Turfgrass GvX, nitrogen harvest, and fertilizer adjustment

Is there a way to estimate how much nitrogen is harvested in clippings? Is there an equation to use to adjust a nitrogen fertilizer rate based on the Turf GvX?¹ Yes, there are ways to do both of these. I described them during an ATC Office Hours episode with Bjarni Hannesson and Jason Haines.

• Watch the episode
• Listen to the episode

These are the equations, or calculations, I described.

Estimating nitrogen harvest in clippings

When you have clipping volume, this is a straightforward estimate.

On average, the dry weight of the clippings will be 6% of the uncompressed fresh weight.² That is, for every 1,000 cm³ of fresh clippings, you can expect them to have a mass of about 60 g.

Then, you can estimate the N percentage of the clippings. It is reasonable to use a value of 4% for N content of the clippings.³ You can adjust this if you have some data from your own site. The 60 g of dry clippings that we have estimated from the 1,000 cm³ of fresh clippings will be 2.4 g of N.

I have recommended for a long time to make this calculation in reverse, starting with a N application rate of say 1 g/m². That much N could produce about $\frac{1}{0.04}=$25 g of dry clippings, and 25 g of dry clippings would be about $\frac{25}{0.06}=$420 mL/m². I find it useful to think of how many clippings one might want to have over a given time period, and then think about how much N should be applied to produce that.

Adjusting nitrogen rate using GvX

In our conversation, Jason made the good point that if the grass is growing faster than his desired GvX, he won’t apply N. If the grass is growing slower than his desired GvX, he will apply N. That’s a simple approach and there’s a lot to be said for making adjustments in that way.

I have the idea that if the standard N rate is appropriate—and by that I mean low enough—then one can apply that rate adjusted by the ratio of desired GvX to actual GvX. Chris Tritabaugh shared that equation in one of the comments, and I discussed it, and I wanted to explain that a bit more here as well.

Let $N_o$ be the standard amount of nitrogen that one would apply in an application. Now this can be figured out in various ways. For example, one might apply the same rate every time, but adjust the spacing between applications. Or one might have a daily amount that gets summed based on past or forecast GP. Or one might estimate N harvest and replace it. Whatever the method is, there is some amount of N that one would be planning to apply in an application.

I’m suggesting that $N_o$ can be adjusted based on the ratio between the desired GvX, expressed as $Gv{X}_o$, and the actual GvX, $Gv{X}_a$. You decide what you want $Gv{X}_o$ to be, and then use a trailing average⁴ of $Gv{X}_a$ to tell you what the actual GvX is.

That adjustment can be expressed in this equation, adjusting the standard amount of N to get the actual amount of N to apply in this application—call $N_a$ the actual to apply—based on whether the actual GvX is above or below the desired GvX.

$$N_o(\frac{Gv{X}_o}{Gv{X}_a})=N_a$$

When the desired GvX is higher than actual, then the nitrogen rate gets adjusted up. When the grass is growing more than the desired GvX, then the nitrogen rate gets adjusted down.