Nitrogen is critical for plant growth, though. It is a fundamental part of chlorophyll, and when leaves contain sufficient nitrogen, photosynthesis occurs at high rates. That’s why one of the important warning signs of nitrogen deficiency is yellowing, pale green leaves—especially if this chlorosis develops in the oldest leaves—and indolent plant growth despite fine weather.
Some plants are particularly heavy “feeders” or users of nitrogen, including roses, corn, lettuce, tomatoes, squash, cucumbers, and cabbage. While organic compost contains nitrogen, compost alone often does not provide an adequate supply for these plants. Gardeners who grow nitrogen-hungry plants may have to replenish this element regularly.
One great way to boost nitrogen is by planting leguminous cover crops, such as alfalfa, clover, hairy vetch, or peas. These plants collaborate with soil bacteria to absorb nitrogen from the air and deposit it in tiny root nodules—a process called nitrogen fixation. A cover crop planted in the fall or early spring and turned into the ground can provide most if not all the nitrogen needed by the crop planted afterward.
Nitrogen is also available in a range of organic amendments, particularly those that contain proteins—grains, seeds, legumes, and animal by-products—since the building blocks of proteins, amino acids, are made from nitrogen. The amount of nitrogen each contains, and speed of uptake, varies from one material to the next.
Here are the most commonly used nitrogen sources, their pros and cons, and how they’re best used.
The nitrogen content of manure from grass-eating animals varies, but it is a great source of both nitrogen and organic matter. Because raw manure can burn plants and may contain weed seeds and pathogens, compost it in a hot pile or age it for at least 6 months. To use fresh manure, spread it over the soil in fall and turn it into the top 6 inches a month before spring planting
Manure from chickens and other poultry is an excellent source of nitrogen, potassium, and phosphate. Fresh manure contains a lot of ammonia and is “hot,” so till it in at least 4 months before planting, or compost it first. Pelletized, composted poultry manure is commercially available.
This fast-acting fertilizer, made from slaughterhouse waste, is a potent, easy-to-find source of nitrogen. Blood meal can burn plants, especially young seedlings. Mix it with water and apply it through your irrigation system or a watering can, or dig it lightly into the soil before planting.
This liquid fertilizer is very fast-acting (and can be quite odorous!). Drench the soil with a solution of fish emulsion every month or so, or dilute it heavily and mist foliage lightly.
This meal, made from shells and offal of blue crabs, is rich in the protein chitin, which enhances beneficial soil microorganism populations; it also suppresses pest nematode activity. Till it into soil in early spring.
Dried pelletized chicken feathers contain keratin, a tightly structured protein that’s not easily broken down by soil bacteria, making this an excellent, long-term source of nitrogen. Incorporate into soil before planting in spring.
This excellent source of nitrogen, phosphorus, and potassium also encourages beneficial microbes. Alfalfa decomposes rapidly, generating heat, so do not use it directly in planting holes or in contact with fragile roots. Scratch it lightly into the soil surface.
This slow- to medium-release source of nitrogen is sometimes used as a lawn fertilizer.
The ground meal from cottonseeds provides nitrogen in a fairly slow-release form and is slightly acidic, so use on plants that require a low pH. Some organic gardeners avoid cottonseed meal because, unless the cotton crop was grown organically, it may have come from a field of genetically modified cotton or contain residues of toxic pesticides. The same can be true of other plant-derived fertilizers.
This soluble mineral salt, mined from a desert in northern Chile, is high in nitrogen (16-0-0). Unlike plant- or animal-derived nitrogen, the nutrient does not need to be biologically processed, so it is available even in cold soils. This salt is not for use in arid and semiarid regions, where soils are high in sodium. Readily soluble, it leaches easily from the soil and can damage water tables.