Monthly maximum N or annual maximum N

I sometimes talk about maximum N. You may have heard me refer to the standard amount of N. I discussed that with Michael Bekken in this ATC Doublecut episode. What do I mean by these quantities of N? There was a question on the ATC YouTube channel about that, asking whether my mentions of a 30 kg/ha or 3 g/m² value for Poa annua is a monthly or annual amount. The comment might have been deleted, or YouTube’s inscrutable comment filter may have disallowed that. I thought it was a good question, and I wanted to start an explanation of it here.

The PACE Turf growth potential (GP) converts air temperature to a scale of 0 to 1 (or 0% to 100%, if you prefer).

When temperatures are close to an optimum for net photosynthesis, the GP will be close to 1. When temperatures are far away from the optimum temperature, the GP will be 0. If the grass is dormant in winter, the temperatures will convert to a GP that will be almost 0. When the grass won’t stop growing and you can’t mow it fast enough in spring, the GP will be close to 1.

The grass is using—and losing–nitrogen based on how many clippings are being removed by mowing. When the grass is not growing, it is not being mowed, and the N use is zero. When the grass is growing rapidly, and is being mown frequently, then a lot more N is being removed from the grass. The GP scale goes from 0 to 1, and we can assume that there is a linear relationship between GP and growth.¹

When I am talking about maximum N, or standard N, what I mean is the quantity of N that I expect will work well to produce high quality turfgrass when multiplied by the GP. I generally refer to this amount as a monthly amount. For example, I use 2 g N/m² as a monthly maximum for bentgrass, 3 for Poa annua, and 4 for bermudagrass. These are based on observation of the rates of N that turfgrass managers use to produce good turf.

These values give a really good starting point. For example, I spoke with Fraser Brown, the course superintendent at Lake Karrinyup Country Club in Perth, Western Australia. He said that he’s been applying an annual N rate that is about 180 to 200 kg/ha (18 to 20 g N/m²). When I checked the temperatures there, and calculated the predicted N using the standard rate of 2 g/N/m² monthly, the annual total comes to 170 kg/ha.

Using this procedure to check N rates can get one very close to what is working to produce high quality turf at other places around the world, but adjusted to the exact temperature conditions of one’s site.