals come to 10 g m-2 in London, 13 in Minneapolis, and 29 in Sydney. That's 2, 2.6, and 5.8 lbs N 1000 ft-2, respectively.

Annual N rate, comparing the same location year to year? I like it. Annual N rate, comparing different locations, or different species, or both, as a curiosity, or to point out how different they are? I like it. But for making a plan of how much N to use at a particular site? I like to break it down to a monthly scale, or weekly, and don't worry about annual N except as the sum of those fine scale applications.