Organic gardeners feed soil life, which then feeds plants. Prefer compost, composted manure, alfalfa meal, fish emulsion, kelp, blood meal (slow nitrogen), feather meal, bone meal or rock phosphate (phosphorus), and sulfate of potash or kelp (potassium). Skip 5-10-10, 10-8-4, and 5-10-5 synthetic blends in this book’s recommendations.

Why organic fertilizers — nitrogen is not just nitrogen

To a plant, a molecule of ammonium is ammonium, wherever it came from. The difference is how it arrived. Conventional nitrogen fertilizer is made with large amounts of fossil fuel. That is a climate cost, and the nitrogen is all immediately available — plants cannot use most of it at once, so the rest evaporates or washes into groundwater and streams. Organic fertilizers come in larger particles that break down slowly. Plants may get less of a jolt on day one; they get a longer, quieter supply, and less nitrogen leaves the garden. The same is true of other nutrients.

Reading the bag

Organic macronutrient fertilizers are labeled with three numbers: nitrogen, phosphorus, and potassium, in that order (N-P-K). A bag of composted turkey manure labeled 4-6-4 means that in 100 lb of product, about 4 lb is plant-available nitrogen, 6 lb phosphorus, 4 lb potassium. The rest is mostly carbon plus traces. Because organic fertilizers are made from once-living things, even a “macro” bag contains some micronutrients.

Calcium, magnesium, and sulfur are also macronutrients, but they are more stable in soil than N, P, and K, so they are not always on the front of the bag. Many soils benefit from calcium; test first. You can have too much — excess lime raises pH and locks micronutrients. Blossom-end rot is more often uneven water than a calcium shortage in the bag.

Begin with one 40–50 lb bag of a balanced organic fertilizer (numbers roughly even, and adding up to about 15 or less). If one number is over 8, or the three add up to more than 15, it is probably not organic — bat guano is a rare exception. Fresh raw manure, especially poultry, is too hot to put on crops; compost it or mix it into soil a week or two before planting.

Three ways to apply

1. Broadcast. Before you dig a bed, scatter fertilizer on the surface and work it in. Roots will find it. For lettuce, radishes, and carrots this is often enough. If the bag has no vegetable-garden rate, a starting point is about 5 lb per 100 square feet (roughly a 40-inch bed, 30 ft long). If you fertilize only one way, do this.

2. In the hole. Large-fruited transplants — tomatoes, eggplant, melons — benefit from extra in each planting hole, mixed into the soil so it is not a salt pocket against the roots. Still broadcast the bed as well. See the crop pages for amounts.

3. Side-dress. Onions, broccoli, cauliflower, cabbage, and kale are too small at planting to use a concentrated hole. Wait until they are about halfway to maturity, then scatter again at the bed-prep rate and water in.

More is not better. Follow label rates on any bagged organic fertilizer; they exist because over-applied even organic products can burn and pollute.