2012 22.4 Estimating Plant-Available Nitrogen from Cover Crops

Information in this article is from a new PNW Extension Publication 636 (in press): Estimating Plant-available Nitrogen Release from Cover Crops.
Cover crops serve many functions in a crop rotation. An important benefit for organic farmers is the ability of legumes to fix atmospheric nitrogen and supply plant-available nitrogen (PAN) to subsequent crops. The authors have been working with organic vegetable farmers in the Willamette Valley to find ways to estimate cover crop PAN and understand the relative costs of cover crops and organic fertilizers.

Does cover cropping pay?

Based on calculations using the Oregon State University (OSU) Organic Fertilizer and Cover Crop Calculator, the short answer is yes, even when PAN is the only benefit taken into account, cover cropping pays. Legumes are a much more economical source of Nitrogen (N) than specialty organic fertilizers.
Consider the case of a 30-acre organic farm that is growing broccoli. The farmer uses a 70 hp tractor and pays $4.00 per gallon for diesel, $10.00 per hour for labor, $.70 per pound for common vetch seed, and $1,000 per ton for feather meal. Assume they drill the seed and irrigate once to establish the cover crop; then they flail mow, chisel plow and disc to incorporate the cover crop. Sixty pounds per acre PAN from a healthy stand of vetch would cost about $138.45 per acre ($2.30 per pound PAN) whereas the same amount of PAN from feather meal costs about $325.92 per acre ($5.40 per pound PAN) including application costs. Readers can use the calculator to make these estimates for their own farm by visiting http://smallfarms.oregonstate.edu/calculator.

Cover crop Plant-available nitrogen

During decomposition a cover crop can increase or decrease the N fertilizer requirements of the following crop. In general, legumes have higher N content than cereals, while leafy plant tissues have higher N concentrations than stems and more mature plant material. For legumes like common vetch that are high in N (e.g. 3%), about half of the cover crop N is released as PAN, because the cover crop has more N than needed to “build” soil organic matter. For non-legumes like cereal rye that are low in N (e.g. 2%), the release of PAN is small, because most of the cover crop N goes into soil organic matter. As cereals mature and start heading, their N content drops (e.g. 1%) and PAN is immobilized (negative PAN) during decomposition. Most of these changes in N levels occur in the first 4-6 weeks after plowdown.
Trials over the last few years give new insight into the amount of N that is mineralized (positive PAN) or immobilized (negative PAN) by cover crops. Amy Garrett and John Luna of OSU compared the response of broccoli to different rates of N fertilizer (applied as feather meal, 12-0-0) and cover crops. They found that common vetch cover crops replaced about 110 pounds of total N from feather meal, and that an additional 50 pounds total N from feather meal was needed to counteract immobilization by an oat cover crop.
We conducted laboratory incubations to measure the amount of N mineralized by different cover crop residues and field trials to verify the results of the lab work. Our results were consistent with research done in Kansas to predict PAN from crop residues. Total N content of a cover crop can be used to provide useful estimates of cover crop PAN (Table 1). Our estimates for cover crop PAN are applicable to Oregon and Washington west of the Cascade Mountains and should be used with caution in other cropping systems.

When to kill cover crops

PAN from any cover crop is minimal when it is very small (e.g., in March). For solo cover crops, the best time to kill the cover crop to maximize PAN depends on whether the cover crop is a legume or a non-legume.

  • PAN from a good stand of legumes (Figure 1) peaks at budding growth stage (e.g. May). PAN declines slowly for legumes as reproductive growth continues.
  • PAN from cereal residues is positive early in the spring (through tillering; mid to late March). As stem elongation proceeds (jointing), PAN from cereal residues declines. By the time the flag leaf (uppermost leaf) emerges from the stem (Feekes growth stage 8 or Zadoks 37), PAN from cereal crop residue is near zero. When cereal heads are visible (Figure 2), PAN from cereals is negative.

To maximize PAN from cover crop residues, kill cereal cover crops early, but wait until bud stage to kill legumes.
In cereal-legume mixtures, the best crop growth stage for maximum PAN benefit depends on the percentage of legume in the stand.

  • When the cover crop has mostly legume (e.g. 75% legume), it behaves much the same as does a pure legume cover crop. The PAN from crop residue increases until cereal boot stage. After cereals reach boot stage, PAN declines.
  • When the cover crop has more cereal than legume (e.g. 25% legume), it follows a similar PAN curve as a solo cereal crop, but negative PAN is usually not observed until the cereal reaches boot stage (around mid May). A cover crop with at least 25% legume can be allowed to grow until early May (boot stage for cereal) without danger of N immobilization (negative PAN).

Because we can’t plan ahead on the weather, we may not always be able to kill a cereal crop early enough to avoid negative PAN. Seeding legume and cereal mixes instead of a solo cereal crop allows greater flexibility in timing of cover crop incorporation without the consequences of negative PAN.

site-specific method to estimate

cover crop N uptake and PAN Our recommended field sampling and analysis method is based on a whole-plant above ground sample. The cover crop is harvested from a known area in the field, weighed wet, then sub-sampled. The sub-samples are sent to an analytical lab for determination of % dry matter (DM), cover crop biomass (dry weight), and total %N in DM.

Advantages of a site-specific method

  • Improved accuracy of cover crop N “credits” and the ability to fine-tune N fertilization practices.
  • Accuracy of this method has been documented extensively for winter cover crops harvested in March-May in the Willamette Valley.
  • Site-specific method is especially useful for mixed cover crop stands.


  • Collecting and weighing four quadrat samples from a field typically requires one to two hours
  • Laboratory analysis for DM and %N in DM costs $20.00 to$40.00 per sample.
  • Additional time is required to send samples to analytical lab, enter data, and use the worksheet or Organic Fertilizer and Cover Crop Calculator.

How to estimate site-specific cover crop N uptake and PAN

To make a PAN prediction for a cover crop, obtain:

  • Dry matter yield of the cover crop (measured or estimated).
  • A cover crop analysis for %N in DM. If there isn’t a N analysis for the cover crop, use typical values for the cover crop, but note the PAN prediction will be less accurate. If using typical values for mixed cover crop stands, estimate the portion of cereal and legume in the stand.

Measuring cover crops in the field requires a sampling frame, scale with about 20 pound capacity and 0.1lb accuracy, sharp knife or sickle (i.e. lettuce harvesting knife), about 4 large paper bags (i.e. grocery bags), and a 1 gallon zippered freezer bag for submitting the sample. The frame can be any size (we use 2’ x 2’ frames) and can be made from any readily available material.
Step 1. Select analytical lab. We recommend working with a laboratory that will dry and grind the whole sample submitted.
Step 2: Select the cover crop sampling areas in the field (quadrats). For most fields, four quadrats will give an adequate estimate of cover crop field weight and species mix. Choose sample areas that represent the species mixture and plant biomass in the field. Record the quadrat area sampled (Figure 3).
Step 3. Harvest cover crop. Cut cover crop leaving about an inch of stem above ground.
The best method for harvesting quadrats depends on the type of cover crop stand. Any harvest method can be used that gives a clean plant sample with a known harvest area. In short stands we worked the metal quadrat through the cover crop stand and cut the plants that root within the frame (Figures 4 and 5).
In tall and viney cover crops we pushed down the cover crop in one direction and cut through the cover crop lying on the ground to get a representative sample (figure 6).
A sickle bar mower (i.e. in front of a walk-behind tractor) can also be used to cut through a standing cover crop.
Step 4: Weigh quadrat samples. Weighing can be done in the field, or in a barn with hanging scale or platform scale. A 20 pound capacity and an accuracy of about 0.1 pounds is sufficient.
Step 5: Collect sub-sample for laboratory analysis. Combine the field samples from each quadrat on a tarp or clean, flat surface and vigorously mix the cover crops together. Chop or tear apart large plants. When the sample is thoroughly mixed, collect a large handful that fits loosely in a one-gallon bag (half full) and weighs about a pound. This is the lab sample.
Step 6: Ship sample to lab for determination of percent dry matter and total percent nitrogen. Ship the sample so that the lab receives a fresh plant sample for determination of % DM. Ship samples overnight or with blue ice to keep them cool in transit.
Step 7. Review laboratory analyses. Tracking cover crop lab analysis values for multiple fields over two or three years will help develop a running average that reflects the management system. After there is consistent data for three years of cover crops under management, growers may be able to reduce the frequency of lab analyses.
Step 8. Estimate PAN using worksheet or OSU Organic Fertilizer and Cover Crop Calculator. We use the following input data to estimate pounds PAN/a from a cover crop:

  • Cover crop dry matter (DM, ton/a and percent).
  • Total N concentration in cover crop dry matter.

We calculate cover crop total N uptake (lb N/a) as: biomass wet weight (ton/a) x (% DM in biomass ÷ 100) x (% N in biomass DM ÷ 100). % PAN is estimated using Table 1 or the OSU calculator.
The Extension publication (see Resources) has a worksheet that guides users through the calculations needed to estimate cover crop PAN. The calculator is an Excel spreadsheet that is available online at http://smallfarms.oregonstate.edu/calculator. It calculates the nutrient content of different organic fertilizer programs and estimates PAN released from organic fertilizers, compost and cover crops. It can help match fertilizer programs to fertilizer recommendations. The spreadsheets also allow quick comparisons of the cost of different fertilizer and cover crop programs. The tool is designed to help find the most cost effective cover cropping and fertilizer programs that meet crop nutrient requirements.
D.M Sullivan and N.D. Andrews (in press). Estimating plant available nitrogen release from cover crops. PNW Extension Publication 636.
Andrews, Sullivan, Julian and Pool. Organic Fertilizer and Cover Crop Calculator. Online at: http://smallfarms.oregonstate.edu/calculator.
This work was financially supported by Oregon Tilth, Inc. and Western Sustainable Agriculture Research and Education.

Tags: Cover Crop, Nitrogen, Nitrogen Cover Crop Calculator