How to find the nutrient values of organic fertilizers

clover field photo by Tiffany Woods
Cover crops generally release their nutrients slowly and include crimson clover (pictured), alfalfa and others. Photo: T. Woods
Last Updated: 
June 29, 2012

CORVALLIS – Gardening resources often recommend chemical fertilizers with N-P-K (nitrogen, phosphorus, potassium) ratios, but for organic gardeners, the numbers can be frustrating. Manure and other organic materials often don't come with N-P-K ratings, especially if purchased in bulk.

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

His report, "Values of Organic Fertilizers," is online.

It has information on the top sources of organic fertilizers, their NPK content, release speed and effectiveness.

"Nutrient values vary greatly among organic fertilizers," Penhallegon said. "They also vary greatly for a given organic fertilizer." Differences include variations in the age of organic material, decomposition, application method, timing, incorporation time, time exposed to the elements (rain, sun).

"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," Penhallegon said. "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 potassium.”

Cover crops generally release their nutrients slowly, over two to six months, Penhallegon said. Nutrient values for cover crops include: alfalfa, crimson clover, Australian winter peas and annual rye.

Bloodmeal, bat guano and many of the manures (variable nutrient contents) release their nutrients over two to six weeks. Burned eggshells, fish emulsion and urea (urine) are the fastest-acting organic fertilizers, lasting only a couple of weeks.

To boost the nitrogen content of your soils, apply nitrogen rich urea, feathers, blood meal, bat guano or dried blood. Manures are usually less expensive than other animal by-products. Organic amendments highest in phosphorus include rock phosphate, bone meal and colloidal phosphate. High in potassium are kelp, wood ash, granite meal and greensand.

To make soil less acidic, use 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 printed copy of Penhallegon's report, send a request along with a self-addressed, stamped, legal-sized envelope to: Lane County Office, OSU Extension Service, 783 Grant St., Eugene, OR 97402 and a check for $2 to cover the publication and mailing costs.

Author: Judy Scott