What are the nutrient values of organic fertilizers?

Last Updated: 
April 30, 2008

CORVALLIS - Do you ever read garden books that recommend chemical fertilizers with certain N-P-K (nitrogen, phosphorus, potassium) ratios?

If you are an organic gardener, these numbers can be frustrating. Manure and other organic materials often don't come with N-P-K ratings, especially if they are purchased in bulk quantities.

Organic gardeners are in luck. Ross Penhallegon, horticulturalist with the Oregon State University Extension Service, has collected information about the nitrogen (N), phosphorus (P) and potassium (K) content of many of the organic substances commonly used as fertilizer in Oregon, including green manure crops such as crimson clover and alfalfa.

His report, "Values of Organic Fertilizers," also contains information about how quickly an organic fertilizer releases available nutrients and a reference list on organic gardening.

"One of the most difficult things to determine for an organic gardener is how much organic fertilizer to use, say on a 1,000 square feet of garden," said Penhallegon. "For a fertilizer with an N-P-K ratio of 12-11-2, this means 12 percent is nitrogen, 11 percent is phosphorus and 2 percent is potassium. In simple terms, this means each 100 pound bag of the fertilizer would contain 12 pounds of nitrogen, 11 pounds phosphorus and two pounds nitrogen.

"For example, using 12-11-2 fertilizer, if we knew we wanted to apply one pound of nitrogen, we would use 1/12th of 100 pounds," continued Penhallegon. "This equals about 8 pounds of this fertilizer applied to get one pound of nitrogen out there in the soil."

Cover crops generally release their nutrients slowly, over a period of two to six months, said Penhallegon. Nutrient values for cover crops include: alfalfa (2.5 -0.5 - 2), crimson clover (2-0.2-2), Australian winter peas (3-0-1), annual rye (1-0-1).

Bloodmeal (12.5-1.5-0.6), bat guano (8-5-1.5) and many of the manures (variable nutrient contents) release their nutrients over a period of two to six weeks.

Burned eggshells (0-.5-.3), fish emulsion (5-1-1) and urea (urine) (46-0-0) are the fastest-acting organic fertilizers, lasting only a couple of weeks.

To boost the nitrogen content of your soils, apply nitrogen rich urea (42-46 percent N), feathers (15 percent N), blood meal (12.5 percent N), bat guano (12.3 percent N) or dried blood (12 percent N). Manures are usually less expensive than other animal by-products.

Organic amendments highest in phosphorus include rock phosphate (20-33 percent P), bone meal (15-27 percent P) and colloidal phosphate (17-25 percent P). High in potassium are kelp (4-13 percent K), wood ash (3-7 percent K), granite meal (3-6 percent K) and greensand (5 percent K).

To make soil less acidic, gardeners want materials rich in calcium, including clam shells, ground shell marl, oyster shells, wood ashes dolomite and gypsum (all are at least 30 percent calcium carbonate or straight calcium).

To obtain a copy of Penhallegon's "Values of Organic Fertilizers," send a request along with a self-addressed, stamped, legal-sized envelope to: Lane County Office, OSU Extension Service, 950 West 13th Ave., Eugene, OR 97402.

Author: Carol Savonen