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In the Garden

Deep six the 10-10-10

by AMY STEWART

NOW THAT THE END OF SUMMER IS DRAWING near, it seems like every gardening magazine and newspaper is running a story about the need to feed plants to keep them blooming through fall. "Give your shrubs a dose of 10-10-10, and hit the roses with 20-20-20, and don't forget some of that 10-10-10 for the annuals, too." It sounds easy enough: Just mix a little of that blue powder into your watering can, or sprinkle some of those little brown pellets around the garden, and your plants will perk right up through fall. It can't hurt, right?

Well, I'm not so sure. I've always viewed synthetic fertilizers with suspicion (and fertilizers referred to as "10-10-10" or "20-20-20" are almost certainly synthetic, but I'll get to that in a minute). A garden is, after all, a little piece of nature, and spraying a petroleum-based chemical around just seems to go against the spirit of the enterprise. Also, even though synthetic fertilizers are often seen as a quick fix, they can't work miracles, as a friend of mine found out when she planted a garden around her new home. The contractor who built the house warned her that the dirt in the newly formed flowerbeds was actually poor-quality fill dirt from the excavation. "Just be sure to fertilize, and it'll be fine," he told her. For the last three seasons, she has filled the beds with flowers, given them a hearty dose of the blue stuff, and then watched them die. "I'm beginning to think that contractor didn't know what he was talking about," she told me. No kidding.

Hers may be an extreme example, but it makes the point: A plant cannot live on 10-10-10 alone. A garden thrives on rich soil that is teeming with life. One 4-by-4 vegetable bed can contain more earthworms, mites, springtails, ants and spiders than there are people in Humboldt County. Include nematodes and the population of that vegetable bed starts to rival that of California. But the other tiny, unseen microbes in the soil -- bacteria, fungi and protozoa -- outnumber all those other creatures combined. A teaspoon of soil can hold a billion bacteria.

I once explained all this at a composting workshop and offered some suggestions for increasing the bacterial population of compost. "Bacteria!" a woman shrieked from the back row. "In MY compost?" She was appalled. But the fact is that the microscopic population of your soil is what supports and sustains plant life, preventing disease, transforming nutrients into a form that is easier for plants to use and improving soil texture. Once you know that, feeding your plants chemical fertilizers starts to seem pretty silly. It would be like you trying to survive on a multivitamin alone, and not a very good multivitamin at that -- one that contains, for instance, only vitamins A, C and D. Not only that, chemical fertilizers can be toxic to some soil-dwelling creatures and reduce the soil complexity you want to nurture.

Now, back to that 10-10-10 designation. The three numbers on a package of fertilizer describe the concentration of three major nutrients: nitrogen, phosphorus and potassium. A 10-10-10 fertilizer contains, by weight, 10 percent nitrogen, 10 percent phosphorus and 10 percent potassium. Synthetic fertilizers tend to have higher concentrations of these nutrients than organic ones, which is why a 10-10-10 or 20-20-20 product is almost certainly synthetic. Organic fertilizers, which are derived from natural sources like bone meal, alfalfa and fish emulsion, tend to have lower concentrations like 5-5-5 or even, in the case of a product like kelp meal, 1-2-1. But even though organic fertilizers offer up smaller doses of those major nutrients, they are derived from natural sources that actually feed the soil over the long term, rather than giving the plants a short-term boost.

Those three major nutrients are important, no doubt about it. Nitrogen promotes green leafy growth and is helpful in the spring when plants are leafing out. Phosphorus encourages flowering and fruiting; you might use more phosphorus in the summer when vegetables are just setting fruit. And potassium is critical for strong root development and for helping plants to resist diseases and withstand extreme heat or cold. But those aren't the only nutrients plants require: Secondary nutrients like calcium, magnesium and sulfur are essential for plant growth, and micronutrients -- or "trace elements" -- like zinc, iron, copper, boron and manganese are also necessary in small quantities for chlorophyll formation and stimulation of plant growth hormones.

Scientists who study soil and agriculture are examining the role that microorganisms play in helping plants to use these nutrients. One scientist told me, "We used to think that plant roots sucked up nutrients the way you'd drink through a straw. Now we know that it's much more complicated than that." In other words, all those microbes living in the soil consume organic matter and help convert nutrients into a form that is easier for plants to use. When you hear gardeners talk about "feeding the soil," what they're really talking about is feeding this army of minute soil-dwelling creatures, who will, in turn, feed the plants.

If you're not already convinced to ditch the blue stuff, I'll give you a few other reasons:

Chemical fertilizers are often petroleum-based, creating even more demand on a dwindling resource and putting farmers and gardeners at the mercy of worldwide petroleum prices.

Chemical fertilizers are salt-based and can kill soil microbes. In fact, ammonium sulfate is so good at killing off earthworms that it is often recommended as a way to green up golf courses and rid them of unsightly worm burrows at the same time.

It is easy to overdose on synthetics and burn plants. Excessive nitrogen in particular can pollute groundwater and streams.

Synthetic fertilizers can lead to overabundant growth in plants, stressing them and shortening their lives.

So what are my recommendations for a late summer fertilizer boost? Mulch your plants well with aged compost. That will keep moisture in and add plenty of beneficial microbes to the soil. To support fruiting and flowering, consider using bat guano or bone meal. Try a foliar spray, a watery mixture of kelp meal or fish emulsion applied directly to the leaves of the plants. And look for one of the new organic fertilizers that contain inoculations of good bacteria or fungi, such as Foxfarm's Peace of Mind or Dr. Earth's Organic 7. Your garden -- and the billions of creatures living in your soil -- will thank you.

garden-related announcements and news to Amy Stewart.

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