Nitrogen is the most ephemeral of the key plant nutrients. Its availability in the soil depends on factors including the temperature and moisture of air and soil, the composition and condition of soil, and the activity level of soil organisms. It’s so transient that most soil labs don’t include it among the parameters they test for in a soil sample.
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.