Cover Crop Calculator
By Nick Andrews
Nutrient availability from organic fertilizers and cover crops is difficult to predict. Organic fertilizers have different fixed ratios of plant nutrients. So unlike conventional farmers, organic farmers can’t order custom blends of fertilizers that perfectly match their fertilizer recommendations. The guaranteed nitrogen (N) analysis on fertilizer labels reports total N content. For conventional farmers this is generally equivalent to plant-available nitrogen (PAN). But the N in organic fertilizers must be converted into plant available mineral N (i.e. nitrate or ammonium) during the decomposition process. It is difficult to estimate the fraction of total N that is mineralized during decomposition. Estimating the fertilizer value of cover crops is more difficult, because it isn’t easy to estimate the “application rate” of cover crop residues, and no guaranteed N analysis is available. Cover crops also pose the same challenge of estimating plant available nitrogen (PAN).
The OSU Calculator was developed to help with this decision-making process. It is available for free download at: http://smallfarms.oregonstate.edu/calculator. The website also has instructions on how to sample and test cover crops and use the calculator. Equations built into the calculator predict how much PAN will be released from cover crops, organic fertilizers and compost within the growing season.
The cover crop stands in figures 1 and 2 were seeded at similar ratios of cereal to legume. In figure 1, the oats dominated, in figure 2 the vetch held it’s own. We tested these stands and although the cover crop in figure 1 was taller, it did not have as much total N or as much PAN, because legumes were more sparse. Cover crop stands vary widely in their N contribution and we recommend testing the cover crop and using the OSU Calculator until you get a good feel for the PAN contribution of cover crops you use.
Cover crop sampling is quite simple and the lab fees are not expensive. We recommend sampling a relatively uniform cover crop stand in four or five areas just before you incorporate it. I use a 2’ x 2’ metal frame, but you can use a frame of any size. If the stand is not too tall, I work the frame down to the soil surface and cut the plants that root within the frame at ground level. In a tall or viney stand I prefer to push the crop down flat for about 10 linear feet and place the frame in the middle of the flattened area. You can also use a sickle bar mower to cut the cover crop, but be sure to measure the area cut. Collect all of the cover crop in large bags and record the total area sampled.
On a tarp or inside, remove any soil from this field sample and quickly mix the plants together and pull apart large plants. This shouldn’t take any more than 2-4 minutes. Weigh the total field sample, then take about a 1 lb sub-sample and weigh it accurately. Place the sub-sample in a paper bag and send it to your soils lab and ask them to give you the percent dry matter and total percent N. When you get the results back from the lab, enter your information into the OSU Calculator to estimate total N contribution and PAN per acre.
The calculator has five worksheets. Enter the price and guaranteed nutrient analysis of the fertilizers you want to compare in the fertilizer analysis sheet. Typical analyses for common organic fertilizers are provided, but we recommend using the actual analysis of the products you use. Enter the results of your cover crop analysis on the cover crop analysis sheet. In order to estimate the cost of managing cover crops and to compare the cost of cover crop and fertilizer PAN, complete the your costs spreadsheet. This sheet asks several easy to answer questions to estimate the total cost of managing a cover crop, including fuel cost, labor cost and equipment depreciation. Next enter the cost per pound of the fertilizers you want to compare in the cost comparisons sheet. You can use this sheet to find the cheapest fertilizer for a particular nutrient and to compare the cost of fertilizer and cover crop PAN. Finally, enter your fertilizer recommendation at the bottom of the nutrients provided sheet. Select the cover crop field you’re interested in from the drop down menu and enter application rates of different fertilizers to find the most cost effective and balanced fertilizer program. Refer to the cost comparisons sheet to find the most cost effective and balanced fertilizer program for your crop.
You are asked to complete a short and voluntary registration form and survey when you download the calculator. This helps us learn how best to improve the calculator and understand whether it is useful. You must complete the simple math problem at the end of the form, it protects you from spam.
We hope that the calculator helps you to realize some of the value of your cover cropping program by fine tuning your fertilizer program. The calculator is a planning tool that can help you find the best supplemental fertilizer program for your farm. However, environmental conditions strongly influence nutrient availability in the field. We recommend visual monitoring and appropriate plant and/or soil testing during the season to make sure that your fertilizer plan is working well.
The OSU Organic Fertilizer and Cover Crop Calculator was developed by Nick Andrews, Dan Sullivan, Kristin Pool and Jim Julian from Oregon State University Extension. Contact Nick at firstname.lastname@example.org or (503) 678-1264 x149 for more information.