As plants grow, their roots absorb nutrients from the soil and use them to produce leaves, flowers and fruit. Over time, a plant can exhaust the nutrients in its growing environment. Traditionally, the answer to that problem has been to provide fertilizers, which return essential nutrients to the soil. With growing concerns about the negative effects of fertilizer runoff, however, organic alternatives to fertilizers provide inexpensive, easy and sustainable options.
In nature, composting breaks down dead plant material and returns available nutrients to the soil for use by living plants. Home composting replicates that process, creating humus-rich soil that can be returned to gardens to restore soil nutrients in lieu of fertilizer. Much of what is discarded as garbage in many households can be used to create compost. Compost forms when nitrogen-rich "green" ingredients combine with carbon-rich "brown" ingredients. Green ingredients include grass clippings, vegetable scraps, coffee grinds and pulled garden weeds. Brown ingredients include dead leaves, paper and straw. Simply piling green and brown ingredients together will result in compost, although special composting units are also available to speed up the process or reduce problems with animals foraging in your compost heap. Add compost on top or mix it into the first few inches of soil for a fertilizer-free way to restore soil nutrients.
Crop rotation is an ancient practice in which a specific plot is planted with successive different crops in order to replenish the nutrients of the soil. Some plants require more of a particular nutrient than others, while other plants return certain nutrients to the soil. Planting many successive crops of a single plant in a single location tends to result in depletion of specific nutrients in that plot of soil. By introducing plants with different needs into the area, depletions are not as severe, giving soil time to recover nutrients needed for healthy plant growth. In a simple crop rotation plan, nitrogen-loving plants, such as tomatoes, should be planted the year after legumes that return nitrogen to the soil. Plants that don't consume many nutrients, like herbs and root vegetables, grow well when planted after "heavy feeders" like lettuces, according to "Rodale's Ultimate Encyclopedia of Organic Gardening."
Similar to crop rotation, planting cover crops--also called green manures--involves planting crops that will restore nutrients to the soil. Cover crops are planted during off-seasons, such as the winter or early spring, to return nutrients needed for spring and summer food crops and prevent nutrients from leaching from the soil. Legumes, like clover, are popular choices for winter cover crops in northern climes, as they restore nutrients while also preventing soil erosion. Peas and beans may also be suitable winter cover crops in warmer locales. Green mulches are another type of cover crop that are planted in the summer among the vegetable crop. In addition to keeping soil nutrients in balance, green mulches also suppress the growth of weeds, according to The National Sustainable Agriculture Information Service
Vermicomposting refers to composting with earthworms. If you don't have the room for a compost pile or bin, vermicomposting is an option for producing nutrient-rich soil to use instead of fertilizer in your garden. As an earthworm feeds on organic matter, it creates castings--small granular droppings--that are rich with nutrients plants need to thrive. A single earthworm produces its weight in castings each day. Kept indoors in a cool, dark place, a worm bin turns discarded kitchen scraps into castings that return essential nutrients to the soil.