Sunday, February 20, 2011


As spring barrels down upon us, today we start a multi-part tutorial on supplemental fertilization. This is a longer-than-usual post, but so worthwhile. Honest.

In an ideal world, plants get everything they need from the air, soil and water around them. In environments where perfectly adapted plants choose their own spots to grow, this is exactly what happens. Once we gardeners start imposing our personalities on a space, however, things change. Managing our landscapes includes removing “unsightly” leaf litter and other plant debris, disrupting the cycling of nutrients and organic matter to the soil. To make matters worse, many soils fall to the left or right of dirt’s bell-curve, in the somewhat-nutrient-deficient zone. No worries; we can just toss some fertilizer around, right?

Well, yes and no. And, from where I stand, mostly no. So first and foremost, get a soil test (kits available from your local Cooperative Extension agent). How you gonna know what to add if you don’t know what’s missing?

In my opinion, fertilizers head the list of most-abused gardening products. The two main problems: 
1)      most people never think about why they fertilize; and 
2)      the combined powers of habit and advertising.

It’s just not as simple as the fertilizer industry wants you to believe. Never assume all plants need identical nutrient casseroles (horticulturally speaking, a nutrient is an elemental substance necessary to the metabolism of a plant, or one that allows it to complete its life cycle). Some plants actively resent supplemental fertilization, and some pests are actually encouraged, or the plant’s natural resistance lessened (scientists aren’t sure which way it works), by the practice.

So why fertilize? Here are five reasons for bothering to amend our soils:

·         to ameliorate visible nutrient deficiencies;
·         to eliminate deficiencies that may not be visible but have been detected by soil tests;
·         to maximize plant growth, flower and/or fruit production; 
·         to improve overall plant health; and
·         to reduce potential disease and pest problems.

Who wouldn't buy Miracle-Gro
on this guy's say-so?

           Where do habit and advertising fit in? Let’s assume, for argument’s sake, that you subscribe to the soil amendment mindset picked up from your parents’ behavior and from TV commercials featuring genial James Whitmore. Most of us base our fertilizing behavior on these types of things. To overcome your early conditioning, ponder this list of pertinent questions as you stand, soil-test results in hand, before the miles of shelf-space dedicated to plant food at your neighborhood home-improvement store.
  • What does that N-P-K number mean?
  • What’s the difference between complete, balanced, and single-element formulas?
  •  Should I choose a timed-release or an quick-release product? And what does it matter?
  •  How about those tree spikes? And my old favorite, Miracle-Gro?
  •   Do I want organic stuff or are inorganic formulations just as good?
  •  Is there anything else I need to consider or should I just take my headache home for a nice nap?       
      To demystify the topic of topical fertilization, I’m going to run through a brief, ridiculously basic overview of plant chemistry. (Don’t stop reading: “brief and basic” represents the sum-total of my knowledge on the subject. I promise it will be practically painless.)
      For optimum health, growth, reproduction, and resistance to pests and diseases, plants require the presence of 16 essential elements.
1.      three structural elements: carbon (C), hydrogen (H) and oxygen (O), obtained by the plant from air and water;

2.       three primary elements: nitrogen (N), phosphorus (P) and potassium (K)—to vastly oversimplify, nitrogen produces lush, green foliage; phosphorus encourages blooming; and potassium aids in building strong healthy roots;

3.       three secondary elements: magnesium (Mg), calcium (Ca) and sulfur (S); and

4.      seven trace elements: iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo) and chlorine (Cl).

(There are also seven important-but-nonessential trace elements: sodium, cobalt, vanadium, nickel, selenium, aluminum and silicon, but they come under the rubric of too much information.)

Structural, primary and secondary elements are called macronutrients, because plants need relatively larger quantities of them. Trace elements, only needed in tiny amounts, are micronutrients.

Plants take in primary, secondary and trace elements from the soil. Just like in humans, a serious lack of any of these elements results in one or more deficiencies, adversely affecting health and appearance.

Your basic commercial

            Commercial fertilizers emphasize the three primary elements, and express them as an N-P-K number, those three digits connected by hyphens emblazoned on most plant-food packages. This number, known as the “guaranteed analysis,” refers to the percentage by weight of each element per bag of mixture.

            For example, a 40-pound bag of 10-10-10 works out to ten percent (four pounds) of N (nitrogen), ten percent (four pounds) of P (phosphorus), ten percent (four pounds) of K (potassium), and 70 percent (28 pounds) of inert ingredients, in this particular case, 28 pounds of itty-bitty rocks. In the interest of thoroughness, 50 to 60% of the active ingredients (6 to 7.2 pounds out of 12) are volatile, meaning they turn gaseous and dissipate upon contact with soil and water. You also lose some of those Ns, Ps and Ks to run-off. And we’ve already covered how much of that 40-pound bag is rocks. Think before you buy.

Read labels closely to determine if what you’re buying is complete, i.e., containing the secondary macronutrients and trace minerals in addition to N, P and K. To up the confusion ante a little further, complete is different from balanced. Balanced means the fertilizer has some amount of each of the primary nutrients, although not necessarily any of the secondary or trace elements.

Osmocote's nine-month
balanced formula's
guaranteed analysis
Huh? you say.

Osmocote, with an N-P-K of 19-6-12, is a balanced blend, but it isn’t complete. Why not? Because it doesn’t contain any magnesium, calcium, sulfur, or the trace guys. Rose-Tone, with an N-P-K of 6-6-4, is both balanced and complete. Confused? See the guaranteed analysis panels at left. 

And then there are single-element formulations, such as blood meal (12-0-0), superphosphate (0-45-0) and greensand (0-0-.1). Some secondary and trace nutrients, such as garden lime and Ironite, are sold as single-element formulations too. These particular soil additives have N-P-Ks of 0-0-0 because there aren’t any primary elements in the bag. In our example, lime provides the secondary element calcium and Ironite (not surprisingly) gives you the trace element iron.

complete formula
guaranteed analysis
(click on the picture
to make it larger)

Still with me?

Okay. You’ve decoded your soil test and written down what the state says will be most beneficial for your shrubbery/flower beds/vegetable garden/lawn. (By the way, don’t waste a lot of time looking for the exact N-P-Ks the state recommends, because many of them don’t exist commercially. Soil tests are primarily a tool for farmers, who custom-blend their chemicals.) The next big thing to think about is whether you should buy a quick-release or timed-release product. Regular ol’ 10-10-10, for instance, is quick-release, meaning that the Ns, Ps and Ks get set loose all at once in the presence of water and/or high temperatures. Timed-release blends such as Osmocote keep their Ns, Ps and Ks in little sacs made of inert material that require the nutrients to osmose out over time, once the soil temperature reaches 70˚F.

 What does it matter, fast or slow? When it comes to synthetic fertilizers, speedy delivery—damningly called nutrient dumping—increases the likelihood of your plants' foliage and/or root system suffering fertilizer-burn. Any time you mess with roots, you’re asking for trouble. Just like people, plants can process only so many nutrients at a time: unlike us, they can’t vomit or take Alka-Seltzer to relieve the effects of over-indulgence. Although more expensive by the pound (although not necessarily by available nutrients), timed-release blends are much better for the health of your plants.

Just say no to tree spikes

I don’t want to hurt anybody’s feelings here, but please avoid tree fertilizing spikes. Aside from the fact that they go all slimy and nasty after a short time in the ground, research indicates that supplementally feeding established trees is probably not just unnecessary but deleterious. Renowned arborist, former chief scientist for the U.S. Forest Service, and iconic author of many books on tree biology and care, Alex Shigo tells us that trees shut down all their other life systems, like photosynthesis and defense mechanisms, in order to convert inorganic fertilizers into a form they can use. Beyond a small of shot Plant- or Holly-Tone at planting, Tim and I only use poop mixture around trees. (What's poop mixture? See February 8th's "Spring Prep" post.)

As for Miracle-Gro and other water-soluble products, well, they don’t hurt anything (if you don’t count compromising the water table). The problem lies in how water goes through your soil. In sand, water drains away rapidly, so plants can only snatch at nutrients as they flow by on their way to the ocean. If your petunias and hollies are looking the other way at the time, they’re out of luck. In swamp-muck and other heavy soils, nutrient retention is higher, but I’d discourage liquid fertilizing until you’ve improved the soil to where it drains more freely, to decrease the likelihood of your darlings drowning. Water-soluble plant foods are fine for containerized plants, however; pots leach out water and nutrients quickly, so a shot every other week or so would be appreciated by their occupants. Besides, the “soil” in most containers is actually soil-less (read: “nothing alive in it”) anyway. As always, read and follow label directions.

That’s all I have to say about inorganic—a.k.a. “chemical” or “synthetic”—plant foods. (Literalists point out that everything is, at base, chemical. They’re right, of course, but they should lighten up.) Next time I’ll reveal what I really think about fertilizing vis-à-vis increasing sustainability in our gardens, and, by extension, on our planet.

Thanks for dropping by. Stay tuned.