Author: Natalie Hoidal, Extension Educator, local foods and vegetable crops
Reducing soil disruption is one of the most significant challenges for vegetable farmers who wish to improve their soil health and resilience. I often hear growers expressing shame about the degree of tillage they use on their farms. However, the impacts of tillage are less black and white than they may seem, and other management practices can be equally important. I did a deep-dive literature review this fall to understand how tillage impacts five key aspects of farming: soil structure, water dynamics, pests (including insects, diseases, and weeds), yields, and climate change resilience.
Before discussing these dynamics, it’s worth noting that tillage is never black and white, or as simple as tilling vs. not tilling. Tillage is simply a spectrum of disturbance. On one end is the Moldboard plow, and on the other is no disturbance at all; in between those extremes are practices like broadforking, strip tilling, subsoiling, chisel plowing, and disk plowing.
On the most basic level, tillage breaks soil aggregates apart into smaller pieces. In the process, it aerates the soil by turning it over. In the short term, this results in fluffy soil and a nice, even seedbed, but in the long term, it results in compaction and erosion. Smaller particles don’t allow oxygen, water, and roots to pass through as easily since they become more tightly packed than larger particles. Smaller particles are also more easily carried away by strong winds and heavy rain.
Tillage also has an impact on soil organic matter. When you aerate the soil through tillage, soil microbes respond by consuming plant residues and soil organic matter. In this process, some of the carbon in those residues is released into the atmosphere as carbon dioxide. In a no till system, there is no sudden burst of oxygen availability, so decomposition occurs more slowly, and more of the carbon stays in the top layer of soil as soil organic matter. More recent research suggests that no till systems do not actually retain more carbon overall (see below for a more detailed discussion), but they do retain more soil organic matter in the top 10cm (Lou et al., 2010). In conventional tillage systems, the soil is turned over, and so more carbon is buried deeply in the soil profile
Reducing soil disruption is one of the most significant challenges for vegetable farmers who wish to improve their soil health and resilience. I often hear growers expressing shame about the degree of tillage they use on their farms. However, the impacts of tillage are less black and white than they may seem, and other management practices can be equally important. I did a deep-dive literature review this fall to understand how tillage impacts five key aspects of farming: soil structure, water dynamics, pests (including insects, diseases, and weeds), yields, and climate change resilience.
Before discussing these dynamics, it’s worth noting that tillage is never black and white, or as simple as tilling vs. not tilling. Tillage is simply a spectrum of disturbance. On one end is the Moldboard plow, and on the other is no disturbance at all; in between those extremes are practices like broadforking, strip tilling, subsoiling, chisel plowing, and disk plowing.
Tillage and soil structure
Soil structure is developed through aggregates: soil particles and pieces of plant matter stuck together with sticky exudates and hyphae from soil bacteria and fungi. Soils with more aggregates tend to have more diverse and abundant microbial communities (Willekens et al., 2014), and they contain larger pore spaces, which allow oxygen and water to pass through.On the most basic level, tillage breaks soil aggregates apart into smaller pieces. In the process, it aerates the soil by turning it over. In the short term, this results in fluffy soil and a nice, even seedbed, but in the long term, it results in compaction and erosion. Smaller particles don’t allow oxygen, water, and roots to pass through as easily since they become more tightly packed than larger particles. Smaller particles are also more easily carried away by strong winds and heavy rain.
Tillage also has an impact on soil organic matter. When you aerate the soil through tillage, soil microbes respond by consuming plant residues and soil organic matter. In this process, some of the carbon in those residues is released into the atmosphere as carbon dioxide. In a no till system, there is no sudden burst of oxygen availability, so decomposition occurs more slowly, and more of the carbon stays in the top layer of soil as soil organic matter. More recent research suggests that no till systems do not actually retain more carbon overall (see below for a more detailed discussion), but they do retain more soil organic matter in the top 10cm (Lou et al., 2010). In conventional tillage systems, the soil is turned over, and so more carbon is buried deeply in the soil profile
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| Reducing tillage helps to build soil structure. Photo: Karl Hakanson |
More soil organic matter in the top 10cm means increased stability, less nutrient leaching, soil erosion, and soil compaction, and improved drainage, water infiltration, and biological diversity. To build soil organic matter, reducing tillage alone may not be sufficient. Reduced tillage should be combined with compost addition and cover crops to achieve these benefits (Willekens et al., 2014).
A field that takes longer to dry in the spring will be more difficult to enter on time. Surface residues retain moisture better than bare soils, and reduced tillage systems tend to retain moisture for up to a few weeks longer than bare soils in the spring. This soil moisture also translates into reduced soil temperatures (Hoyt 2016). Cool, damp spring soil has implications for yield loss, soil-borne pathogens such as damping-off, and some insects such as cabbage maggot and seedcorn maggot. These factors are discussed more in-depth below.
Despite the challenge of cooler, wetter spring soils, building SOM is important for longer-term resilience. Minnesota climate predictions, based on the last 100 years of data, include more water overall and more intense rain events, but also more time between rain (i.e. more droughts). Soils that are higher in organic matter retain water more efficiently, meaning less runoff during heavy rainstorms, and more resilience to drought. While tillage is not the only factor influencing SOM, a holistic approach to building soil organic matter on your farm, including the use of cover crops and the addition of compost, can mean more long-term resilience to climate change.
Tillage and water dynamics
Reduced tillage systems are better at holding on to water, which is a double-edged sword. Water retention in reduced tillage systems occurs for two reasons: surface residues take longer to break down when they are not incorporated, and these residues retain moisture. In the longer term, reduced tillage systems retain more carbon in the upper portion of the soil profile, and soils with higher levels of soil organic matter (SOM) can retain water more than those with lower organic matter (this characteristic is also determined by soil texture).A field that takes longer to dry in the spring will be more difficult to enter on time. Surface residues retain moisture better than bare soils, and reduced tillage systems tend to retain moisture for up to a few weeks longer than bare soils in the spring. This soil moisture also translates into reduced soil temperatures (Hoyt 2016). Cool, damp spring soil has implications for yield loss, soil-borne pathogens such as damping-off, and some insects such as cabbage maggot and seedcorn maggot. These factors are discussed more in-depth below.
Despite the challenge of cooler, wetter spring soils, building SOM is important for longer-term resilience. Minnesota climate predictions, based on the last 100 years of data, include more water overall and more intense rain events, but also more time between rain (i.e. more droughts). Soils that are higher in organic matter retain water more efficiently, meaning less runoff during heavy rainstorms, and more resilience to drought. While tillage is not the only factor influencing SOM, a holistic approach to building soil organic matter on your farm, including the use of cover crops and the addition of compost, can mean more long-term resilience to climate change.
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| Floods may become more common in Minnesota due to climate change. Photo: Anne Sawyer |
Tillage and insects, diseases, and weeds
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.Diseases
There is a basic assumption that burying plant debris and removing residues helps to reduce disease incidence because residues break down faster when they are chopped and incorporated, or heated in a compost pile. While this is true for many diseases, it does not necessarily mean that growers using reduced tillage will face substantially more disease pressure, especially if they are also using disease management best practices such as crop rotation and resistant varieties.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, is consistently cited as more problematic in reduced tillage systems (Turner, 2020). 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, thus helping to reduce both disease incidence and impact.
No till systems, especially organic no till systems, are sometimes weedier than conventional fields. As such, weed pressure may contribute to increased disease incidence due to the presence of alternative hosts for pathogens, which create a “bridge” for the pathogen to survive in a field between rotations of susceptible crops.
All of that said, some diseases are actually reduced in no till systems. Because these systems have higher microbial abundance and diversity, there can be more competition between microbial communities in the soil (Turner, 2020).
For pathogens that are specific to crop families, sufficient rotation is more important than your tillage practice (Sumner et al., 1986). Where genetically resistant or tolerant varieties are available, using them may also have a more significant effect on disease pressure than your tillage practice (ibid).
Insects
In a meta-analysis (basically a paper summarizing existing studies to-date) of 29 studies about pest insects and 30 studies about predator insects in different tillage systems, Rowen et al. (2020) summarized the following about how tillage impacts insect communities. While results may vary in individual studies, overall, there does not seem to be a difference in the abundance of insect pests between reduced till and conventional till systems. However, foliar insect pests tend to be more abundant than soil dwelling pests in conventional till systems.Studies about predator insects were quite variable, but in general, the overall abundance of predators was the same between reduced till and conventional till systems. The difference between these systems was most clear in soil-dwelling predators; there were more soil collected predators in no-till systems (Rowen et al., 2020).
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| Soil-dwelling carabid beetle eating larvae in a no-till field. Video: Claire LaCanne |
Weeds
Reduced tillage systems, particularly no till systems, tend to favor perennial weeds over time. Tillage agitates the soil and can disrupt the root systems of perennials, and so in the absence of tillage, these plants tend to become more prominent. When choosing an area for reduced tillage, start with a relatively clean field, and avoid moving to no-till until you have challenging perennials under control.Tillage and yields
In another meta-analysis of reduced till vs. conventional till systems, Cooper at al. (2016) found an overall yield reduction of around 4.5% in European cropping systems when using conservation tillage. In organic systems, yields were 19-26% lower. The authors attributed this discrepancy to the lack of organic herbicides and quick-release fertilizers. They did find a significant interaction between yields and crop rotation, as well as residue retention; farms that practiced good crop rotation saw less substantial yield reductions, and farms that removed residues from the soil surface saw fewer yield reductions. Keep in mind that removing residues means that you are not adding carbon back into your soil.It is worth noting that the studies involved in this meta-analysis focused primarily on field crops. Vegetable growers have certain advantages over larger field crop producers including shorter cropping windows (meaning more flexibility with timing for non-chemical weed control strategies like mechanical cultivation), and the ability to incorporate strategies like flail mowing tarping to expedite residue breakdown. Vegetable growers also have the option of starting seeds indoors and transplanting, which provides a wider window of opportunity to get crops in the field. Technologies such as paperpot transplanters have only increased these opportunities by allowing farmers to reduce the space and labor involved with transplanting.
In a series of studies specifically on vegetables in North Carolina, Hoyt (1999) found that tillage mattered more for short-season crops. Tilled fields consistently showed higher yields for short-season crops such as broccoli and snap beans, and they attributed the lower yields to lower soil temperatures in reduced tillage fields. However, for longer-season crops like tomato and winter squash, the yields were actually higher in reduced till fields. These crops had a chance to “catch up” with hot summer temperatures, and the surface residues seemed to provide benefits such as moisture retention (Hoyt, 2016).
Tillage and climate change
Reducing tillage is often cited as a mitigation strategy for climate change, but this may be an oversimplification. While reducing tillage can definitely contribute to adaptation, the mitigation effects are less clear.Mitigation
The impacts of tillage on soil carbon sequestration, or the ability of soil to keep carbon stored in the soil, is well documented in surface soils. Initial studies showing higher amounts of carbon in surface soils helped to create some of the buzz around reduced tillage practices as a form of mitigating climate change. However, more recent studies have looked at carbon storage deeper in the soil profile and found less significant differences between tillage practices. In general, reduced tillage systems retain more carbon in the upper portion of the soil profile (~10 cm), but conventional tillage systems retain a similar amount of carbon, it’s just buried and distributed more evenly throughout the soil profiles (Cooper et al., 2016 & Lou et al., 2010). If you are adding carbon over time (through residue retention, cover crops, compost addition, etc.), you can build your soil carbon stores. However, reduced tillage alone does not seem to result in overall soil carbon (Willekens et al., 2014). | ||
| Soil organic carbon is higher in reduced till systems if you only look at the top 10cm of the soil profile, but in studies that look deeper, stored carbon does not differ substantially. Original image from NRCS. |
Another important factor in the climate mitigation question is how nitrogen behaves under different tillage strategies. Mei et al. published a meta-analysis in 2016 including 212 observations of nitrous oxide emissions under different tillage strategies from 40 different publications. Overall, they summarized that conservation tillage had higher N2O emissions than conventional tillage (~17.8% more emissions), especially in tropical areas. This is significant because N2O has 298x the warming potential of CO2, and N2O emissions in agriculture account for 61% of all anthropogenic, or human-caused N2O emissions.
The basic principle they described was:
- Denitrification is an anaerobic process, meaning it occurs in the absence of oxygen. Denitrification is the transformation of NO3- (nitrate) into N2O (nitrous oxide) and N2 (nitrogen).
- When oxygen is present, denitrification is limited. Tillage briefly aerates the soil, introducing oxygen.
- Under limited tillage, the soil becomes more anaerobic, creating more opportunities for denitrification and N2O emissions. (This is true for a few years until the soil structure becomes more porous in reduced till systems, creating spaces for oxygen to permeate the soil).
All of that said, the study noted an important caveat, that in longer-term studies on soils that had been under conservation tillage for more than ten years, there was no difference in emissions between conservation tillage and conventional tillage. The take-away was that in the first years of reduced tillage, soils become more anaerobic (oxygen-depleted) because without tillage, the soil is not being actively aerated. However, over time due to the activity of plant roots, invertebrates, and microbes, the soil begins to form more aggregates and pore spaces, which allows for better oxygen infiltration (Mei et al., 2016 & Page et al., 2020).
My take-away from all of this: there’s a lot still left to learn, but the assumption that reducing tillage can mitigate or reverse the impacts of climate change may be a bit too simple to apply broadly.
Adaptation
Even though mitigation impacts are unclear, reducing tillage can still be an important strategy for climate change adaptation. In other words, while changing your tillage practice may not reverse climate change, it can help you to be more resilient.Because reduced tillage results in improved aggregation and soil structure (see discussion above), fields experiencing reduced tillage will eventually become more resilient to heavy rainfall events and erosion. While reduced tillage alone is not likely enough to improve soil carbon sequestration, combining reduced tillage with cover crops and compost addition can lead to healthier soils with increased organic matter as well as increased stability.
The fact that carbon is more concentrated in the upper 10cm or so of soil in no till systems is also significant; having more organic matter in the top few inches of soil (soil organic matter is ~58% organic carbon) means that the top layer of soil is better able to hold onto water and nutrients. This will become increasingly important if our climate changes in accordance with projections, which predict more intense rainfall events, more rainfall overall, and more periods of drought between rainfall events.
Moving forward
Throughout the winter, we'll be posting more about soil health. We also have four upcoming farmer to farmer events where you can learn from innovative farmers about their soil-building practices. If you have questions about soil health or ideas for articles you'd like to see, feel free to reach out anytime!
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