Crop Report – Week of July 31st, 2023

Managing Cover Crop Nitrogen / Residue Breakdown

Across Ontario, wheat harvest continues, opening the best window for planting cover crops in most field crop rotations. You can find more information on the  2023 winter wheat quality update and achieving success with cover crops after wheat at fieldcropnews.com. But can those cover crops be managed to supply nitrogen to the next crop?

The amount of nitrogen a cover crop could supply to the following corn crop is a factor of the total amount of nitrogen it contains and how much of that will be released in time for the corn to take it up.

Cover Crop Nitrogen

The amount of nitrogen in a cover crop stand is determined by the amount of biomass (crop residue, roots) and its nitrogen (N) concentration. More cover crop biomass usually means more N. Planting early, using winter-hardy cover crop species that continue growing in the spring, and delaying termination can all increase biomass and total cover crop N.

The highest amount of nitrogen usually comes from a good stand of pure legumes. Most legumes are 3-4% N, and a good stand can easily contain over 150 lb N per acre.

But all cover crop species will accumulate nitrogen from the soil in their biomass. Scavenging nitrogen left over from fertilizer is a primary cover cropping goals in some situations. Even legumes, which we typically think of as fixing their own N, will first take up residual mineral N before doing much fixation. When that N is held in organic form in biomass, it is safer from loss to the environment, but it then needs to be released to be available for the following crop, ideally when the crop has high N demand.

Cover Crop Nitrogen Availability and Release

Accumulated N in cover crop biomass is released after decomposition and mineralisation. The rate and timing of N availability depends on the carbon to nitrogen ratio (C:N) of the biomass, soil texture, moisture, and temperature.

The soil microbes that decompose cover crop residue need carbon and nitrogen in a certain balance. A C:N ratio around 24:1 has the balance of carbon and nitrogen that microbes need to decompose residue efficiently. With higher C:N residues, microbes take the extra N they need from the soil, which can lead to competition with corn N uptake. The microbial N is eventually released, but too little too late for corn. If the C:N ratio is much lower, N is released quickly, sometimes before the crop can use it.

Cover crop species vary in their C:N. Legumes and brassicas have lower C:N ratios than cereals and grasses. This provides the opportunity to manage cover crop C:N by mixing species. An experiment in Maryland, US that mixed cereal rye and hairy vetch in six different proportions, a 50:50 split between rye and vetch biomass was able to achieve a C:N ratio between 25 and 30. (The pure cereal rye had a C:N of 86, while the C:N of pure hairy vetch was 16). Predicting biomass proportions from relative seeding rates is tricky and depends on timing, weather, and soil N availability. The C:N ratio of established cover crop mixes can be measured by lab analysis of biomass samples collected around cover crop termination time. Harvest from a measured square area to be able to convert results to a per-acre basis (Figure 1).

Figure 1. Cover crop biomass harvested from a 0.25m2 area for tissue analysis.

The composition of cover crop biomass carbon also matters. Carbon in biomass is a mix of carbohydrates (sugars), cellulose, and lignin. As cover crops mature, C:N tends to increase, as does the proportion of cellulose and lignin, which are harder for microbes to break down. This is especially true for cereals as they mature. This starts at tillering and accelerates after stem elongation. Finally, tillage can speed up residue decomposition and nitrogen release. Residue  sizing, increased soil contact, and soil aeration from tillage cause a quick rise in residue decomposition rate and can produce a flush of nitrogen release, whereas residue left on the surface breaks down and releases nitrogen gradually over the season (Figure 2).

Figure 2. Cover crop residue decomposition peaks quickly after tillage and is more gradual in no-till scenarios (as modeled by the University of Illinois Cover Crop Decision Support Tool based on trial data). Figure from https://farmdocdaily.illinois.edu/2021/02/introducing-an-update-to-the-cover-crop-decision-support-tool.html.

Predictive Tools for Nitrogen Release

Accurately predicting the amount and timing of nitrogen release from cover crops is challenging. Several tools have been released in the last few years that build on decades of research to help manage that challenge.

The University of Illinois and Purdue University produced a tool to guide cover crop termination timing by showing how cover crop C:N changes in the spring and how that timing influences decomposition rates under different weather scenarios.

Penn State created a tool for adjusting corn N rates based on cover crop biomass N content and C:N ratio that also considers soil texture and organic matter. This is a good tool to see how much cover crop biomass is needed for a large N credit.

A tool developed from the USDA’s, Natural Resources Conservation Service (NRCS) research, available from the Northeast Cover Crop Council includes lab-analyzed N, carbohydrates, cellulose, and lignin to estimate the amount and timing of nitrogen released from cover crop residues (Figure 3).

Note of caution: These tools have not been tested or calibrated in Ontario and should not be used for direct N credit recommendations, but rather may show trends of cover crop N release to help with on-farm trials and decision making on your farm.

Figure 3. Outputs from the Cover Crop Nitrogen Calculator showing the nitrogen credit and approximate release timing from a hypothetical cover crop.

Take-home Messages

• Experiment on your own fields to see how your cover crops affect optimum nitrogen rates.
• For a sense of what to expect from a cover crop mix, send a sample for lab testing of the C:N ratio at cover crop termination.
• Most cover crops grown in Ontario don’t provide a significant N credit next season, but consistent cover cropping can build soil organic nitrogen pools and increase soil N supplying potential over time.

Weather Data – July 24 – 30, 2023

Location	Year	Highest Temp (°C)	Lowest Temp (°C)	Rain (mm)	Rain (mm) April 1st	GDD 0C April 1st	GDD 5C April 1st	CHU May 1st
Harrow	2023	29.4	13.7	55.0	344	1937	1343	1985
	2022	29.0	13.6	1.0	302	1993	1421	2147
	10 YR Norm (11 – 20)	28.0	16.0	26.6	378	1983	1380	2141
Ridgetown	2023	30.7	12.3	38.2	418	1841	1256	1850
	2022	29.9	12.8	2.4	211	1874	1310	1974
	10 YR Avg. (2011-20)	27.4	13.8	34.8	335	1864	1267	1994
London	2023	29.8	13.0	72.4	410	1832	1247	1847
	2022	28.6	11.8	8.8	231	1817	1262	1912
	10 YR Avg. (2011-20)	27.2	14.5	14.5	338	1842	1248	1975
Brantford	2023	29.6	12.3	61.2	356	1822	1236	1836
	2022	29.8	11.2	1.9	231	1822	1256	1866
Welland	2023	29.6	13.0	77.8	379	1842	1250	1858
	2022	28.6	12.0	15.8	264	1871	1295	1977
	10 YR Avg. (2011-20)	27.3	15.0	18.8	321	1850	1255	1987
Elora	2023	29.1	10.7	27.2	335	1682	1113	1687
	2022	28.5	9.0	12.4	196	1664	1111	1706
	10 YR Avg. (2011-20)	26.2	12.0	18.9	335	1657	1076	1762
Mount Forest	2023	28.6	11.8	28.9	326	1681	1117	1693
	2022	26.0	11.6	28.3	260	1664	1117	1740
	10 YR Avg. (2011-20)	26.1	12.3	20.5	339	1637	1064	1759
Peterborough	2023	29.5	8.7	44.1	344	1710	1128	1675
	2022	27.8	10.6	7.4	262	1683	1115	1752
	10 YR Avg. (2011-20)	28.4	12.1	19.0	297	1682	1101	1768
Kemptville	2023	29.2	9.1	56.0	371	1833	1257	1826
	2022	27.6	13.0	10.0	385	1802	1215	1896
	10 YR Avg. (2011-20)	28.9	13.3	21.0	327	1764	1184	1881
Earlton	2023	27.4	8.3	13.3	266	1552	1023	1595
	2022	27.7	11.6	28.7	277	1515	997	1636
	10 YR Avg. (2011-20)	26.4	10.3	14.7	281	1410	913	1553
Sudbury	2023	28.6	9.4	36.5	438	1578	1050	1626
	2022	26.2	13.0	13.1	257	1519	991	1643
	10 YR Avg. (2011-20)	26.7	12.6	17.0	310	1512	993	1662
Thunder Bay	2023	31.4	8.2	12.8	212	1390	883	1456
	2022	28.7	8.9	11.0	398	1291	800	1345
	10 YR Avg. (2011-20)	26.1	11.0	16.0	310	1325	816	1414
Fort Frances	2023	31.5	6.3	23.1	254	1550	1036	1655
	2022	28.6	8.3	15.3	521	1368	879	1490
	10 YR Avg. (2011-20)	26.7	10.5	23.1	309	1451	925	1571