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In the 1860s, German researcher Julius von Sachs added different nutrients to plants growing in water. He was trying to discover what plants need to live.
Today we know that nitrogen, potassium, phosphorus, sulfur, magnesium and a few other elements sustain plant life. Before man came into the picture, plants got those nutrients from fallen leaves, rotten logs, dead animals and animal waste. Those aren't things most of us would like in our gardens, so we apply fertilizers, often without knowing much about them. Here's a quick primer on the different types of fertilizers and how they can help -- or hurt -- your garden.
Compost is the most natural source of nutrients that you can provide for your garden. Since compost is made mostly of dead plants, it contains the elements that plants need in the amounts they need.
Other organic fertilizers, such as seaweed extracts, fish emulsions and guano, also work well and are less bulky than compost. And some of these natural, unprocessed fertilizers are renewable resources (fish emulsion and seaweed extracts), while others (rock phosphate and greensand) are not.
Two organic fertilizers that have been getting a lot of attention are corn gluten meal and compost tea. Corn gluten meal, a byproduct of corn processing, is more concentrated than most organic fertilizers. It's typically used to feed lawns, but it also helps to control weeds. Corn gluten meal may not work quite as well as synthetic fertilizers and herbicides, but it's proving to be popular with many gardeners who want chemical-free alternatives.
Compost tea is "brewed" by soaking a sack of compost in a bucket of water. In theory, the compost is supposed to release helpful microbes and nutrients into the water, which is then applied to lawns and gardens. There's no doubt that compost tea adds some nutrition, but there's very little evidence that good microbes come with it. What's more, it's possible that E. coli and salmonella can be spread in compost tea. That's why I tend to shy away from this concoction.
Since the 1920s, synthetic fertilizers have become popular. These fertilizers supply the same sort of nutrition that organic fertilizers do, but in a much more concentrated form. Synthetic fertilizers come from natural materials, but they are chemically processed. For example, the nitrogen in these fertilizers comes from air, which is treated with natural gas at a very high temperature and pressure, to produce ammonia, which is made into fertilizer. (Ammonia itself is not a good fertilizer.) Phosphorus and potassium come from mined rocks that are treated with acids and other chemicals to form granules that will dissolve and reach plant roots quickly.
While almost all synthetic fertilizers contain nitrogen, phosphorus and potassium, some also contain the elements that our plants need in lower quantities. Synthetic fertilizers are a cheap, efficient way to get nutrients to your plants, but because they're not a renewable resource, many gardeners prefer organic fertilizers.
Plenty of gardening books suggest adding beer, Epsom salts or milk to your garden. Most of these homemade fertilizers are a waste of time. Beer and soda contain carbohydrates (also known as sugars), which plants produce by themselves. If you add carbohydrates to soil, you can stimulate the growth of bacteria that break down the sugars. These bacteria use nitrogen, in effect stealing it from plants.
Epsom salts (magnesium sulfate) can be useful, but only if you have a magnesium deficiency in your soil. And you can only determine that if you have your soil tested.
Surprisingly, milk can be useful as a fertilizer. It's loaded with nutrients intended for young cows. And these same nutrients are also good for plants. The problem with using milk is that it tends to smell bad a few days after it's applied.
Jeff Gillman is an associate professor of horticulture at the University of Minnesota. He's also the author of two books, "The Truth About Garden Remedies" and "The Truth About Organic Gardening" (Timber Press, $12.95).