1. Grass clippings (from untreated lawns!) are very effective. They also can be used as mulch to prevent weeds and conserve moisture. Nitrogen content varies, however clippings from unfertilized lawns will probably be around 2% N. For the most part, half an inch of clippings (about six 5 gallon buckets per 100 square feet) mixed into the soil, or 1-2 inches used as surface mulch will provide all the nutrients most of your crops will need for the season.
2. Compost usually contains about 1% N (composted manure contains closer to 3%), and it releases nutrients very slowly, over a period of years. Also, many strains of fungi and bacteria from the compost form beneficial relationships with plant roots, helping them absorb (or develop) nutrients. Compost also helps soil hold moisture. As a rule of thumb, each time a crop is finished, spread at least half an inch of compost over the soil to maintain fertility (add more to increase fertility).
3. Organic matter is essential to soil health. The more organic matter in your soil, the more your soil can hold on to nutrients, as well as maintaining soil moisture. Organic matter also helps to suppress soilborne diseases. Using shredded leaves, old straw/hay, or grass clippings as mulch will increase your soil’s organic matter as the mulch decomposes into the soil. You also grow cover crops (like buckwheat or clover) on soil not being used to increase soil fertility.
Adding too much fertilizer can be just as bad for your crops as not adding enough. So when should you add extra? In the spring or fall, a quick releasing fertilizer (like fish emulsion or blood meal) helps support strong, early growth of hungry cool-weather crops, like brassicas. In addition, for plants that take all season, like tomatoes and peppers, you might need to mix in a light application of fertilizer into the top inch of soil over the root zone mid-summer. You could also replenish your grass clipping mulch. Also be aware that seedlings started indoors may need some fertilizer (fish-based fertilizer works well, mixed half ration with water) two to three weeks after sprouting, as they will have started to use up the nutrients available in their container.
When planting/planning your garden, be aware of which plants are heavy feeders (i.e. corn takes a lot of nitrogen).
Info taken from Mother Earth News, Vol. 6, Spring 2010
