Soil Test Recommendations; Be Very Careful with Big Numbers!
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Q. I recently had my lawn soil analyzed by the soil test lab at Penn State University. Their suggestion for the amount of limestone I should add to bring up my soil's pH is quite clear, but they give endless recommendations on fertilizers, none of which seem to actually match the numbers on any bags of lawn food at garden centers.
Here are just a few of the "N-P-K" ratios they listed: 32-4-10; 33-3-10;
30-3-10; 30-6-12; and 29-3-4
They suggest I pick a fertilizer that matches (or is close to) one on the list and use it three times during the growing season. Some of the offerings in a famous "Four Step" program come close at 30-0-4 and 28-1-4; but nothing matches any of Penn State's numbers exactly. What defines "close" regarding ratios? Is 28-6-6 "close" to 28-1-4? I appreciate your advice!
---David in Havertown, PA
A. Well, first I want to assure our diehard organic listeners that I emailed David back about what those numbers mean, and he quickly decided to use organic fertilizers instead of the super-high potency chemical ones that were recommended.
How do I know that, by definition, those numbers refer to chemical fertilizers? That's easy. Every package of fertilizer, by law, displays three numbers. The first is the relative amount of Nitrogen in the product, the second is Phosphorus, and the third is Potassium. (These are often known by the abbreviations "N-P-K".) The highest known sources of natural nitrogen—things like corn gluten meal, poultry manure and bat and seabird guanos—top out around 10; maybe as high as 12 or 13 for a really potent guano, but that's about it. Any first number that's up in the 20s and 30s is definitely so super-concentrated that it has to be a chemical salt of Nitrogen.
Now, I found it odd that Penn State was still recommending these old-school chemical fertilizers, as they're currently being outlawed in many other parts of the country to try and protect local water supplies. So I called the soil test lab at Penn State, and the manager explained that they in the lab have no control over the recommendations that get printed out on the tests; that information is dictated by Penn State's Turfgrass department.
She gave me the number of the turfgrass professor in charge of the recommendations, whose name I'll choose to withhold here. I called him up, introduced myself, tried to have a conversation about the recommendations, and got yelled at for about ten minutes, then hung up on. (The lab people were very nice, however, and I even called them back to warn them not to refer anyone else to the Turfgrass Tyrant in the future.) And I'll add that he is the exception in University horticulture departments; I've spoken to dozens of others in his position and while some didn't agree with anything I said, all were polite and professional.
But enough about mean people.
Now, it's hard to estimate exactly how many pounds of nitrogen these chemical fertilizers would supply in real life. To know that, you need the weight of the bag and its avowed coverage area. But it's clear that these are the exact kinds of fertilizers that are being outlawed in many parts of the country to try and protect wildlife and water supplies from nasty chemical runoff.
So what's a homeowner who wants a nice lawn without damaging local waterways and wildlife to do? Think '10'—as in 10% nitrogen. Fresh grass clippings are about 10% nitrogen by weight, and a ratio of 10-1-1 is about the ideal for a lawn fertilizer; a nice amount of nitrogen and a pinch of phosphorus and potassium. So that's what you want to see on a bag of slow-release, natural lawn fertilizer—a first number that's between, say, a low of eight and a high of twelve; and zeroes, ones or twos for the other two numbers.
For cool-season lawns (fescue, bluegrass and rye), you'd use a natural fertilizer that supplies about one pound of slow-release nitrogen per thousand square feet of turf early in the Spring, and the same amount again in the late summer or early fall. (If you use corn gluten meal, you'll also get some natural pre-emergent herbicide effect, defeating dormant crabgrass seeds in the Spring and preventing dandelions, chickweed and clover in the fall.)
Warm season grasses (like Bermuda, zoysia and St. Augustine) should get two or three equal feedings while they're nice and green over the summer—just the opposite of the timing for cool-season lawns (because these grasses truly are the opposites of each other).
Or use compost instead of a bagged fertilizer for any of the feedings (especially the fall feeding for cool-season lawns). Compost doesn't have big numbers, but an inch raked into the surface of your lawn creates some of the healthiest, greenest grass you'll ever see. The power of organic matter in action!
Now some praise for the sane people in the turfgrass department at Penn State. In the 'maintaining a home lawn' recommendations at their website, they urge homeowners to follow the same good practices I've been espousing for decades: To use less fertilizer than more, to never feed a cool-season lawn in the heat of the summer, and to return grass clippings to the lawn rather than collect them.
Like I said earlier, those clippings are full of nitrogen (as am I), and returning them to your lawn (especially in the pulverized form a true mulching mower provides) gives your lawn a nice natural feeding every time you mow. Penn State says it provides a third of the food your lawn requires in a season, which would eliminate the third feeding that they recommend for cool-season lawns. (A feeding I don't recommend.) It's also the only safe way to dispose of clippings if your lawn has been treated with the chemical herbicides I hope you don't foolishly use.
And Penn State (and many other State Extension Service Soil Test Labs) will provide organic alternatives to their fertilizer recommendations to anyone who asks—either when they send in the test sample or after they get the results. Here's a link to Penn State's current 'organic recommendation' brochure. It's pretty good!