Reducing Tillage in Vegetable Crops

This spring and summer our team visited 100 small-scale vegetable farms in Minnesota to do soil health assessments, and one of the key drivers of healthy soil was tillage. Soils with less tillage had better aggregate stability, faster water infiltration, and less compaction. While tillage is harmful in the long-term, it provides short-term benefits that can make growing vegetables easier. This article covers some of the main reasons that farmers till their soil and provides other ways to get the same benefits.

Tillage is the practice of disturbing soil by digging, stirring, or turning. It is commonly used in vegetable farming to loosen compact soils, add residues to the soil, prepare fields for planting, incorporate fertilizers, and manage weeds.

Reducing tillage provides long-term benefits to soil health. Soils that have less tillage tend to:

• Have more stability
• Resist compaction
• Hold more water
• Have less erosion
• Have enhanced biological activity

Reason 1 for tilling: Loosening compact soils

Loosening compact soils is one of the primary reasons that people till their soil. Tillage can be used to create new plots in areas like lawns or pastures. It is also used annually to help loosen soils before planting.

Alternative 1: Cover crops

One of the most important strategies growers can use to manage compact soils is planting deep-rooted cover crops. Cereal rye does an excellent job of loosening compact soils. Other common choices include cover crops with taproots like tillage radish and sugarbeet, or cover crops with very deep root systems like sorghum sudangrass (sometimes called Sudex).

Do not plant cover crops in the same families as your main vegetable crops. This helps to avoid disease and insect pest pressure. For example, planting tillage radish in the fall might be risky if you will plant broccoli in the same field the next year because they are in the same family and experience the same diseases and insect problems. 

Alternative 2: Less aggressive tillage implements

Tillage implements like rototillers or moldboard plows cause a lot of soil disturbance. Switching over time to tools that disturb the soil less can help reduce compaction. Light-intensity tillage implements include tools like broad forks and tilthers. Medium-intensity tillage implements that can help loosen compact soil include chisel plows, discs and vertical tillage.

Keep in mind that while tillage aerates soil in the short term, it breaks up beneficial soil aggregates and leads to more compaction in the long-term. Lower and medium intensity tillage implements are better than traditional tillage tools like the moldboard plow. But, they can lead to similar problems if used too often. In some cases, growers find that they need to make multiple passes with these tools to get the benefits they need.

Each farm is different, and the success of these less intensive tillage implements will depend on soil type and other management factors.

Alternative 3: Permanent beds

Some Minnesota growers, especially growers on heavier soils, use permanent raised bed systems. Instead of tilling the soil and making new beds or rows each year, growers can design their fields so that the walkways are always in the same places. Permanent beds are beneficial for many reasons.

Foot traffic and equipment is confined to the pathways, which reduces compaction in the beds.

Pathways are typically covered with mulch such as wood chips or straw, or planted with cover crops such as white clover or crimson clover.

Farms vary significantly in how they manage the beds in permanent bed systems. Some use tillage in the beds, others use moderate or reduced tillage practices, and others use no tillage at all in the beds.

Reason 2 for tilling: Residue and cover crop management

Incorporating residues (leftover plant material from crops and cover crops) into the soil is important for a few reasons. Plant residues on the surface can serve as hosts on which insect pests and crop pathogens survive between growing seasons. Decomposing green matter, like crop residues or cover crops, can also compete for nutrients in the soil.

It is ideal to have most of the residues from previous crops or cover crops broken down at the time of planting. Tillage helps speed up the process by breaking residues into smaller pieces and burying them in the soil. This makes them more accessible to microbes. However, tillage also compacts the soil and degrades soil structure over time. There are a few ways growers can manage residues with limited tillage.

Alternative 1: Using winter-killed cover crops

Sticking to winter-killed cover crops can reduce or even eliminate the need for tillage to incorporate cover crop residues. Fall-planted cover crops like oats, field peas, or radish, and warmer season cover crops planted in late summer will naturally die from cold winter temperatures. Usually, these cover crops are fairly well decomposed by the time spring planting comes.

Alternative 2: Mowing

Mowing cover crops and crop residues can be an effective way to kill existing crops / cover crops if done in a timely manner. For growers who do not use herbicides, mowing tends to be most effective when the cover crop reaches the flowering stage. Mowing before this point may result in re-growth.

Mowing can also help quicken the breakdown process because it cuts plant residues into smaller pieces, which are easier for microbes to consume. This is especially important if you plan to plant soon after terminating your cover crop or harvesting your current crop.

Flail mowers in particular chop residue into very small pieces and can help to spread residue evenly across the soil surface. Flail mowers are not a good fit in all cases. For example, when spring rye is terminated with a flail mower, it can form a thick mat on the soil surface, so a sickle bar mower is a better choice. But, in most cases, flail mowers work and can quicken the breakdown process. 

Alternative 3: Tarps

Solarization (applying clear plastic sheets to the landscape) and occultation (applying opaque plastic sheets to the landscape) are simple and effective ways to kill vegetation. These are impractical for large-scale plantings. But, they can be very useful for starting new plots for small-scale growers.
Common materials include silage tarps (typically 50’ x 100’) and billboard tarps (typically 15’ x 50’).
Depending on existing vegetation and weather conditions, solarization tends to take around 2 to 3 weeks to kill existing vegetation, and occultation takes about 4 to 6 weeks.
In very small-scale systems such as home gardens, cardboard boxes can be used. 

Alternative 4: Targeted tillage

Growers using intensive tillage implements such as rototillers and moldboard plows should consider switching to less intensive implements. Chisel plows and field cultivators are good choices for terminating cover crops and incorporating crop residues. That is because they have both horizontal and vertical action. They leave limited residue on the surface of the soil.

Some reduced tillage approaches like vertical tillage and coulter carts are less effective. That’s because they do not incorporate horizontal tillage. They tend to leave at least 50% of the residue on the soil surface. These implements are useful for other purposes such as cutting up residue and creating smooth seedbeds, but they will not effectively incorporate cover crop residues.

Strip tillage is another reduced-tillage practice to consider. The basic idea is to only till narrow strips and leave the area between the strips untilled. The area between strips often has a cover crop, either living or dead. Strip tillage allows growers to:

• Loosen compaction
• Apply fertilizers
• Create a nice, even seedbed in the areas they plan to plant

while the soil still benefits from not tilling between rows. These benefits include improved water infiltration and better weed control from cover crops. Dr. Ajay Nair’s lab at Iowa State University has spent many years examining strip tillage options for vegetable growers. They have videos and other resources on their website.

Alternative 5: Herbicides

Herbicides can also be used to kill cover crops. Herbicides are most effective when crops are young and actively growing, during warm, dry conditions. Refer to the Midwest Vegetable Production Guide for up-to-date herbicide recommendations for vegetable crops.

Alternative 6: Livestock

Some vegetable farmers use livestock to graze cover crops in the fall. For food safety reasons, growers should only bring livestock into vegetable fields that will not receive another crop that season. Typically, livestock do not fully terminate cover crops. They essentially “mow” the cover crops until they are winter killed. They also reduce the amount of residue in the field in the fall. Their manure adds valuable organic matter to the soil. When the cover crop dies over the winter, less breakdown needs to occur in the spring.

Reason 3 for tilling: Preparing fields for planting

It is commonly thought that a perfectly smooth seedbed is needed for good germination, especially for small-seeded crops like carrots and beets. Clods on the surface can smother small seedlings. Also, an uneven soil surface can result in variable seeding depths and access to moisture. In recent years, no-till and reduced till growers have adopted two main strategies to avoid the need for perfect seedbeds:

Alternative 1: Using a row cover

Cover the soil with a row cover right around the time that you expect seedlings to germinate. Leave it on for a week or two. This helps to hold moisture near the soil. While this doesn’t solve the issue of clods, it does help maintain moisture in the top couple of inches of soil. Growers seem to have success germinating small-seeded crops like beets and carrots this way in reduced tillage systems. Regardless of the smoothness of the seedbed, small-seeded crops are finicky and require constant moisture. Invest in irrigation for the first couple of weeks after planting. This is especially important in soils that are prone to crusting. Keeping the soil moist helps to promote seedling emergence by preventing crusting on the surface.

Alternative 2: Transplanting instead

Transplanted vegetables do not require the same smooth seed bed that direct seeded crops need. More growers are using PaperPot transplanters for small-seeded crops like beets (though this is an uncommon practice for carrots and mesclun). This technology has made transplanting more affordable and less labor intensive.

Alternative 3: Shallow cultivation tools

Tools like tilthers can break up clods on the surface to create an even seedbed surface without disturbing the soil below. These shallow tillage implements tend to till the soil down to about 2 inches, and so the integrity of the soil structure below is preserved. If the soil is very compact, using a broadfork can help to aerate the soil right before tilthing and planting. 

Alternative 4: Planting into residues

Growers who wish to reduce tillage can plant directly into cover crop residues. Large-seeded crops like pumpkins can be planted directly into the residues. Smaller seeded crops can be transplanted. The most common example of this system involves terminating a winter rye cover crop at the time of anthesis (flower formation) with a mower and sometimes herbicides.

Pumpkins are then planted directly into the residues, which serve as a mulch and help to reduce weed pressure. The mulch keeps the pumpkins clean and can reduce cleaning costs, especially for growers who sell to wholesale markets that demand very clean pumpkins. The mulch also acts as a barrier between plants and the soil, which can help limit soil borne disease pressure.

Keep in mind that straw mulch tends to keep the soil slightly cooler than bare ground in the summer (but slightly warmer in the winter). So, this method may be less suitable for heat-loving crops. In very wet years, this system can result in excess moisture. It is most commonly used in sandy soils that drain quickly. 

Reason 4 for tilling: Incorporating fertilizer

Ideally, fertilizer should be incorporated into the soil for maximum plant uptake and minimum losses to the environment. Applying nitrogen on top of the soil can lead to denitrification, and applying phosphorus on top of the soil can lead to runoff. So how can we incorporate nutrients into the soil while still minimizing soil disturbance?

• Banded applications: Apply fertilizer in furrows at a depth of 4 to 6 inches using a coulter, or in small-scale systems, a Dutch or Warren hoe. There is still some degree of disturbance with this approach, but it is far more targeted than tillage. Do not apply directly into furrows where you are seeding to avoid burning plants. Make your furrows for fertilizing a few inches away from your crops.
• Injecting fertilizer into the soil: This is a common approach among field crop growers, who directly inject anhydrous ammonia and ammonium nitrate in field crops. These types of fertilizers are not as common in vegetables.
• Strip tillage: Strip tillage and permanent bed systems allow for more traditional incorporation of fertilizers while still maintaining beneficial residue levels across the field.
• Fertigation: Fertigation refers to the application of fertilizers through irrigation water. Many vegetable farmers already use fertigation in high tunnels. This is an effective way to inject fertilizer directly near the root system. The water helps to pull nutrients deeper into the soil.

Reason 5 for tilling: Disease, insect and weed management

Tillage may have less of an impact on insects and pathogens than is commonly thought, but it does have a significant impact on the dynamics of your weed seedbank and timing of emergence of weed seedlings.

Diseases

Reduced tillage will not necessarily result in more or less disease. Some diseases are likely to cause fewer problems in reduced tillage systems, but others may become more problematic. The pathogens most impacted by reduced tillage are those that are soil-borne and impact a wide variety of crop families. Most notably, Rhizoctonia solani, a pathogen involved in damping off as well as various root and stem rots, can be worse in reduced tillage systems. This pathogen has a wide host range and survives in colonized plant debris in the soil. Vegetable growers may choose to start seedlings indoors and transplant rather than direct seed if damping off is a problem on their farms, to help reduce both disease incidence and impact.

For pathogens that are specific to crop families, sufficient rotation is more important than your tillage practice.

Insects

Overall, there does not seem to be a difference in the number of insect pests between reduced-till and conventional-till systems.

Weeds

The final reason that growers may wish to till their soil is for weed management. Tillage helps to bury weeds and is an important weed management strategy for organic growers. In general, a shift to reduced tillage tends to favor perennial weeds. Tillage agitates the soil and can disrupt the root systems of perennials. Without tillage, these plants tend to become more prominent. Read more about weed seedbank dynamics under various tillage systems here.

Reduced and no-till growers have a few ways to manage weeds:

• Shallow cultivation: While cultivation is a form of tillage, shallow cultivation disturbs only the top inch of soil. Shallow cultivation essentially uproots recently germinated weeds. It is most effective when weeds are in the “white thread” stage, only a quarter of an inch tall. Growers who rely on mechanical cultivation should cultivate early and often until the crop canopy fills in. Mechanical cultivation is most effective on annual broadleaf weeds. It is likely not enough for farmers with a lot of grasses or perennial weeds.
• Stale seedbed: Stale seedbeds are a simple but important management strategy for all growers. The basic concept is that prior to planting a crop, a grower prepares the field, and then allows it to rest for anywhere from 1 to 3 weeks. In that time, the first weeds will emerge, allowing the grower to kill the weeds using shallow cultivation, tarps, flaming or herbicides. Following that pass, the main crop is planted. If it has a long germination window, growers often do another shallow cultivation or flaming pass immediately prior to germination.
• Herbicides: While most Minnesota specialty crop growers are organic or use organic practices, herbicides are an effective option for growers who choose to use them. Growers should consult the Midwest Vegetable Production Guide for Commercial Growers for up-to-date herbicide recommendations.
• Solarization / occultation: Using tarps to kill weeds is a common strategy among no-till growers. It is an especially common practice for growers who have tricky patches of perennial weeds like thistle. Keep in mind that while tarping a field at the beginning of the season can prevent perennial weeds from emerging during the remainder of the season, they are unlikely to die after only one season of solarization.