Sooner or later, every gardener discovers that for good results -- whether in the vegetable garden, perennial border, or lawn -- replenishing soil nutrients is necessary. And one of the key choices is whether to use organic or synthetic fertilizers. Synthetic fertilizers are manufactured. Organic fertilizers are derived from plants and animals, and from naturally occurring mineral fertilizers.
One advantage of organic fertilizers is that their nutrients are doled out as a steady diet in sync with plant needs. Because the nutrients come from natural sources, a portion of them may be temporarily unavailable to plants until released by a combination of warmth and moisture -- the same conditions plants need to grow. Released slowly, the nutrients from organic fertilizers are unlikely to burn plant roots or be leached away by water. And a single application may last a whole growing season.
You also might choose organic fertilizers for philosophical or environmental reasons. Organic fertilizers generally place fewer demands on energy resources, and they offer opportunities to recycle "garbage".
The more concentrated a fertilizer (even an organic one), the less organic matter it contains. Fertilizers containing high concentrations of nitrogen, when used alone, can actually deplete soil organic matter, so if you use any such fertilizer, apply plenty of bulky organic matter, too. Dig materials such as straw, peat, compost, and leaves into the soil, or lay them on as mulch.
Naturally occurring mineral fertilizers are organic in the "not-synthetic" sense, but because they don't contain organic matter, they're not included in this list. Among them are Chilean nitrate, rock phosphate, greensand, and sulfate of potash magnesia.
Synthetic fertilizers do have some advantages. They cost less, are easier to transport, and are more uniform in nutrient content. All but controlled-release synthetic fertilizers are more quickly available to plants than organic fertilizers.
Why fertilize? Fertilizers are necessary make up for nutrients that are naturally carried down into the groundwater by rainfall, carried off into the air as gases, and carried into the kitchen by you. At least 16 nutrient elements are necessary for plant growth, but plants need three -- nitrogen, phosphorus, and potassium (referred to by the elemental symbols N, P, and K) -- in relatively large quantities. Most soils contain large reserves of the other 13 nutrients -- especially calcium, magnesium, sulfur, iron, zinc, and manganese -- that might also hitchhike along when you fertilize with "the big three."
The only way to know for sure if your garden requires fertilizers is to have the soil tested. The cooperative extension services in most states test garden soil for a nominal fee. Also check telephone directories for "soil testing laboratories."
When to Apply. The best times to apply organic fertilizers are early spring and fall -- or even a few months -- before planting, because that allows time for soil microbes to digest the organic matter and transform nutrients into forms plants can use.
How to Apply. When you apply organic fertilizers, there's no need to dig them deep into the soil. Plants's feeder roots are mostly near the soil surface, and low oxygen levels deep in the soil would retard microbial growth, slowing nutrient release from organic fertilizers. Make an exception to that no-dig rule if a soil test shows that phosphorus levels are low. This nutrient moves very slowly, so the only way to spread it quickly through the root zone is to mix it into the top 6 to 12 inches of soil.
Always wear a dust mask when you apply bonemeal, guano, or any other type of fertilizer that's dusty. All dusts are potential lung irritants.
How Much to Apply. The actual amount to apply will vary, depending on the results of a soil test and the rate of nutrient release from a particular fertilizer. A rough rule is: Apply approximately 2 pounds of actual nitrogen (100 pounds of 10-10-10 contains 10 pounds of "actual" nitrogen) per 1,000 square feet, or 0.2 pounds per 100 square feet. Apply the other key nutrients plants take from soil -- phosphorus and potassium -- at about one-tenth this rate, unless a soil test specifies otherwise.
In catalogs and garden centers, you can find many different kinds of organic fertilizers. Other kinds are either custom blends, or materials that are available in limited quantities or only regionally. All fit into one of the basic categories -- plant, animal, compost, or manure -- that are further described below.
Plant Substances or By-products. Fertilizers that are plant substances or by-products are often rich in nitrogen, sometimes in potassium. These fertilizers can be considered renewable resources, but you should take into account the resources that may have been needed to grow as well as process or transport them. Some, such as beet pulp and cottonseed meal, are by-products of other industries.
Animal processing by-products. Industries such as dairy farming and meat processing generate waste materials that are dried or minimally processed into fertilizers. None of these materials is derived from "certified organic" animals.
Composts. These are the "Cadillacs" of organic fertilizers. Although making compost from a variety of raw materials is possible, the finished products are remarkably similar in their final concentrations of nitrogen, phosphorus, and potassium. Composts generally contain a good balance and wide spectrum of nutrients, and they're rich in humus -- so rich in humus, in fact, that their actual nutrient concentrations are relatively low.
Composts are available commercially or can be homemade. They can be used along with other fertilizers. Ingredients in commercial composts include various kinds of animal manures and lawn and garden wastes. Homemade fertilizer is a way to deal with "waste" and make fertilizer simultaneously -- and you always know what ingredients went into the finished product.
Manures. Manures used as organic fertilizers are derived from humans, animals, and in one case, insects; manures are available fresh or dried; however, use composted manure whenever possible.
The composition of various manures vary not only with the kind of animal source, but also with the age of the animal, the bedding, and method of manure storage and application.
Much of the benefit of organic fertilizers comes not from the nutrients, but from the organic matter -- the bulk -- the fertilizers contain. Among other benefits, organic matter helps soils hold water and air, makes nutrients already in the soil more available, and helps prevent diseases.
Don't spurn organic fertilizers that are low in nutrients, because they're rich in organic matter that turns to valuable humus in the soil.
Lee Reich lives in New Paltz, New York, and writes a regular garden column for the Woodstock Times.
Photography by John Goodman
Article published on June 23, 2008.