Soil Tests Sick Trees and Fertilizers: Why 10-10-10 is Not the Answer to Anything?

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Q. I've been having issues with a pin oak, and one of the suggested fixes was an application of 10-10-10. I had a bit left over afterward and sprinkled some around two ten-inch high cherry tomato plants as an experiment. All my other plants kept growing, but the two I gave the 10-10-10 to first looked stunned, then wilted and are now about to expire. How could a "fertilizer" be the end of these plants?

---George in Norristown, PA

A. We'll get to that. But first we asked George where he got the advice to give chemical fertilizer to an ailing tree.

George: "It was recommended by the lab at Penn State after they ran a soil test for me."

So I called the director of the lab, and he agreed with my assertion that when a tree is failing, the first thing you want to look at is its physical environment, especially any mulch. Commercial arborists have been warning for years that piling mulch up against the trunk—so-called 'decorative mulching'—is the biggest cause of tree problems and premature death.

But landscapers don't listen, homeowners sometimes insist, and so it seems like everybody does it. Reality check: Almost everybody in America used to smoke—and they used to smoke almost everywhere except in church! Covering the bark of a tree with chipped wood makes just as much sense as the ashtrays that used to be next to every hospital bed in America.

OK; rant over for this week. So you should remove any mislaid mulch before you even think about a soil test or fertilizer. But what if there isn't any mulch on the trunk?

If the tree is growing near a lawn that's being treated with chemical herbicides, you have to suspect that the herbicides are doing their job of killing any plant that isn't grass. Fertilizer won't help there. (This does not, however, apply to non-chemical herbicides like corn gluten meal—which would actually provide a gentle and healthy feeding to any trees growing in or very close to the lawn.)

And fertilizers of any kind won't help a tree that was planted in a wet spot or with its roots all strangled up in burlap or other nursery wrappings.

So is lack of food ever the problem?

My experience is that trees are pretty good at feeding themselves; they got deep roots to find water and nutrients down low and lots of leaves up high to collect the solar energy that helps the tree use that food efficiently. And if food was the answer, it would not be in the form of the ubiquitous '10-10-10'; '20-20-20'; or any of the other so-called 'balanced' chemical salts, because those tens and twenties are only balanced in the mind of the person buying the fertilizer.

As most of you know, by law all fertilizers must have three numbers on the label—their NPK rating. The N stands for the total amount of Nitrogen in the bag or bottle, while the P and the K—the Phosphorus and Potassium—represent the amounts of those nutrients that will be delivered to plants in the first year.

That's because Nitrogen gets used up fast, but Phosphorus and Potassium persist in the soil for several years, so there's no need to keep adding high amounts of them every time you fertilize. In fact, there's good reason NOT to, as they can build up to levels that cause serious problems, especially if you fertilize with large amounts more than once a year.

And no plant uses equal amounts of these three nutrients!

Research has shown that a truly balanced fertilizer for flowering plants would have a ratio of 3-1-2; so a 6-2-4 on a label would indicate a perfectly balanced fertilizer for fruiting plants like tomatoes, peppers, cukes and squash; and if the ingredients are all natural, it would be doubly ideal. For lawns a 'balanced' fertilizer' would be almost all Nitrogen—something like a 9-0-0. (Note: Don't go crazy looking for these exact numbers; they're just a guideline. Use the natural fertilizer with the numbers closest to those.)

And trees? I just can't wrap my head around the idea of 'feeding' a tree. If there was ever a poster child for "just give it a little compost", it'd be a tree.

OK; so now: what happened to those poor tomato plants?

At ten inches high, tomato plants are still babies. I suspect that the powerful chemical salts in that 10-10-10 just "burned" them up. Too much Nitrogen can "burn" any plant, especially young plants—although the director of the soil lab told me that he feels strongly that the fertilizer itself was safe, and that our listener must have used too much.

But why did the soil lab recommend the fertilizer in the first place?

Because that's what they do. Most of the Land Grant Universities that handle soil tests for their individual states were established with fees collected on fertilizer sales, and many still have a pro-chemical bias. In this specific case, our listener's soil tested a little low in nutrients so the lab recommended a common form of chemical fertilizer that they knew would be easy for him to find. 10-10-10 may not make sense to a sustainable advocate like me, but it is widely available.

And like many state labs, this lab didn't offer an organic option out of the box; you have to ask for that up front or search through their website. And even then I'm not quite sure what kind of information you'd get in response. And the organic advice you get from a soil lab is going to vary greatly from state to state. Some state labs are very hip to organics; others are woefully behind.

The bottom line is that these labs are essential for testing soils for lead and other contaminants; and they're a great low cost way to learn the pH of your soil and information about various nutrients. But folks who want an organic, sustainable landscape should take the recommendations they provide with 'a grain of chemical salt' and instead use the information archived in these articles and other reliable organic sources.