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Can You Have Too Much Organic Matter in Your Soil?


You Bet Your Garden
Question of the Week © 2019 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Can You Have Too Much Organic Matter in Your Soil?

Q. Henrietta in Pennington, New Jersey writes: "My husband planted a large bed of one hundred roses along the driveway 25 years ago. Initially he started off with screened top soil and shredded pine bark mulch. He fertilized with Vigoro and Rose Tone. When planting new bushes, he mixed mushroom compost into the hole.

"At my urging a few years back, he began mulching with leaf compost; and then two years ago he had the soil tested. The results showed that there was too much organic matter (28% rather than the recommended 10%). Phosphorus, potassium, magnesium and calcium were also off the charts. However, nitrogen was needed. He decided to put down dried blood and corn gluten meal to try to correct the nitrogen deficiency.

"His question is what, if anything, should he do about having too much organic matter? Should he mix sand or clay soil into the beds? Should he ever use compost mulch again? The roses look fine and healthy. Thanks for your help."

A. Your husband needs a new hobby, Henrietta. Woodworking, making big ships in little bottles, trying to watch every Major League baseball game for an entire season, restoring an abandoned red '68 Chevy Impala Convertible with black leather upholstery and an eight-track player that's been stuck on side six of 'In a Gadda Da Vida' since Gerald Ford was president...anything other than messing with those roses anymore!

Takeaway lesson #1: Trust your plants more than a soil test. If the roses look healthy and are blooming nicely, everything is fine and adding more kitchen sinks to the mix could easily send the poor plants into decline.

Now let's look at these issues and inputs in depth--or at least as deep as I ever get:

• "Screened top soil mulched with shredded pine bark. Fertilized with Vigoro and Rose Tone. When planting new bushes, he mixed mushroom compost into the hole."

'Screened top soil' is very variable in quality. Vigoro is not a specific product; it's a company that makes chemical fertilizers. Rose Tone is a natural fertilizer from the people who make Holly Tone. 'Mushroom compost' could mean several things: Fresh mushroom compost is too 'hot' to use, especially down in the planting hole. Aged mushroom compost is better, but:

Takeaway # 2: There is never any reason to {quote} "improve the soil in the planting hole". You want the roots of your plants to spread out into the surrounding crappy soil. If they have the option of staying inside a nice warm embryo of improved soil, they may do just that--and even after several years have gone by you can lift those pampered plants right out of the ground because their roots are more shallow than I am.

Although it may seem counter-intuitive, you should backfill planting holes with the same soil you dug up to make the hole. Any improvements should be applied to the surface, especially with roses, which greatly appreciate a mulch of disease-preventing compost at the soil surface.

So let's add up the score: It's hard to imagine top soil improving the organic matter content very much. Mushroom soil would, but it sounds like hubby didn't use that much. Both the chemical and natural fertilizers would have added nitrogen (and other nutrients), but not organic matter--that comes from bulk material like compost and composted manures.

• Henrietta continues: "At my urging, he began mulching with leaf compost, and then had the soil tested. The results showed that there was too much organic matter (28% rather than the recommended 10%). Phosphorus, potassium, magnesium and calcium were also off the charts. However, nitrogen was needed. He decided to put down dried blood and corn gluten meal to try to correct the nitrogen deficiency."

Wow. I want him to take me out to dinner and pick up the check. I'll wear bulky clothes stuffed with Zip-Locs and keep ordering racks of lamb, lobster tail, caviar and truffles.

The leaf compost is, of course, a great idea. It's the absolute right thing to do--especially with roses--and would raise the organic matter content of the soil considerably. And--with no apologies to the soil test lab--there is no such thing as too much organic matter in your soil; its like saying your house is lacking in lead and asbestos. The excesses of potassium, magnesium and phosphorus are likely due to overuse of those fertilizers. Drop the Vigoro and have a lighter hand with the Rose Tone.

And finally, the phrase "Nitrogen was needed" points to a poor-quality soil test. Nitrogen is ephemeral; it constantly re-enters the atmosphere, and soil measurements one day can mean nothing the following week. Plus--an excess of Nitrogen is bad for flowering plants like roses. Nitrogen makes plants grow bigger but too much can inhibit the development of flowers and fruits. Blood meal and corn gluten meal are almost pure nitrogen, so back off the blood and go easy on the corn gluten.

The bottom line here is that most labs don't even test for nitrogen; its just too ephemeral. Instead they tell you the organic matter content of your soil, which in your husband's case is exceptional. Most organic farms would envy his 28 percent.

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