Spring is the time for thinking about fertilizers. Organic options are a great way to go.
Organic fertilizers such as manures, compost or bone meal are derived directly from plant or animal sources, according to Ross Penhallegon, horticulturist with the Oregon State University Extension Service. Inorganic fertilizers such as ammonium sulfate or ammonium phosphate are often called commercial or synthetic fertilizers because they go through a manufacturing process, although many of them come from naturally occurring mineral deposits.
Inorganic fertilizers usually contain only a few nutrients — generally nitrogen, phosphorus, potassium, sulfur and sometimes micronutrients, either singly or in combination. These nutrients are in a form readily available to plants. However, since they are lost from the soil quickly, you may have to fertilize plants several times during the growing season unless you use a specially formulated, slow-release type.
Some nutrients, such as nitrate, are quickly available for uptake by plant roots, Penhallegon said. If you need only a certain element such as nitrogen and want it to be quickly available to your plants, an inorganic fertilizer such as ammonium nitrate might be in order.
Organic fertilizers usually contain plant nutrients in low concentrations. Many of these nutrients have to be converted into inorganic forms by soil bacteria and fungi before plants can use them, so they typically are more slowly released, especially during cold weather when soil microbes are not as active.
But organic fertilizers have advantages. They don’t make a crust on the soil as inorganic fertilizers sometimes do. They improve water movement into the soil and, in time, add structure to the soil. Organics feed beneficial microbes, making the soil easier to work. But they may cost more than chemical, or inorganic, fertilizers because they are less concentrated, supplying fewer nutrients pound for pound.
Since many chemical/inorganic fertilizers are concentrated and very soluble, it’s easier to apply too much and damage your plants. Fresh, non-composted manure can damage your plants as well, because some manure contains harmful amounts of salts. They can also be a source of weed seeds.
Penhallegon 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. 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 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 2 pounds nitrogen.
“For example, using 12-11-2 fertilizer, if we knew we wanted to apply 1 pound of nitrogen, we would use 1/12th of 100 pounds,” he said. “This equals about 8 pounds of this fertilizer applied to get 1 pound of nitrogen out there in the soil.”
Blood meal (12.5-1.5-0.6) releases 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 to 46 percent N), feathers (15 percent N), blood meal (12.5 percent N) or dried blood (12 percent N).
Organic amendments highest in phosphorus include rock phosphate (20 to 33 percent P), bone meal (15 to 27 percent P) and colloidal phosphate (17 to 25 percent P). High in potassium are kelp (4 to 13 percent K), wood ash (3 to 7 percent K), granite meal (3 to 6 percent K) and greensand (5 percent K).
To make soil less acidic, gardeners want materials rich in calcium, including clamshells, oyster shells, wood ashes, dolomite and gypsum (all are at least 30 percent calcium carbonate or straight calcium).
Many garden centers and feed stores carry organic fertilizers and amendments for gardens.