HOME & AG: Organic fertilizers can still harm water sources

Apr. 7—When it comes to fertilizer, you can have too much of a good thing.

Whether growers are managing cash crops, a backyard garden or a single flower bed, professionals suggest following recommendations for fertilizer quantities in order to maximize results and prevent runoff.

"Organic gardening has been growing in popularity for many years now," said Bradley Secraw, Cleveland County Oklahoma State University Extension agriculture educator. "Hundreds of professional horticulturists and gardening experts have promoted benefits such as protection of pollinator species, reduction of human exposure to pesticides, and increase in nutrient content of foods produced."

Secraw said it can be difficult for consumers to decide from the thousands of products on the market, and just because a product is labeled "organic" does not mean a grower can use it indiscriminately.

"Even in the world of organic production, moderation is necessary," he said.

The three most popular kinds of fertilizers that address soil fertility can be sorted into three categories: plant sources, such as alfalfa meal and corn gluten; animal sources, such as blood meal and fish emulsion; and mineral sources, such as epsom salts and rock phosphate.

"However, the most popular organic nutrient source is compost," Secraw said. "It can be derived from plant sources, animal sources or a combination."

For most gardeners, adding compost may not be enough to achieve optimal results, as compost is classified as a soil amendment, not a fertilizer. Soil amendments are materials that improve the physical property of the soil to increase water retention, permeability, water infiltration, drainage, aeration, and structure, according to a factsheet provided by the Cleveland County OSU Extension Office.

Secraw said commercial compost additives follow a 1:1:1 analysis, which means it contains 1% nitrogen, 1% phosphorus and 1% nitrogen. As an amendment, he said this isn't destructive to crops, but if used as a fertilizer, the ratios for growing crops is thrown off.

One inch of coverage over a 1,000 square-foot garden will require 84 cubic feet of compost, which offers 3,100 pounds of compost per 1,000 square feet.

"With that 1:1:1 nutrient analysis in our compost, that gives us 31 pounds of nitrogen, phosphorus and potassium per 1,000 square feet or 1,350 pounds of each per acre," Secraw said. "To put that into perspective, OSU horticulturists will recommend an application of 1 lb of nitrogen per 1,000 square feet for a garden if the soil test nitrogen is at or near zero."

In other words, this kind of compost will add 30 pounds of unnecessary nitrogen if used as a fertilizer, which can harm crops.

"Granted, much of this nitrogen is in an organic form that will be released over several seasons, but at these levels, it is still excessive," he said.

Secraw said these mixes also offer too much phosphorus and potassium, which will harm growth, or run off.

"In the case of nitrogen, it does not bond to the soil, so it leaches through the soil profile and ends up in the groundwater," he said.

Secraw said a problem in Oklahoma farmers who operate small farms don't consider themselves a threat to the environment because of the scale of their operations, and because they use organic fertilizers and soil amendments.

"Imagine if these small gardens and farms were scaled up to hundreds or thousands of acres," he said. "Though not their intention, individuals engaged in these practices would be inadvertently contaminating groundwater and surface waters."

The best way to avoid runoff is to run a soil test so growers will know how much nitrogen they need to apply before it becomes saturated and runs off.

"Based on soil test results, compost can be inappropriate while other organic fertilizers may provide the needed nutrients without causing an excess accumulation of others," he said. "There are many, easy ways to fertilize and care for a great organic garden or lawn without it becoming a potential source of pollution."

Brian King covers education and politics for The Transcript. Reach him at bking@normantranscript.com.