Plant Food: the basics

No area of horticulture is wrapped in a greater mystery or filled with more confusion and misinformation than fertilization.
Some people are fairly obsessed with the idea of “feeding their plants.” In truth, plants feed themselves. First, they take in carbon and oxygen – as carbon dioxide – through their leaf pores. Next, they combine these elements with the hydrogen in water to produce carbohydrate or sugar, which is the only nourishment they need.
Plants are unique in the living world because of this autotrophic (self-feeding) capacity. Most other living organisms, from bacteria to human beings, are heterotrophic, which means we have to forage for our supper.
If you first developed an interest in horticulture while living in an apartment, and had a pet at the time, you could not help but err in your initial assessment of a plant’s nutritional requirements. Every time you fed your cat or dog or canary you probably thought, “What about my pet Ficus; shouldn’t I be feeding it, too?” And, if you fertilized every time or every other time you watered, your plant probably grew right out of the pot, and so you were convinced that plants, like animals, need to be properly fed to thrive.
But the fertilizer you were applying was not food; it consisted of mineral compounds that, when broken down into their constituent elements and absorbed by the plant, simply performed the self-feeding process – known as photosynthesis. Nitrogen also is found in plant DNA and proteins.
The other major elements – the so-called micronutrients – that plants require are phosphorus potassium, calcium, magnesium and sulfur. There are seven elements, in minute quantities, also needed by plants. These micronutrients or trace elements are iron, zinc, magnese, copper, chlorine, molybdenum and boron. The major elements, added together, make up no more than 15 percent of a plant’s dry weight, while the minor elements, collectively, make up less than one-tenth of 1 percent of the plant’s dry weight. This autumn, when you pile up leaves, recognize that 85 percent of what you’re raking is sugar or, more precisely, cellulose – a cell wall constituent that consists of many sugar molecules linked together.
Acknowledging that plants are basically sugar may prompt you to position your plants in such a way that photosynthesis – the sugar-making process – is maximized. In the Valley, this means keeping plants out of unobstructed full-day sun, no matter what the tag at the nursery says. The plant has yet to be invented that, in full sun exposure, is not stressed by the kind of heat we have witnessed this summer and fall.
Stress of any kind reduces the photosynthetic capacity of plants. This stress can come from excessive heat or wind, or from standing water around roots when soil drainage is poor. So often people think that fertilizer will green up their plants when, in fact, it’s the soil that needs improvement.
Hazel Luke of Canoga Park writes of “brown patches in my lawn. Last year,” she says, “I used Scott’s anti-fungus fertilizer, which helped, but the brown patches are back again even worse. Is there some other product I can use that might do the trick?”
Virtually every adverse condition in garden plants and lawns may be traced to cultural conditions. Brown spots in lawns are attributable to one or more of the following: soil compaction, inadequate sprinkler coverage, insufficient light or dogs.
Marie Dailey of Echo Park inquires about the Chinese elm. “One grew out of a retaining wall and is now higher than the house.” Aside from the Shamel ash, the Chinese elm is the biggest “weed tree” in Los Angeles. Unlike the Shamel ash, however, the Chinese elm is a magnificent tree with gorgeous mottled bark and curving, sculptural scaffold branches. It does produce seeds that germinate all over the lot, so uproot young seedlings unless you want to live in a forest of 60-foot-tall trees that, come to think of it, is a rather intriguing possibility.
Tip of the week: To decompact soil under a lawn, spread compost and gypsum over the grass on a regular basis, the more often the better. Alternatively, decompaction may be achieved through aeration of the soil, whether with a gas-powered aerator – which is available at rental yards – or, for small areas, with a two-pronged aerator that may be purchased at hardware stores for around $15.

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