ROO Learning Series: Selecting Cover Crop Species for Your Context

Watch “Selecting Cover Crop Species for Your Context” with Ruth Knight here:

Here is an audio version of the session for listening on the go.

Here is the slide deck used in Ruth Knight’s presentation on Selecting Cover Crop Species for Your Context.

Key Takeaways

• Cover crops are an adaptive tool, not a fixed solution.
• Context is everything – no single cover crop works for all situations.
• Diversity improves soil resilience.
• Cover crops are part of a long-term system – Expect benefits to compound over time.
• Soil health is a journey, not a destination.

What Are Cover Crops?

Cover crops include plants that serve as a management tool for soil health and agronomic goals. They connect air to soil, create the carbon cycle by bringing in carbon, and feed microbes that are central to soil health. Cover crops are used to cover bare soil, either in fall, early spring, full growing season, or inter-seeded into a cash crop.

• Shoulder season cover crops: between cash crops – Fall or early Spring.
• Full season cover crops: dedicated to soil-building.
• Targeted areas: across entire fields or in select zones.
• Under-seeded/interrow: grown within a cash crop.

Regenerative Agriculture Principles

When managed properly, cover crops support all of these regenerative principles to improve soil function, regenerate ecosystems, and enhance productivity.

• Know your context: every farm and every field is different.
• Minimize soil disturbance: tillage and biocides disrupt the soil’s biological system.
• Keep soil covered: bare soil is an invitation for erosion and weeds.
• Maintain living roots: the more time roots are in the ground, the better the microbial life.
• Increase diversity: diverse plants feed diverse microbes, leading to healthier soil.
• Integrate livestock: livestock add biology to the system through manure and grazing.

Adaptive Management

By introducing cover crops, we can disrupt weed cycles, improve soil microbe biodiversity, and create resilient cropping systems.

• Rule of compounding: positive soil effects build over time.
• Rule of diversity: more species = stronger ecological function.
• Rule of disruption: cover crops can disrupt weed and disease cycles.

Ecological Processes

Cover crops are not a magic bullet, but they interact with every major ecological process (energy
flow, water cycle, mineral cycle, and community dynamics) to improve soil functions. They help
increase carbon inputs, cycle nutrients, and enhance soil aggregate stability.

Iterative Management Approach

Plan, monitor, evaluate, and adapt. Cover cropping is a continual learning process that requires
adjusting based on observed results. Attempting to assess the effectiveness of a cover crop in a
single growing season is not recommended.

Soil Monitoring

In-field assessments help identify limiting soil functions, such as compaction, infiltration issues,
crusting, and ponding. Monitoring aggregate stability highlights how cover crops contribute to soil
structure improvement. Soil respiration monitoring can indicate microbial activity, as microbes work
to decompose organic matter and cycle nutrients. Field tests, including soil infiltration and
compaction assessments, help determine cover crop effectiveness.

Functions of Soil Organic Matter

Soil organic matter is built through plant residues and microbial interactions. Cover crops
contribute to carbon capture, produce root exudates, increase microbial activity, and are essential
for long-term soil health.

Cover Crop Planning

The six steps for cover crop planning include:

1. GOAL: Know your goal.
2. WINDOW: Find your rotation window.
3. SPECIES: Determine what plant species to grow.
4. SEEDING: Choose seeding strategy.
5. TERMINATION: Choose termination strategy.
6. EVALUATION: Evaluate and plan for next crop.

Step 1: Define Your Goal

The first step is to define what you want to achieve with cover cropping. Goals influence species
selection, termination methods, and nutrient cycling. If you have multiple goals, rank them most
important to least, or short-term to long-term.

• Immediate Goals (First-Year Impacts)
• Reduce erosion: keep soil anchored against wind and water erosion.
• Scavenge nutrients: prevent excess nitrogen from leaching by holding them in plant and microbe biomass.
• Improve soil structure and water infiltration: cover crops break compaction, improve porosity and support the microorganisms that build soil aggregates.
• Increase biodiversity: a wider variety of root types/plant species creates a healthier soil microbiome by feeding a more diverse community of beneficial soil microbes.
• Provide livestock forage: cover crops can serve as high-quality grazing material.

Short-Term Goals (1-3 years)

• Fix nitrogen: legumes like clover and peas add nitrogen naturally.
• Suppress weeds: root competition keeps weeds under control.
• Improve soil compaction: deep-rooted crops like radishes break through compacted layers.

Long-Term Goals (3+ years)

• Increase soil organic matter: soil organic matter builds over time through microbial activity.
• Carbon sequestration: root-derived carbon is key to long-term soil health.
• Reduce input costs: healthy soils need fewer inputs over time.

Step 2: Identify Your Rotation Window

Timing is critical — cover crops must fit within your crop rotation. Being aware of which window is going to suit you best is key. Some options for cover crop integration are:

• Shoulder seasons: planted after harvest or before spring planting.
• Full season: used in place of a cash crop or fallow period for soil-building.
• Inter-seeding: planted within a standing crop (e.g., clover in wheat).

Considerations:

• Where in your rotation is it going to fit into your typical crop rotation and harvest dates?
• Water availability: drought conditions can impact cover crop success.
• Fertility compatibility and nutrient budgeting: the next crop’s needs must be factored in.

Step 3: Select Cover Crop Species

When selecting cover crop species, consider your primary goal. Species are categorized into grasses, legumes, broadleaves, warm-season, and cool-season crops.

• Grasses: readily available, provide rapid ground cover, and absorb excess nutrients.
• Legumes: (e.g., clovers, field peas, and alfalfa) fix nitrogen in the soil through symbiotic relationships with rhizobia bacteria.
• Broadleaves: (e.g., brassicas or chicory) often have deep taproots that enhance soil structure.
• Warm-season crops: (e.g., sorghum or millet) require higher temperatures for germination and growth and undergo peak growth during the heat of summer.
• Cool-season crops: cool-season species grow in spring and fall.
• Perennials, biennials and annuals: consider whether species that survive more than one growing season are a desirable addition to your mix.

Consider potential disease bridges when selecting species to avoid carrying over pathogens between crops. For example, avoid including brassicas in your mix if you’ll be planting canola the next year and canola-related pests are a concern.

Diversity Matters
Mixtures of multiple species help increase biodiversity, improve nutrient cycling, and reduce establishment risks (e.g., you’re ensuring something will survive regardless of environmental conditions).

C:N Ratio
Balancing the C:N ratio in cover crop mixes helps optimize soil fertility and facilitate plant residue breakdown. C:N ratio of cover crop residue is important; high-carbon crops (e.g., rye, straw) can immobilize nitrogen, while low-carbon crops (e.g., fresh alfalfa, buckwheat) decompose faster and provide nutrients to the next cash crop. Using a low C:N cover crop before a high C:N residue crop improves nutrient cycling and prevents nitrogen lock-up.

Step 4: Choose Seeding Strategy

How you choose to seed will primarily be dependent upon the equipment available to you for seeding the cover crop.

Equipment Considerations
Drill, broadcast, air seeder, planter, aerial seeding.

Seeding Rates, Timing & Costs Considerations

In general, if you’re preparing a 3 species cover crop mix, include 1⁄3 of the recommended seeding rate for each species. For a 4 species mix, use 1⁄4 the recommended rate. Seeding rate should be adjusted based on the planting method. For broadcasting, increase the seeding rate to account for lower seed-to-soil contact. For later fall planting, increase the rate to improve soil coverage and germination potential. For mixtures where you have small and large seeds, will you blend the seed to put it into one box? Or will you put it in multiple tanks? This really has an impact on your method of seeding and timing in terms of seeding rate. Take into account the seed size, as larger seeds need to be a greater part of the mix by weight than the smaller seeds. Blend expensive species with common, cost-effective ones.

Step 5: Choose Termination Strategy

Options for Termination:

• Winterkill: some species die naturally with frost.
• Grazing: animals convert biomass into nutrients.
• Tillage: consider how aggressive tillage needs to be to avoid excessive soil disturbance.
• Roller crimping: used as an organic termination method but can have variable success.
• Mowing/mulching: useful for weed suppression and residue management.

Regardless of your termination strategy, if you do not want to see the cover crop come back next year, make sure you prevent it from going to seed prior to termination. Timing is everything; terminate cover crops two weeks before planting to avoid moisture loss. If you had a very dry period, terminating cover crops close to planting cash crops may use up soil moisture. If you’re going along with the plan, sometimes you have to think of plan B or C if conditions are too wet for tillage or termination plans. Tarping and mulch are effective but usually only appropriate for smaller types of enterprises.

Step 6: Evaluate & Plan for Next Year

Regenerative farming is a cycle of observation and improvement. Evaluation Questions:

• Did you achieve your goal(s) with the cover crop? How do you know?
• Did you make any observations on erosion, weed control or other potential side effects of cover crops?
• How did the cash crop respond? Did you observe any changes in yield or crop health?
• How much biomass was produced?

Measuring biomass is actually a great way of an overall assessment of your cover crop, how well it’s growing, and your whole nutrient cycling. No system is perfect. We learn by doing, monitoring, and adjusting.