Nutrient recommendations with clippings removed or with clippings returned

This question arrived recently:

How do you estimate soil nutrient depletion and provide a recommendation if clippings are being returned? Example: lawns or roughs.

That’s a good one. See “How to get a quick estimate of grass nutrient use” for some background information about this—specifically about the a + b - c formulation to find the quantity of a nutrient to apply.

I responded with this:

Good question. Basically I look at change in soil test values over time in relation to nutrient applied. From those two values one can make a site-specific calculation of depletion rate.

For a single soil test snapshot, it’s a bit trickier, and I generally make a recommendation based on MLSN and then adjust it when the soil is tested a second time and I can calculate the change.

I haven’t thought about this a lot recently. I did think about it some time ago. And I think what made sense to me then was to make a rough estimate that 50% of the nutrients returned through clippings could be considered like fertilizer, and 50% maybe assume they are lost somehow. That wasn’t based on any calculation, just basically saying ok if you return clippings, we essentially cut the fertilizer rate in half and I feel pretty comfortable that that is safe.

As far as testing interval, on sand, I want to test every year, regardless of clippings removed or not. On soil, I’d typically suggest testing once every 3 years. But if you are somehow close to MLSN values for P or K, maybe test more frequently.