Types of Cover Crops That Improve Wisconsin Soil Fertility

Cover cropping is a practical, cost-effective strategy for Wisconsin farmers and landowners who want to improve soil fertility, reduce erosion, and increase resilience to extreme weather. This article reviews the principal types of cover crops suited to Wisconsin conditions, explains how they improve soil fertility, and provides concrete management recommendations for establishment, mixes, and termination in common cropping systems across the state.

Why cover crops matter in Wisconsin

Wisconsin’s climate and soils present both opportunities and challenges for maintaining soil fertility. Long winters, spring runoff, heavy summer storms, and repeated corn-soybean rotations can deplete organic matter, leach nutrients, and compact topsoils. Well-chosen cover crops help by:

• capturing and recycling residual nitrogen and phosphorus before they leave fields;
• adding organic matter and stimulating soil biology (microbial activity, earthworms);
• fixing atmospheric nitrogen with legumes, reducing fertilizer needs;
• improving soil structure with fibrous roots and deep taproots that break compaction;
• reducing erosion and surface runoff on slopes and tile-drained fields;
• suppressing weeds through rapid canopy closure and allelopathic grasses;
• providing forage, grazing, or pollinator resources when managed appropriately.

Choosing species that match the planting window, termination options, and management goals is essential. Below we break cover crops into groups and provide specific recommendations for Wisconsin users.

Major groups of cover crops and how they build fertility

Legumes: biological nitrogen fixation and high-quality residue

Legumes are the primary choice when nitrogen addition is a priority. In Wisconsin rotations, legumes can supply 30 to 150 lb N/acre over a season depending on species, biomass, and fixation efficiency.
Common legume cover crops for Wisconsin:

• hairy vetch (Vicia villosa): winter-hardy, fixes 60 to 120 lb N/acre when established early and allowed to grow in spring; excellent in mixes with rye; seeding rate 20-30 lb/acre; fall or late summer seeding.
• crimson clover (Trifolium incarnatum): winter-tender in northern Wisconsin, useful in southern areas or as a summer cover; fixes 40-80 lb N/acre; seeding rate 10-15 lb/acre; planted in late summer.
• red clover (Trifolium pratense): long-lived, can be used as a short-term forage or multi-year sod; fixes 50-120 lb N/acre; seeding rate 8-10 lb/acre; spring or late summer seeding.
• field peas and Austrian winter peas (Pisum sativum): good for spring or early summer, fix 30-60 lb N/acre; seeding rate 40-80 lb/acre.

Management notes: inoculate legume seed with the correct Rhizobium strain; allow sufficient growth before termination so nitrogen accrues in biomass; be mindful of residue C:N ratios–legume-dominant residues mineralize faster and supply available N to subsequent cash crops.

Grasses: biomass, erosion control, and nitrogen scavenging

Grasses excel at producing large amounts of carbon-rich biomass, protecting soil and capturing leftover nitrate. They are critical when erosion control and soil structure are priorities.
Key grass cover crops for Wisconsin:

• cereal rye (Secale cereale): extremely winter-hardy, produces abundant biomass, aggressive nitrate scavenger; seeding rate 60-90 lb/acre for fall seeding; plant as early as possible after harvest (August-October); ideal for reducing spring nitrate loss and forming mulch for no-till soybean.
• winter wheat and triticale: similar to rye but less vigorous as volunteer suppressors; seeding rate 60-90 lb/acre.
• oats (Avena sativa): winter-kill in most Wisconsin winters–useful where spring termination is preferred; good for quick biomass and smothering weeds; seeding rate 60-90 lb/acre; plant late summer for autumn growth.
• annual ryegrass (Lolium multiflorum): deep root system, excellent soil structure improvement and compaction alleviation; seeding rate 10-25 lb/acre; can be winter-hardy depending on variety and location–be cautious about persistence.

Management notes: grasses tend to immobilize nitrogen when incorporated (high C:N), so planting legumes or mixing grasses with legumes can balance nutrient dynamics for the following cash crop.

Brassicas: deep foraging roots and nutrient capture

Brassicas, including radishes and turnips, provide intense rooting that fractures compacted layers, captures nutrients, and creates macropores that benefit following crops.
Common brassica cover crops:

• forage radish (Raphanus sativus var. longipinnatus, e.g., “Daikon” or “Tillage” radish): deep taproots that penetrate compaction, scavenge nitrogen, and rapidly decompose in spring; seeding rate 2-8 lb/acre; fall seeding preferred; often winter-kills in Wisconsin but check microclimate.
• turnips: similar benefits to radish with additional surface biomass; seeding rate 2-4 lb/acre.
• canola/rapeseed (Brassica napus): provides biomass and deep rooting; seeding rate 3-6 lb/acre; may overwinter in milder winters.

Management notes: brassicas are excellent in mixes to create channels in the soil; because they decompose quickly, they can release captured nutrients early in the season. They do not fix nitrogen.

Other covers: buckwheat, phacelia, and mixes for specific functions

• buckwheat: very fast-growing warm-season broadleaf for weed suppression and phosphorus mobilization; seeding rate 30-50 lb/acre; short season, useful between cash crops in summer.
• phacelia and other pollinator-friendly species: increase biodiversity and provide flowering resources.

Mixes combining legumes, grasses, and brassicas often give the best balance of nitrogen fixation, scavenging, biomass, and soil structure improvement.

Practical cover crop mixes and seeding rates for Wisconsin situations

Below are example mixes with target seeding rates and intended benefits. Adjust seeding rates downward when using drill vs broadcast seeding and adapt for northern vs southern Wisconsin.

• Winter protection and nitrogen supply (post-harvest to spring): 30 lb/acre cereal rye + 20 lb/acre hairy vetch. (Rye dominates biomass and scavenges N; vetch fixes N and adds diversity.)
• Compaction relief and quick spring nutrient release (fall plant): 6 lb/acre tillage radish + 20 lb/acre oats + 15 lb/acre field peas. (Radish opens soil; oats and peas add biomass and N.)
• Grazing or forage plus fertility (summer planted): 30-50 lb/acre oats + 15-30 lb/acre crimson clover. (Quick forage, moderate N fixation.)
• Short-season summer cover (between double-crop): 40 lb/acre buckwheat. (Rapid weed suppression and P mobilization.)
• Long-term soil-building mix for no-till soybeans: 60 lb/acre cereal rye drilled + 10 lb/acre annual ryegrass + 20 lb/acre hairy vetch. (High biomass mulch, compaction relief, and N.)

Seeding windows and establishment tips for Wisconsin

Timely establishment is critical to success in Wisconsin’s climate.

• Fall seeding windows: For winter-hardy species like cereal rye and hairy vetch, early fall seeding (late August to mid-September) produces the best root and crown development before winter. Later fall seeding is possible but reduces biomass and N fixation potential.
• Post-harvest options: After corn silage or soybean harvest, prioritize rapid ground cover–drilling improves establishment compared to broadcasting. If broadcasting, reduce seed size adjustments and consider light raking or rolling.
• Spring seeding: For spring-planted covers (e.g., oats, field peas), plant as early as soils are fit to capture spring moisture and grow before summer heat.
• In-season options: Interseeding (e.g., interseed crimson clover or radish into standing corn at V5-V6) can be effective but requires careful management and compatible herbicide choices.

Termination strategies and fertility outcomes

How and when you terminate cover crops directly affects nutrient availability and planting success for the next cash crop.

• Winter-kill species (oats, some radishes) simplify spring management but provide less long-term ground cover in mild winters.
• Roller-crimper into soybeans: For cereal rye, roller-crimping at anthesis produces a thick mulch that suppresses weeds and gradually releases nutrients. Successful roller-crimping requires rye to be at anthesis and soybeans to be planted into the mulch.
• Herbicide termination: Glyphosate or other herbicides can terminate most covers prior to spring planting. Timing matters: terminating too early loses biomass and soil protection; terminating too late can delay or reduce cash crop emergence.
• Mechanical termination and incorporation: Tillage kills and incorporates residues, accelerating decomposition but risking erosion and loss of soil structure gains.

Fertility outcomes: legume residues release N faster (lower C:N), grasses immobilize N until microbial decomposition catches up (high C:N). For corn after a high-rye stand, consider applying starter N or terminating early to reduce N tie-up.

Integration with manure, tile drainage, and erosion control

Cover crops and manure work well together in Wisconsin. Manure applied to cover crops in fall or spring adsorbs to root and surface biomass, reducing runoff and denitrification losses.
Best practices:

• Apply manure to actively growing covers when possible; the cover will take up nutrients and incorporate them into stable organic matter.
• On tile-drained fields, cereal rye and annual ryegrass are particularly effective at capturing nitrate before spring tile flow.
• On steep slopes and highly erodible land, prioritize high-cover species (rye, oats) and plant as early as feasible to protect soil through winter and spring runoff.

Potential risks and how to manage them

Cover crops are not a cure-all. Common challenges and mitigations include:

• Competing with cash crop: Terminate at the correct stage and use appropriate methods (roller-crimper timing, herbicide window).
• Volunteer issues: Rye and some brassicas can create volunteer or volunteer-suppression problems; plan rotations and termination correctly.
• Nitrogen tie-up: Mix grasses with legumes or time termination to allow mineralization before crop uptake; consider starter N for corn in heavy-grass residues.
• Winter survival variability: Recognize that oats and some clovers will winter-kill in Wisconsin; choose species by region and risk tolerance.

Practical takeaways for Wisconsin growers

• Match species to goal: choose legumes when N is the main objective, grasses for erosion control and nitrate scavenging, brassicas for compaction relief.
• Seed early for maximum benefit: earlier fall seeding yields greater biomass and more N fixation for winter-hardy covers.
• Use mixtures: combining grasses, legumes, and brassicas balances C:N, biomass, and rooting benefits and reduces risk.
• Pay attention to termination timing: it directly affects nutrient availability and cash crop performance–plan termination to minimize N immobilization and planting delays.
• Integrate with manure and tile management: cover crops can significantly reduce nutrient losses when coordinated with manure applications and tile drainage management.
• Start small and monitor: test mixes and termination methods on a few fields, monitor biomass, soil nitrate, and planting outcomes, and scale practices based on measured results.

Cover crops are a powerful tool for building soil fertility and resilience in Wisconsin systems. With careful species selection, proper seeding timing, and thoughtful termination, growers can capture nutrients, build organic matter, relieve compaction, control erosion, and gradually reduce reliance on purchased fertilizers while improving crop performance over the long term.