For healthy plants and a healthy planet, fertilize wisely.

Since the dawn of agriculture, people have known that healthy plants need healthy soil. But modern practices have made even organic fertilizing more complicated than it used to be. Here’s the latest on the age-old art of nourishing your soil organically.

Why organic?

“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 fertilizer? Think of it as feeding your soil, not just your plants.

When you apply organic fertilizers and 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 fertilizers dally in the soil, providing a steady supply of nutrients.

Organic 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.