Estimating Plant-Available Nitrogen Release from Cover Crops

PAN basics
Applicability of this guide
A site-specific method to estimate PAN
Case studies
Appendices of cover crop research

Cover crops reduce soil erosion, improve soil tilth, and increase soil biological activity. Legume cover crops maintain nutrient balance in organic cropping systems becasue htey are one of the few organic inputs that supply nitrogen (N) without phosphorus (P) or potassium (K). Cereal cover crops maintain vegetative cover in winter (to reduce soil erosion) and to take up residual soil nitrate-N from the summer crop that might otherwise leach to groundwater. A key benefit of cover crops is their ability to supply plant-available nitrogen (PAN) for the following crop. PAN consists of ammonium-N + nitrate-N. The PAN provided by a cover crop can replace purchased N inputs such as fertilizer, compost, or manure. But to take advantage of this benefit, you need to know how to predict the PAN value of the cover crop. How much PAN is provided? When is PAN provided? What is the best way to predict the PAN-supplying ability of various cover crop species and mixtures?

Publication highlights

Legume cover crops provide up to 100 lb PAN/a. To maximize PAN contribution from legumes, kill the cover crop at bud stage (early May).
Cereal cover crops immobilize up to 50 lb PAN/a. To minimize PAN immobilization from cereals, kill the cover crop during the early stem elongation (jointing) growth stage (early April).
Legume/cereal cover crips mixtures provide a wide range of PAN contributions, depending on legume content. When cover crop dry matter is 75 percent from cereals + 25 percent from legumes, PAN is usually near zero.
A laboratory analysis for cover crop total N as a percentage in dry matter (DM) is good predictor of a cover crop's capacity to release PAN for the summer crop.
When cover crops contain a low N percentage (less than 1.5 percent N in DM), they provide little or no PAN.
When cover crops contain a high N percentage (3 percent to 4 percent N in DM), they provide approximately 25 to 40 lb PAN/ton of dry matter.
Cover crips decompose rapidly and release or immobilize PAN rapidly. Most PAN is released in 4 weeks after cover crop kill.
PAN from legume cover crops is usually less expensive than PAN from organic fertilizers.
Values for cover crop PAN listed here are most applicable to winter cover crop/summer vegetable crop rotations in western Oregon and Washington.

Section anchor "pan-basics"

PAN basics

Why cover crop N percentage is a good indicator of cover crop PAN

Cover crops may increase or decrease N fertilizer needs for the following crop in the rotation. Because decomposition happens quickly, so does PAN release or immobilization (negative PAN).

PAN released from a cover crop depends on crop species and crop growth stage. In general, green leafy plant tissues have high N concentrations and high PAN. More mature plant tissues (stems) have low N concentrations and low or negative PAN.

Carbon (C) and N dynamics control PAN release. As soil organisms decompose cover crop residues, a portion of cover crop C is lost from soil (as carbon dioxide). The remaining cover crop C is transformed by the decomposition process, yielding fresh soil organic matter with a C:N ratio of approximately 12:1. For a typical cover crop, 60 percent of cover crop C is lost as carbon dioxide, and 40 percent of cover crop C is incorporated into soil organic matter. Most of this decomposition occurs in the first 4 to 6 weeks after spring plowdown.

Nitrogen percentage in a cover crop is strongly related to PAN release following cover crop incorporation, as illustrated in the conceptual example given in Table 1.

For legumes (e.g., common vetch) that are high in N, about half of cover crop N is released as PAN because the cover crop has more N than needed to “build” soil organic matter.
For non-legumes, such as cereal rye, that contain about 2 percent N in dry matter (DM) during stem elongation, the release of PAN is small, because most of the cover crop N goes into soil organic matte
When cereal crops reach the heading growth stage (1 percent N in DM), PAN is immobilized (made negative) by cover crop decomposition because more N is required to build soil organic matter than is present in the cover crop.

**Table 1.** Nitrogen fate after rapid phase of cover crop decomposition is completed.^1,2
Cover crop (%N in DM)	Growth stage	Biomass DM (lb/a)	Cover crop N uptake (lb/a)	N in soil organic matter (lb/a)	Plant-available N (PAN) NH₄ -N + NO₃ -N (lb/a)
				N fate
Common vetch (3% N)	vegetative	3,000	90	40	50
Cereal rye (2% N)	stem elongation	3,000	60	40	20
Cereal rye (1% N)	heading	8,000	80	107	-27

¹Rapid decomposition typically occurs during the first 4 to 6 weeks after cover crop plowdown.
²Assumptions: Cover crops contain 40 percent C in DM; 60 percent of cover crop C is decomposed (lost as carbon dioxide); all cover crop N is retained (zero N loss); stable soil organic matter has C:N ratio of 12:1

Cover crop roots contribute only a small amount of PAN for the following crop and are ignored in calculations used in this publication. Research conducted in western Washington showed that Austrian pea and hairy vetch cover crops contained approximately 100 lb N/a above ground, but only 10 lb N/a in roots (Kuo et al., 1997). Roots also had low N in DM (%N less than 2 percent; C:N ratio greater than 20), suggesting that PAN release would be near zero.

Other cover crops

Phacelia is sometimes used to replace cereals as a winter cover crop. Like cereals, PAN from phacelia is positive in early vegetative growth, but is near zero or negative at flowering (Luna et al., 2018).

Brassicas provide PAN when killed at flowering growth stage (%N in DM is near 2 percent). Brassica cover crops are atypical in western Oregon vegetable crop rotations because of concerns about cross pollination of Brassica seed crops. Also, Brassica cover crops stimulate soilborne diseases, such as club root, that can infect Brassica cash crops (cabbage, broccoli, turnips, etc.) Mustard (a Brassica species) is used successfully in eastern Oregon potato rotations.

When to kill a winter cover crop to get maximum PAN benefit

PAN from any cover crop is minimal when the cover crop is killed 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 legumes (Figure 1) peaks at budding growth stage (May). PAN declines slowly as reproductive growth continue.
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 (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.

Figure 1. Common vetch in the vegetative stage.

Photo by D. Sullivan

Figure 2. This cereal rye has started to head.

Photo by D. Sullivan

Figure 3. This phacelia is flowering, but the peas are in late vegetative stage.

Photo by D. Sullivan

To maximize PAN, kill cereal cover crops early, but wait until bud stage to kill legumes.

In cereal/legume mixtures (Figure 3), 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 (75 percent legume line in Figure 4), it behaves much the same as does a pure legume cover crop. However, the PAN from crop residue increases until cereal boot stage (Feekes stage 10; Zadoks stage 45). After cereals reach boot stage, PAN declines.
When a cover crop has more cereal than legume (25 percent legume line in Figure 4), it follows a similar PAN curve as a solo cereal crop, but negative PAN is usually not seen until the cereal reaches boot stage (around mid-May). A cover crop with at least 25 percent legume can be allowed to grow until early May (boot stage for cereal) without danger of N immobilization (negative PAN).

Seeding legume/cereal mixes instead of a solo cereal crop allows greater flexibility in timing of cover crop kill without consequences of negative PAN.

Figure 4. Effect of kill date on typical plant available N (PAN) release from cereal, legume, or mixed stands. Based on compilation of field data from Willamette Valley cover crop trials. Source: D. Sullivan.

Estimating PAN

Cover crop N uptake is the total amount of N present in above-ground biomass.Usually, less than half of cover crop N uptake is released as PAN during the first year after incorporation. Figure 5 shows the typical relationship between cover crop N concentration and expected PAN release. Table 2 has the same information in a table format.

Figure 5. Calculator predictions for PAN from cover crop residues; PAN in units of lb/dry ton

**Table 2.** Predicted PAN release from cover crop residues at 4 and 10 weeks after field application¹
Cover crop N concentration	Plant-available N (PAN) predicted by the OSU calculator
% total N in dry matter²	% of total N		lb/dry ton
	4 wk	10 wk	4 wk	10 wk
1.0	-32	-12	-6	-2
1.5	-11	7	-3	2
2.0	11	26	4	10
2.5	28	38	14	19
3.0	34	42	20	25
3.5	39	46	28	32
4.0	45	50	36	40

¹ PAN estimated via OSU calculator prediction equations. Source: OSU calculator
²Obtain cover crop N concentration (on DM basis) from a laboratory analysis report, or estimate cover crop N concentration using the shortcut method.

Example:

You sampled the cover crop using one of the harvest methods and submitted a composite sample of the cover crop to the lab for N analysis. The cover crop biomass you measured was 2 ton DM/a, and the lab analysis was 3 percent N in DM. Using Figure 5 or Table 2, you can see that predicted PAN release from this cover crop is:

20 lb PAN/ton DM after 4 weeks
25 lb PAN/ton DM after 10 weeks

PAN release per acre is calculated as follows:
PAN (lb/a) = cover crop biomass (ton DM/a) x estimated PAN (lb PAN/ton DM)

Example: 2 ton DM/a x Calculator prediction at 10 weeks (25 lb PAN/ton DM) = 50 lb PAN/a

Section anchor "applicability"

Applicability of this guide

The field sampling approaches described to determine cover crop N uptake are applicable to a wide variety of cropping systems.

Research to support our recommendations was performed in the Willamette Valley, where cover crops are normally seeded in fall and killed in spring prior to a summer vegetable crop (see appendices for details).

Compared to results in western Oregon, winter cover crops accumulate less N in western Washington at the same calendar date because of the cooler climate, and some popular Oregon cover crops do not survive the colder winters in western Washington.

We expect a strong relationship between cover crop %N and PAN to be found in most locations. However, the timing of PAN release will differ in regions outside of western Washington and Oregon.

Crop residue decomposition rate and accompanying PAN release rate are primarily driven by soil temperature and moisture content. Decomposition proceeds two to three times faster at a soil temperature of 70°F than it does at 50°F. Decomposition proceeds most rapidly when soil is near field capacity, and it slows as soil dries.

Local research has not specifically addressed whether the method of cover crop kill (tillage, herbicide, roller-crimper, mowing) affects PAN. Research from other regions suggests that cover crop kill method does not affect the amount of PAN release, but these practices may affect the timing of PAN release.

Section anchor "estimate-pan"

Using a site-specific method to estimate PAN

The recommended field sampling and analysis method in this publication is based on a wholeplant above-ground sample from a specified area. The cover crop is harvested from a known area in the field, weighed wet, then subsampled. The subsamples are sent to an analytical lab for determination of cover crop biomass (dry weight) and total N percentage (%N in DM).

Advantages of a site-specific method

Accuracy of cover crop N “credits” is improved, and N fertilization practices can be fine tuned.
Accuracy of this method has been documented extensively for winter cover crops harvested from March through May in the Willamette Valley
A site-specific method is especially useful for mixed cover crop stands

Considerations

Collecting and weighing four quadrat samples from a field typically requires about an hour of labor.
Additional time is required to send samples to the analytical lab, enter data, and use the OSU Organic Fertilizer and Cover Crop Calculator or the Worksheet in Table 3.

Supplies needed

A sampling frame. The frame can be any size (we use 2' x 2' frames) and can be made from metal, PVC pipe, wood, or any other readily available material.
A scale with about 20-lb capacity and 0.1-lb accuracy
A sharp knife or sickle (e.g., lettuce harvesting knife)
About four large paper bags (e.g., grocery bags) for collecting samples
A small tarp for combining cover crop samples (about 6 x 8 ft)
A paper lunch bag for submitting the sample to the laboratory
Blue ice packets for preserving samples during shipment to the laboratory

Step-by-step method

Step 1. Select an analytical lab.

Because you will be collecting perishable cover crop samples, think about shipping details prior to sample collection. Check with your analytical lab to determine shipping options, sample packaging (paper or plastic), analysis cost, and whether the lab will grind the whole submitted sample (see step 5). We recommend working with a laboratory that will dry and grind the whole sample you submit. Grinding the whole sample prior to subsampling for analysis ensures that the lab analyzes a representative sample (a few grams at most) of your cover crop.

Step 2. Select the cover crop sampling areas in the field (quadrats).

It is better to sample a number of representative small quadrats from different parts of the field (Figure 6) than to sample one large area. 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 your field. Record the quadrat area sampled (ft² ).

Figure 6. Place the sampling frame in representative areas of the cover crop.

Photo by K. Pool

Step 3. Harvest the cover crop.

Cut the cover crop, leaving about an inch of stem above ground. Do not harvest small, low-growing weeds, because they typically have adhering soil. Getting soil into a cover crop sample alters its analysis, inflating DM and reducing N percentage.

The best method for harvesting quadrats depends on the type of cover crop stand. Three methods are described below. Any harvest method can be used that gives you a clean plant sample with a known harvest area.

Harvest method A.

Short, upright cover crops can be harvested using a quadrat frame. Work the frame through the canopy to ground level. Sample plants that root within the quadrat (Figures 7 and 8).

Figure 7. After working the frame to ground level, cut the plants that root inside the frame.

Photo by N. Andrews

Figure 8. The finished sample.

Photo by N. Andrews

Harvest method B.

For tall or trailing cover crops, push down the canopy in one direction and cut through the cover crop lying on the ground (Figures 9–10).

Figure 9. Push the cover crop down in one direction.

Photo by K. Pool

Figure 10. Cut through the cover crop to sample all plants lying within the frame.

Photo by K. Pool

Harvest method C.

Use a sickle-bar mower or similar harvesting equipment to cut a cover crop strip from the field (Figure 11).

Figure 11. Using a sickle bar mower to sample cover crops.

Photo by J. Luna

Regardless of the harvest method used, combine all of the field cover crop samples in a large bag or container. Clean plastic tubs or bags (at least 10 gal) work well. Avoid crushing or smashing the cover crop into slime. Protect samples from wilting in the sun or getting soaked by rain.

Step 4. Weigh quadrat samples.

Weighing can be done in the field with a tarp, tripod, and hanging scale (Figure 12). When it is not convenient to weigh in the field, you may want to weigh samples under a roof on a platform scale. An accuracy of about 0.1 lb is sufficient.

Figure 12. Weighing the field sample from all sampled quadrats.

Photo by J. Luna

Step 5. Prepare subsample for laboratory analysis.

Place the combined field samples on a tarp or clean, flat surface and vigorously mix the sample (Figure 13). Chop or tear apart large plants. When the sample is thoroughly mixed, collect a large handful that fits loosely in a paper bag (half full) and weighs about a pound. This is your lab sample.

It might take a couple of rounds of subsampling to reduce the field sample volume to a 1-gal lab sample volume. If you are not satisfied that you are getting a good mix of species, leaves, and stems, slice the plants into 4- to 6-inch pieces before doing the final subsampling.

Step 6. Ship the sample to the lab for dry matter and total N analysis.

Ship the sample so that the lab receives a fresh plant sample. Ship samples with blue ice to keep sample bags cool in transit, or ship the sample overnight. Generally, it is best to ship the sample early in the week, so it can be processed shortly after arrival.

Figure 13. Remove any soil, tear up large plants and mix the field sample.

Photo by N. Andrews

Step 7. Review laboratory analyses.

Tracking cover crop lab analysis values for multiple fields over 2 or 3 years will help you develop a running average that reflects your management system. After you have consistent data for 3 years of cover crops under your management, you may be able to reduce the frequency of lab analyses.

What %N and DM are "typical"?

Typical mid-April DM and %N for cereals and common vetch are given in the sidebar “Shortcut method”. Typical DM for phacelia, clover, and rye/vetch cover crop mixtures is shown in Appendix B.

What range is possible?

Cover crop N can range from less than 1 percent N in cereals after head emergence to 4 to 5 percent N in very young, leafy plants.
DM of 10 percent can be present in young, leafy, wet cover crops such as vetch or clover. Dry matter of 20 to 25 percent is typical for cereals after head emergence.

Step 8. Estimate PAN using Table 3 or OSU Organic Fertilizer and Cover Crop Calculator.

**Table 3.** Worksheet for estimating site-specific plant-available N release from cover crop.¹
Line no.	What to measure/calculate	Example: Vetch	Units
1	Area sampled to determine cover crop biomass: Quadrat area (ft²) x number of quadrats	16	ft²
2	Number of sample areas per acre: 43,560 ft²/acre ÷ Line 1	2,723	sample areas/ acre
3	Wet weight of cover crop field sample (lb)	12	lb wet cover crop
4	Percent DM in cover crop: lab data or your “shortcut” estimate²	15	DM, % in wet cover crop biomass
5	Calculate cover crop DM (ton/a): (Line 2 x Line 3 x Line 4 ÷ 100) ÷ 2,000	2.45	ton DM/acre
6	Cover crop total N percentage: lab data or your “shortcut” estimate (N, % dry wt)³	3.0	N, % in cover crop DM
7	Plant-available N from cover crop decomposition: Find your %N in DM in column 1 of Table 2 or Figure 5 and then find estimated PAN release under the “10 wk” column.	24	PAN, lb/ton DM
8	Calculate plant-available N for summer crop⁴ (lb PAN/acre): Line 5 x Line 7	59	PAN, lb/acre

¹The OSU Organic Fertilizer and Cover Crop Calculator calculates PAN (Line 8) from the input data in lines 1–6.
²See sidebar “Shortcut method” and Appendix C. A closed cover crop canopy retains moisture, so cover crop dry matter is relatively consistent across sampling dates.
³See sidebar “Shortcut method.”
⁴Typical values for PAN are 30 to 70 lb N/a for winter cereal/legume cover crops killed in mid-April (see Case Study 4). Check your calculations if your PAN estimate (Line 8) is greater than 100 lb PAN/a. This is the maximum PAN value observed for excellent vetch cover crops allowed to grow to bud stage (total cover crop N uptake = 150 to 200 lb N/a).

Shortcut method

If you prefer to forego lab analysis, you can harvest and measure cover crop biomass (see steps 1–3 in Table 3) and and use typical values for cover crop DM (step 4 in Table 3) and %N (step 6 in Table 3) to estimate PAN. Values below are typical for cover crops collected in mid-April in the Willamette Valley:

Biomass dry matter:

Common vetch = 12 to 18 percent
Cereals = 15 to 20 percent
50/50 vetch/cereal mix = 15 percent

%N in DM:

Common vetch = 3 to 4 percent
Cereals = 1.5 to 2.5 percent
50/50 vetch/cereal mix = 2.5 to 3 percent

The %N in cereals varies with field history. Fields that have a history of manure/compost application and/ or legumes in rotation have higher %N in cereal than do fields with history of only mineral N fertilizer application.

We always recommend cutting and weighing cover crop biomass to estimate PAN. Visual estimates of cover crop biomass are not very accurate, especially for multi-species cover crop mixes.

Section anchor "case-studies"

Case Study 1. Cover crop growth: Is bigger always better?

Situation

May can be rainy in the Willamette Valley, sometimes delaying cover crop kill by several weeks. Data shown below came from a field where cover crops were ready for killing in early May. Because of weather delays, however, the cover crop wasn’t killed until late May.

Question

What is the effect on PAN of delaying cover crop kill until late May?

Method

Cover crop samples were collected on May 5 and May 27 from the same field, and N concentration in the harvested biomass was measured (lb N/ton DM). Crop biomass (lb/a) increased by about 10 to 20 percent between cover crop sample dates.

PAN was determined via incubation of cover crop samples in moist soil in the laboratory. Nitrate-N accumulated in soil was measured after 4 and 10 weeks of incubation at 72°F.

Results

Delaying cover crop incorporation until late May increased N immobilization (negative PAN) for phacelia and cereal rye. Late May incorporation of legume cover crops also reduced PAN substantially (Figure 14).

Figure 14. Delaying cover crop incorporation until late May reduced PAN. Above-ground biomass was harvest on May 5 (left) or May 27 (right) from the same field in the northern Willamette Valley.

Recommendation

To get the most PAN value from cover crops, we recommend they be killed during the late vegetative growth stage. In western Oregon, this is often the first window of dry weather after about April 15 (about 2 weeks before bud stage for legumes or boot stage for cereals). The PAN penalty for delaying cover crop kill past bud/boot stage (about May 1) is most severe for cereal cover crops.

Case study 2. Soil testing for nitrate-N to track PAN

If you don’t estimate cover crop PAN by following the method described in this publication, you can use soil sampling to estimate the cumulative effects of the cover crop and soil organic matter on N fertilizer needs.

Most of the PAN released by a cover crop is present in the soil by about 6 weeks after incorporation. Thus, early-season (May/June) soil nitrate sampling can be used to assess N supply for a summer vegetable crop.

Soil nitrate-N above 30 ppm (0- to 12-inch depth) just prior to rapid vegetative summer crop growth is usually sufficient to meet yield goals for vegetable crops (Figure 15). When midseason soil nitrate-N is low, summer vegetable crops may respond strongly to PAN from a winter cover crop (Figure 16). In-season soil nitrate sampling can be used to assess PAN supply in both organic and conventional cropping systems. See EM 9221, Soil Nitrate Testing For Willamette Valley Vegetable Production for details.

Figure 15. Lettuce growth did not increase with additional N fertilizer when midseason soil nitrate-N was above 30 ppm. Lettuce field had small plots receiving 0 to 200 lb of preplant N fertilizer per acre, but all plants looked the same.

Photo by N. Andrews

Figure 16. Table beet growth responded strongly to PAN from a winter legume cover crop (upper half of photo) when midseason soil nitrate-N was low (less than 10 ppm nitrate-N without cover crop; lower half of photo).

Photo by N. Andrews