Mother Earth News commissioned Will Brinton - who holds a doctorate in Environmental Science and is president of Woods End Laboratories in Mt. Vernon, Maine - to develop some water-based, homemade fertilizer recipes using free, natural ingredients, such as grass clippings, seaweed, chicken manure, and human urine.
Why and When to use Liquids.
Liquid fertilizers are faster-acting than seed meals and other solid organic products, so liquids are your best choice for several purposes. As soon as seedlings have used up the nutrients provided by the sprouted seeds, they benefit from small amounts of fertilizer. This is especially true if you're using soil-less seed starting mix (such as a peat-based mix), which helps prevent damping off but provides a scant supply of nutrients. Seedlings don't need much in the way of nutrients, but if they noticeably darken in color after you feed them with the liquid fertilizer, that's evidence they had a need that has been satisfied. Liquid fertilizers are also essential to success with container-grown plants, which depend entirely on their growers for moisture and nutrients. Container-grown plants do best with frequent light feedings of liquid fertilizers, which are immediately distributed throughout the constricted growing area of the containers.
Out in the garden, liquid fertilizers can be invaluable if you are growing cold-tolerant crops that start growing when soil temperatures are low -- for example, overwintered spinach or strawberries coaxed into early growth beneath row covers. Nitrogen held in the soil is difficult for plants to take up until soil temperatures rise about 50 degrees Fahrenheit or so, meaning plants can experience a slow start because of a temporary nutrient deficit in late winter and early spring. The more you push the spring season by using cloches and row covers to grow early crops of lettuce, broccoli, or cabbage in cold soil, the more it will be worth your time to use liquid fertilizers to provide a boost until the soil warms up.
Water-soluble home made fertilizers are short-acting but should be applied no more than every two weeks, usually as a thorough soaking. Because they are short-acting liquid fertilizers are easy to regulate compared to longer-acting dry organic fertilizers, though i like using both. With an abundant supply of liquid fertilizer to use as backup, you can use a light hand when mixing solid organic fertilizer into the soil prior to planting.
Remember: If you mix too much nitrogen-rich fertilizer into the soil, you cant take it back. As soil temperatures rise, more and more nitrogen will be released, and you can end up with monstrous plants that don't produce well. In comparison, you can apply your short-acting liquid fertilizers just when plants need them-- sweet corn in full silk, peppers loaded with green fruits-- with little risk of over doing it. Late in the season, liquid fertilizers are ideal for rejuvenating long living plants, such as chard and tomatoes, which will often make a dramatic comeback if given a couple of drenchings.