This is an excellent excerpt from an article on the role of nitrogen in the garden AND how it is important for every garden to plant mindfully. Many cover crops will replenish nitrogen in the soil (aka 'fix' the soil) particularly legumes like favas and peas. It is always a great idea to allow some of your garden beds to rest and remain fallow every few rotations. Plant a crop rotation of peas for both a green manure (like animal manure you add to gardens for fertilizer, but green because it is a plant, see?!) and for the added benefit of nitrogen fixing. Once peas flower, cut the vines into small pieces and turn them completely under the soil to decay. This is like homemade compost + fertilizer!You can plant directly on top or leave it sit for a few weeks - up to you. (I typically cover the beds with burlap and let them sit. Spring (read: NOW) is actually a great time to do this, as you have enough time before tomatoes are planted in June to get a good pea rotation in. TRY IT!
Riding the Nitrogen Cycle by Managing Soil Fertility...from WSU Green Times Newsletter
Got nitrogen? If plants asked questions, that might be one farmers would hear frequently. For plants, nitrogen is food. By talking to the region’s small farmers about the challenges they face, Washington State University researchers learned that understanding soil fertility--the availability of food for plants-–is a top priority. Based on that need, soil scientist Doug Collins is leading a team to develop practical soil fertility management strategies.
Plants require nitrogen to grow and reproduce--but not just any nitrogen. The Earth’s atmosphere is mostly nitrogen, but nitrogen in the air is in a form that plants cannot use directly. The nitrogen plants want must be available in the soil so that their roots can absorb it.
Moving nitrogen from the atmosphere into the soil is part of a complex process called the nitrogen cycle. That process starts when atmospheric nitrogen is “fixed” by soil bacteria that have the ability to convert gaseous nitrogen into compounds, such as ammonium, that are useful to plants. The plants convert those nitrogen compounds into amino acids, chlorophyll and other organic compounds essential to life. All nitrogen in animals can be traced back to the organic nitrogen in plants.
When plants die, the organic nitrogen they contain is converted by bacteria back into ammonium in a process called mineralization. Mineralization once again makes nitrogen available for absorption by plant roots.
Most farmers add nitrogen in the form of fertilizers so that their crops have plenty of nitrogen without having to rely so heavily on the nitrogen cycle. Organic farmers, though, are interested in reducing the input of artificial fertilizers and in relying more on increased organic matter in their soils, so that their plants can be fertilized by the nitrogen cycle more directly.