Organic fertilizers are those that are derived from plants or animals. Almost any organic material can be used as fertilizers, but some are objectionable for various reasons.
(The word “organic” is often used as a synonym for “natural.” Under this definition, rock phosphate is an “organic” fertilizer because it is derived from natural sources. This site uses the definition in the paragraph above, under which rock phosphate is not organic because it is not derived from a plant or animal.)
Organic materials can supply various plant nutrients, depending on what they are derived from. Some are unpredictable because the source material is unpredictable. For example, composted manure is an excellent source of nitrogen if it has been protected from rain. Manure that has spent weeks or months sitting in the rain, however, has had much of the nitrogen leached from it.
Possibly the most valuable effect from organic fertilizer isn’t the nutrients it adds, but the humus it decomposes into. Humus improves any soil, keeping clay from being too sticky and sand from being too droughty. Whether or not you use organic fertilizers, try to add organic material to your soil to improve its structure.
Depending on its state of decomposition, nitrogen in organic fertilizer may be locked up as protein, partially decomposed as urea or ammonium, or in a soluble form as nitrate. Well-composted organic matter will have a larger proportion of its nitrogen nitrified. This adds immediately available nitrogen to the soil, as well as the nitrogen that will be released later, after soil microorganisms break down the remaining protein and ammonium.
Organic forms of potassium are immediately available. Phosphorus becomes available at different rates, depending on its state in the organic matter, but is quickly locked up by the soil, depending on the soil composition.
Some of the most commonly available organic fertilizers are described below.
Bone meal (0–22–0)
Bone meal decomposes slowly, so it releases phosphate slowly. It is traditionally used when planting bulbs. Bone meal is alkaline. It helps neutralize the acidity of peat-based potting mixes, and makes a good additive for alkaline-loving plants such as cacti and succulents.
Cottonseed meal (6–3–2)
This by-product of cotton manufacturing is somewhat acid in reaction. Cottonseed meal is more readily available to plants in warm soils, but there is little danger of over-fertilizing. For general garden use apply 2 to 5 pounds per 1,000 square feet. Used frequently for fertilizing acid-loving plants such as azaleas, camellias, and rhododendrons.
Blood meal (12–2–06)
This is dried, powdered blood from cattle slaughterhouses. It is a rich source of nitrogen—so rich, in fact that it may do harm if used in excess. Be careful not to exceed the recommended amount suggested on the label. In addition to nitrogen, blood meal supplies certain of the essential trace elements, including iron.
Fish emulsion (formula varies, depending on manufacturer)
This well-rounded fertilizer is a partially decomposed blend of finely pulverized fish. The odor is intense, but it dissipates in a day or two. Don’t use it inside. Fish emulsion is high in nitrogen, and is a source of several trace elements. Too strong a solution of fish emulsion can cause plants to “burn,” particularly those in containers.
Activated sewage sludge (7–3–0.5)
Sewage sludge is a recycled product of municipal sewage treatment plants. Two forms are commonly available: activated and composted. Activated sludge has higher concentrations of nutrients than composted sludge, and is usually sold in a dry, granular form for use as a general purpose, long-lasting, non-burning fertilizer. Composted sludge is generally considered a soil amendment rather than a fertilizer. Sludge is an excellent fertilizer, often used on lawns and popular with golf course groundskeepers.
Manures
Commonly available manures include horse, cow, pig, chicken, and sheep products. The actual nutrient content varies widely, depending on the source and how it has been treated. Steer manure, often available as a bagged product, is composted manure from feedlots. Since cattle are fed large amounts of salt in feedlots, this manure is salty and shouldn’t be used where salt or drainage is a problem. In most soils, however, the salt leaches out quickly with normal watering.
In farm country, many types of manure may be free for the hauling. Chicken manure is very high in nitrogen, and is often protected from the weather, which helps conserve nutrients. Cow and hog manure are more likely to be exposed to weather.
Stable manure is often available in non-farm areas. Its quality depends on how weathered it is and how much bedding is included. Bedding (often sawdust, shavings, or straw) adds organic matter, but usually has little nutrient value of its own, so dilutes the nutrient level of the manure.
Fresh manure can be composted by simply piling it in an out-of-the-way place for a few weeks. To speed the process turn the pile over a couple of time to oxygenate it. It will heat up each time you turn it. When it heats only slightly, it is through composting. It can also be added to a mixed compost.
Other materials
Alfalfa pellets, sold as animal feed, are rich in nitrogen and make excellent fertilizer. They can be dug into the soil or added to a compost pile. Byproducts of many animal and plant processing industries can be used as fertilizer, either raw or conditioned in some way, such as chopped and composted. Check processing plants in your vicinity.
Nutrient content of organic materials. Primary nutrients by percentage of dry weight.
| Organic Material | Nitrogen | Phosphate | Potash |
| Manures | |||
| Cow | 2.1 | 3.2 | 3.0 |
| Horse | 2.1 | 3.2 | 2.0 |
| Poultry | 3.0 | 5.0 | 2.0 |
| Sheep | 1.6 | 1.2 | 1.0 |
| Swine | 2.5 | 2.1 | 2.0 |
| Alfalfa hay | 2.5 | 0.5 | 2.5 |
| Blood meal | 12.0 | 2.0 | 0.6 |
| Bone meal | 1.0 | 15.0 | 0 |
| Cottonseed meal | 6.0 | 3.0 | 1.5 |
| Fish meal | 10.0 | 6.0 | 0 |
| Straw | 0.6 | 0.2 | 2.1 |
| Kelp | 1.5 | 1.0 | 4.9 |
| Lawn clippings | 2.5 | 0.3 | 2.0 |
| Leaves | 0.9 | 0.2 | 0.8 |
| Sawdust | 0.2 | 0.1 | 0.2 |
| Soybean meal | 7.0 | 1.2 | 2.0 |
| Wood ashes | 0 | 2.0 | 6.0 |