Small farmers use commercial fertilizers, cover crops, organic fertilizers, compost and other amendments to add nutrients to their soil. But are they getting the best bang for their buck?

A new online tool from the Oregon State University Extension Service does the math so that small-scale conventional and organic farmers can figure the answer to that question more precisely. Nick Andrews, an instructor with the OSU Extension Service’s small farms program, helped develop the free, spreadsheet-based tool, which can found at Organic Fertilizer and Cover Crop Calculator.

It was emphasized that this is not entirely for organic fertilizer calculations.

“The calculator lets you estimate how much nitrogen and other nutrients your cover crops and fertilizers will provide for your next cash crop,” Andrews said. “That could help you cut back on fertilizer use and benefit from your soil building practices.”

Farmers can save money on fertilizer, while also using this information to reduce the risk of nutrient runoff into waterways, Andrews said. On the flip side, farmers might discover that they’re not using enough fertilizer, he said.

Farmers and gardeners who don’t use cover crops can still use the calculator to determine which types and amounts of organic and synthetic fertilizers to use.

The new calculator estimates the amount of nitrogen needed in pounds per 1,000 square feet while taking into account the amount of nitrogen added by cover crops and other soil amendments such as compost. The original 2010 calculator made calculations on a per acre basis.

This new calculator is most useful for small-scale farmers and experienced gardeners who are interested in refining their fertilizer programs. Before using the calculator, be sure to sample your soil. The calculator helps you account for legume cover crop nitrogen contributions and select the most cost-effective fertilizers, Andrews said.

More about cover crops, soil fertility and soil labs information is available in the list of links to  OSU Cooperative Extension publications.