Customized calculators can estimate plant-available nitrogen (N) for large farms as well as for small farms and gardens. Both calculators predict plant-available N release from three types of organic inputs: cover crops, organic fertilizers and compost.
The OSU Organic Fertilizer and Cover Crop Calculator is available on the Small Farms website. It is a downloadable Excel spreadsheet that allows farmers and gardeners to estimate nitrogen release from organic fertilizers and cover crops.
The calculator makes it easy to compare the cost of nutrients from cover crops and fertilizers, and develop a fertilizer plan. This allows you to manage nutrients cost-effectively, and to supply enough nutrients for your crops without overfertilizing, allowing you to save money, optimize yield and reduce the risk of water pollution on your farm.
On the website, you can choose a calculator that shows estimated plant-available N from a cover crop in units of pounds N per acre (large acreage version) or in units of pounds of N per thousand square feet (small farms and gardens version). PNW Extension publication 636 “Estimating plant-available nitrogen release from cover crops” describes the science used to make estimates of plant-available N.
How can you estimate the amount of plant-available nitrogen released from a cover crop?
The amount of plant-available N released by a cover crop depends on its biomass (dry weight of aboveground plant material) and its N concentration (percentage of N in biomass). A typical mixed cereal-vetch cover crop that is killed in mid-April in the Willamette Valley will provide about 50 pounds plant-available N per acre for the summer crop. But, cover crop biomass and N concentration vary depending on a host of site variables. To get a more accurate estimate of cover crop plant-available N, Estimating plant-available N from cover crops (PNW 636) leads you through the process of collecting a cover crop sample, sending it to a laboratory for analysis and interpreting the laboratory results.
To estimate cover crop plant-available N you first need to measure or estimate cover crop biomass, dry matter and its total N concentration. The most accurate on-farm method is to:
- Harvest the above-ground cover crop from a known area and measure the fresh weight (figures 1-2).
- Mix the complete field sample thoroughly for a few minutes and shred large plants by hand (figure 3).
- Collect a small (1 pound) subsample, place it in a paper bag, and send it to an agricultural testing laboratory, following the step-by-step instructions in PNW 636.
- Find testing laboratories provided in the directory of Laboratories Serving Oregon (EM 8677). It generally costs $20-$40 for postage and lab fees to analyze cover crops.
When you have plant analysis results back from the lab, you can use one of the OSU Organic Fertilizer and Cover Crop Calculators to estimate PAN. It is not uncommon for people to see improved crop performance, and to save $200 or more per acre by including cover crop N in their fertilizer plans.
