Since the dawn of agriculture, people have known that healthy plants need healthy soil. But modern practices have made even organic soil fertilizing more complicated than it used to be. Here’s the latest on the age-old art of nourishing your soil organically.
Why organic soil?
“It makes no difference to your tomato plant if the atom of nitrogen it is absorbing has come from a compost pile or a fertilizer factory,” states a University of Saskatchewan department of horticultural science publication. So why use organic soil fertilizer? Think of it as feeding your soil, not just your plants.
When you apply organic fertilizers and organic soil amendments, you feed billions of soil microorganisms, which in turn break down the organic matter into nutrients that plants can use—including certain forms of nitrogen. Unlike synthetic fertilizers, which provide only a few major nutrients that can leach easily out of the root zone, most organic oil fertilizers dally in the soil, providing a steady supply of nutrients.
Organic soil fertilizer does more than just nourish plants. It also improves the soil’s texture, buffers the soil against extremes of pH, and helps plants resist pests and disease. About 90 percent of the time, insects choose unhealthy plants to attack, according to John Jeavons in How to Grow More Vegetables (Ten Speed, 2002). “The insect is not the source of the problem,” he writes, “but rather an unhealthy soil is.” And just like people, he adds, healthy plants on a good diet are less susceptible to disease.
How does your garden grow?
If you’ve ever perused a fertilizer package, you’ve probably noticed three major plant nutrients: nitrogen, phosphorus, and potassium (N, P, and K). Plants use these nutrients in large amounts, so they’re the ones you most often need to replenish. Other important nutrients include calcium, magnesium, sulfur, and trace elements such as iron, manganese, copper, zinc, boron, and molybdenum. Soils that contain ample organic matter provide varying amounts of these nutrients.
Having an adequate supply of nutrients is only part of the picture, though. “A soil loaded with nutrients is no guarantee that they will be recovered by plants,” says Bob Russo, co-owner of Timberleaf Soil Testing in Murrieta, California. Compacted soil may not contain enough of the oxygen that plant roots need to take up nutrients. And if soil is too acidic or too alkaline, nutrients may be unavailable to plants.
Take care, too, not to overfeed your soil. “The majority of unbalanced soil conditions that we’ve seen over the years arise from the excessive use of fertilizers,” says Russo.
Animal, vegetable, mineral
A wild rose petal falls, a passing deer leaves some droppings, a mouse dies, and all the while the underlying rock breaks down slowly into particles of soil. All contribute to the soil’s fertility. When you apply organic fertilizers, you basically add the same substances: plant remains, manures, animal remains, and substances mined from the earth.
Although compost is a moderate source of nutrients, it’s not a fertilizer per se. It’s more of a jack of all trades: It loosens a clay soil, holds together a sandy soil, retains moisture, moderates drainage, builds organic matter, and increases soil biological activity. It’s the “heart and soul” of organic soil care, says Russo. You can buy it or make it—with or without animal ingredients—and you can even compost kitchen waste in a worm composting bin in your garage or basement.
Like compost, manures supply some nutrients but are most useful for improving soil structure. Organic manure won’t be contaminated by pharmaceuticals. But to avoid exposure to dangerous E. coli bacteria and other pathogens, use composted and cured manure, wash your hands after handling manure, wait 120 days between applying manure and harvesting food crops, and wash produce before eating. Green manures are actually plants that you sow into an empty area—for example, a vegetable garden over the winter—then dig back into the soil to improve its texture and add nutrients.
To add specific nutrients to the soil, you can fertilize with plant, animal, or mineral ingredients. Search out organic plant-based fertilizers, especially when it comes to cottonseed meal. Cotton plants are heavily sprayed with pesticides that accumulate in the oil from which cottonseed meal is extracted.
Most processed animal remains are by-products of intensive cattle production or commercial fisheries, so consider whether you’re comfortable with the source of animal fertilizer (see “Animal-Free Fertility,” below). Since the late 1990s, more than 100 Europeans have contracted Creutzfeldt-Jakob disease (CJD), a fatal human brain illness, after eating beef contaminated with mad cow disease. Opinions vary on the likelihood of whether it can be transmitted to humans via inhalation. To err on the side of caution, wear gloves and a dust mask when you apply bone, blood, or hoof and horn meal.
Manure, bone meal, and fish emulsion have long been standard fare in organic gardens. But a movement to eschew animal-based products along with synthetic chemicals is growing, according to Amy Duggan, cofounder of the Center for Vegan Organic Education, of Vashon Island, Washington.
Food safety is one reason. When manure contacts food crops, Duggan notes, E. coli and other bacteria can contaminate fruits and vegetables. Preliminary research also suggests that the antibiotics routinely administered to cattle show up in manure and can stunt the growth of plants, get into food crops, and contribute to antibiotic resistance. Duggan also alludes to several gardeners in Britain who may have contracted mad cow disease from inhaling bone meal.
Pointing to all the resources that go into raising a cow—land, water, and feed—Duggan explains that much of the nitrogen in the grass a cow eats is excreted in the cow’s urine. “Soil fertility doesn’t originate from animals, but from plants,” she says. “Let’s tap fertility at the source, not the back end.”
It’s the rare gardener these days who gets manure “from a happy cow out in a field,” says Duggan. She argues that buying slaughterhouse by-products supports an industry that’s inhumane—and not just to the cattle. “Working conditions are horrible,” she says. In addition to plant-based compost and green manure, Duggan recommends an array of plant- and mineral-based fertilizers, including alfalfa meal, organic cottonseed meal, soft rock phosphate, kelp meal, and wood ashes.
Putting it all together
So how do you know what and how much to use? There are as many answers as there are gardens—and gardening experts. But here are a few basics.
• Look at the plants growing in your soil now. If they’re flourishing, chances are your soil is in good health. Patterns of unhealthy growth and coloration can help you diagnose specific soil deficiencies. For example, yellowing of older leaves is one symptom of nitrogen deficiency, and purplish leaves may signal a phosphorus deficiency.
• In a new garden, soil testing can be useful in a location where plants aren’t thriving or in vegetable beds where you want to maximize production. To get the most thorough information, use a commercial lab, preferably one that recommends organic products. You can also buy kits for home use that test for N, P, K, as well as pH.
• The centerpiece of organic soil care is a source of organic matter such as compost. For fertilization, you can either mix and match basic ingredients or purchase premixed organic fertilizers. In selecting commercial products, inspect ingredients lists and seek out products certified for organic growing.
Feed your bed
Sources of nitrogen
Hoof and horn meal
Fish meal or fish emulsion
Sources of phosphorus
High-phosphorus bat guano
Sources of potassium
Finely crushed granite or granite dust
Calcium and pH
Crushed eggshells (supply calcium)
Gypsum (corrects calcium deficiency without affecting soil pH)
High-calcium limestone (corrects calcium deficiency and increases soil pH)
Dolomitic limestone (corrects calcium and magnesium deficiencies and increases soil pH)
Let It Rot by Stu Campbell (Storey, 1998)
The Rodale Book of Composting, edited by Deborah L. Martin et al. (Rodale, 1992)
Worms Eat My Garbage by Mary Appelhof (Flower, 1997)
The Worm Book by Loren Nancarrow and Janet Hogan Taylor (Ten Speed, 1998)