Gardening Articles: Care :: Soil, Water, & Fertilizer

Organic Fertilizers (page 3 of 3)

by Lee Reich

Kinds of Organic Fertilizers

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.

  • Alfalfa meal and pellets contain nitrogen, phosphorus, and potassium. Some rose growers report good results when it is used as a mulch.
  • The dried, shredded remains of sugar beets after the juices and sugars are extracted are sold as organic fertilizer. They are rich in nitrogen.
  • Corn gluten is a high-nitrogen fertilizer with the unique ability to inhibit germination of seeds. With it you can feed your lawn and prevent crabgrass at the same time.
  • Cottonseed meal is made from the remains of cotton seeds after the oil is pressed out. It is a high-nitrogen fertilizer, but some growers have concerns about pesticide contamination of the meal. Cotton is a heavily sprayed crop, but pesticide-free cottonseed meal is available.
  • The extracts or pulverized parts of several seaweeds and kelp are good sources of minerals, potassium, and sometimes nitrogen. Follow application directions carefully when spraying on leaves, because sea plants can affect plant growth when sprayed directly on leaves.
  • Soybean meal is a high-nitrogen fertilizer that's very similar to its better-known cousin, cottonseed meal. For the best price, look for it at animal feed-supply stores.

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.

  • Blood meal is a rich source of nitrogen that is quickly available, so use with care.
  • Bonemeal is a rich source of phosphorus and calcium, and it supplies moderate amounts of nitrogen. "Steamed" bonemeal has less nitrogen but somewhat faster nutrient availability than "raw" bonemeal.
  • Fish products can be fairly rich in nitrogen, phosphorus, and potassium, but read the labels because nutrient concentrations vary.

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.

  • Cow manure is low in nutrients, but plants can absorb them moderately quickly.
  • Manure from seabirds or bats are rich in nutrients, but not in organic matter. Highly soluble and quickly available nutrients are useful early in season to stimulate vegetative growth. But be careful: high-nitrogen guanos may burn plants. Several types are available: Texan bat (10-4-2), Ancient seabird (0-12-1), and Peruvian seabird (14-11-2) are examples. All guanos can be mixed with water, steeped, and applied as a liquid.
  • Commercially available insect manure, cricket manure, is relatively nutrient-rich.
  • Poultry manure is relatively high in nitrogen, phosphorus, and potassium.
  • Composted sludges are rich in slowly available nitrogen. If industrial wastes are included, contaminants, such as heavy metals, are present. Though approved for vegetable gardening by the Environmental Protection Agency, these fertilizers are not acceptable for organic gardening.
  • Worm castings are similar to compost in their composition and are equally easy to produce at home. Because nutrient levels are so low in worm castings, they are -- like compost -- considered more a soil amendment than a fertilizer.

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

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