The word organic, applied to fertilizers, simply means that the nutrients contained in the product are derived solely from the remains or by-products of a once-living organism. Urea is a synthetic organic fertilizer, an organic substance manufactured from inorganic materials (although urea is also, as the name implies, a consititment of urine). Cottonseed meal, blood meal, bone meal, hoof and horn meal, and all manures are examples of organic fertilizers. When packaged as fertilizers, these products will have the fertilizer ratios stated on the labels.

Some organic materials, particularly composted manures and sludges, are sold as soil conditioners and do not have a nutrient guarantee, although small amounts of nutrients are present. Most are high in one of the three major nutrients and low in the other two, although you may find some fortified with nitrogen, phosphorus, or potash for a higher analysis. Many are low in all three.

In general, organic fertilizers release nutrients over a fairly long period; the potential drawback is that they may not release enough of their principal nutrient at a time to give the plant what it needs for best growth. Because organic fertilizers depend on soil organisms to break them down to release nutrients, most of them are effective only when soil is moist and soil temperature is warm enough for the soil organisms to be active.

Cottonseed meal is a by-product of cotton manufacturing. As a fertilizer, it is somewhat acidic in reaction. Formulas vary slightly, but generally contain 7 percent nitrogen, 3 percent P₂O₅, and 2 percent K₂O . Cottonseed meal is readily available to plants in warm soils, and there is little danger of burn. Cottonseed meal is frequently used for fertilizing acid-loving plants such as azaleas, camellias, and rhododendrons.

Blood meal is dried, powdered blood collected from slaughter houses. It is a rich source of nitrogen - so rich, in fact, that it may do harm if used in excess. The gardener must be careful not to use more than the amount recommended on the label. In addition to supplying nitrogen, blood meal supplies certain of the essential trace elements, including iron.

Fish emulsion, a complete fertilizer, is a partially decomposed blend of finely pulverized fish. No matter how little is used, the odor is intense - but it dissipates within a day or two. Fish emulsion is high in nitrogen and is a source of several trace elements. In the late spring, when garden plants have sprouted, an application of fish emulsion followed by a deep watering will boost the plant’s early growth spurt. Contrary to popular belief, too strong a solution of fish emulsion can burn plants, particularly those in containers.

Manure is also a complete fertilizer, but low in the amounts of nutrients it can supply. Manures vary in nutrient content according to the animal source and what the animal has been eating, but a fertilizer ratio of 1-1-1 is typical. Manures are best used as soil conditioners instead of nutrient sources. Commonly available manures include horse, cow, pig, chicken, and sheep. The actual nutrient content varies widely: the highest concentration of nutrients is found when manures are fresh. As it is aged, leached, or composted, nutrient content is reduced. However, the subsequent reduction in salts will reduce the chances of burning plants. Fresh manure should not be used where it will contact tender plant roots. Typical rates of manure applications vary from a moderate 70 pounds per 1000 square feet to as much as 500 pounds per 1000 square feet.

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 (approximately 6-3-0) 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 used primarily as a soil amendment and has a lower nutrient content (approximately 1-2-0). There is some question about the long-term effects of using sewage sludge products in the garden, because heavy metals, such as cadmium, are sometimes present in the sludge. However, all sewage sludge must be analyzed for heavy metals and meet EPA standars before it can be sold for soil applications.