Author: Nick Andrews

Publish Date: Spring 2014

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 as cost-effectively as possible, and supply enough nutrition for your crops without over-fertilizing. This can help you save money, optimize yield and reduce the risk of water pollution on your farm.

In March 2014 we launched a new “small farms and gardens” version of the calculator that makes calculations on a 1000ft2 basis. The original calculator for larger farms was launched in 2010. It makes calculations on a per acre basis (Oregon Small Farm News, vol. 5(2), spring 2010). Most small scale farmers and gardeners manage their fields in units that are less than 1 acre. This new version is designed to be easier to use on smaller scale operations.

The new “small farms and gardens” version includes all of the features of the “larger farms” calculator. Both calculators predict nitrogen (N) mineralization from organic fertilizers, compost and cover crops. When you use the nutrient management information and cost comparisons for fertilizers and cover crops, you can select the cheapest fertilizers and fertilizer programs for your farm that meet but do not exceed crop requirements. Depending on what you are doing now, this can save $200 or more per acre.

In November 2012, Dan Sullivan and Nick Andrews published PNW 636: “Estimating plant-available nitrogen release from cover crops”. This 23 page publication describes the science used to predict N-mineralization from cover crops, and answers many common questions about N release and immobilization from cover crops.

How much nitrogen can you get from a cover crop?

Figure 1 presents findings from Amy Garrett and John Luna’s research. The graph on the left shows that a vetch cover crop supplied about 110 lbs of plant-available N (PAN) for broccoli. In this example, a farmer could reduce supplemental N fertilizer by about 110 lbs PAN/ac and save up to $600/acre in fertilizer costs. The graph on the right shows that an oat cover crop immobilized about 50 lbs of plant-available N when it was decomposing. This farmer would have to supply more N fertilizer to maintain crop yield. 

How can you estimate cover crop PAN?

To estimate cover crop PAN you first need to measure or estimate cover crop biomass, dry matter and total N content. The most accurate on-farm method is to harvest cover crop foliage from a known area and measure the fresh weight (figures 2-3). Mix the complete field sample thoroughly for a few minutes and shred large plants by hand (figure 4). Collect a small (i.e. 1 lb) sub-sample and send it to a soil and plant laboratory in a paper bag (see Laboratories Serving Oregon). Make sure you don’t send any soil with the sample, and that the lab is willing to dry and grind your whole sample.

When you have results back from the lab, you can use the OSU Organic Fertilizer and Cover Crop Calculator to estimate PAN on a per acre scale, or per 1000ft2. We periodically hold workshops to show people how to sample cover crops and use the calculator. It generally costs less than $20 for postage and lab fees to analyze cover crops. 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. If you prefer not to send cover crop samples to the lab, you can estimate cover crop PAN by measuring cover crop biomass and using typical values to predict PAN. After a couple years’ experience measuring cover crops and using the calculator, some farmers feel comfortable estimating cover crop PAN visually based on their experience.

Figure 2. Harvesting cover crop biomass from a known area (4 ft2 quadrats in this picture) in order to estimate plant-available N release. This is usually done from at least four areas of the cover cropped field.        Figure 3. Weighing the cover crop biomass from the complete field sample. Samples from all four areas of the field are combined.                 Figure 4. Mixing the harvested cover crop to weigh total fresh weight from the 16 ft2 sample we take from each field. Submit about a 1 lb subsample to the lab to measure percent dry weight and percent total N.