I started gardening a long time ago and at that time tilling was standard practice. Most gardeners did not have a tiller so they did it by hand with a shovel. In some circles the idea of not tilling started to make waves. My first introduction was Ruth Stout’s no-till garden. Just cover everything with straw. Better for the soil and a whole lot less work. No-till became more popular in agriculture in the mid 1990 but few gardeners followed suit. Even today, many gardeners have never heard of the technique and continue the practice of spring tilling.
No-till is claimed to be better for soil and the environment since it releases less carbon dioxide into the air.
I have been promoting this idea in gardening circles for 15 years or more and slowly the idea is catching on with gardeners, but it might be time to take a step back and have a close look at the claimed benefits of no-till because science now has a lot more data on this.
The Claims For No-Till
One of the main claims for no-till is that it prevents the negative effect that tilling has on climate change. The story goes like this. Tilling adds air into the soil. This air increases microbial activity which reduces the organic matter (food source for microbes) in soil. This organic matter is converted to CO2 through respiration and then escapes into the atmosphere.
If that was not bad enough, the loss of organic matter reduces the quality of soil. Tilling is blamed for the degradation of agricultural soil.
Even the United Nations Environment Program restates the claim that changing to no-till practices in agriculture, results in an accumulation of organic carbon in soil, thus mitigating climate change through carbon sequestration. You can read this story all over the place online and especially in fringe gardening groups. Unfortunately, the science does not agree.
In order to understand the difference between tilling and no-till it is important to look at various aspects of soil. You can’t just look at the level of organic matter. It is also important to understand that this is a very complex issue affected by location, climate, soil type, crop type, use of cover crops, type of crop rotation. etc. Any single blanket statement is at best an average and exceptions do exist.
Does Tilling Reduce Organic Matter (OM) In Soil?
There have been many studies that compare the soil organic matter in till and no-till systems and some of these have been done on farms that have been under scientific control for many years. I had no trouble finding studies that showed no-till increases OM, but I also found studies that showed tilling and no-till resulted in the same amount of soil organic matter. One metadata review of the global literature put it this way, “only about half the 100+ studies comparing soil carbon sequestration with no-till and conventional tillage indicated increased sequestration with no till”. Another meta study compared conventional tillage to no-till for 69 paired-experiments, where soil sampling extended deeper than 40 cm and found no increase in total organic matter with no-till.
Another meta study that looked at a lot of studies but did not eliminate ones that were not paired experiments found that the OM decreases in the upper level and increased in lower levels, however the overall change still showed a decrease in organic matter. The amount of decrease was related to soil type with some showing no decrease and others showing some decrease.
Moving to no-till may increase the total OM level or it might not depending on soil types and other parameters. Tilling does increase the OM level at lower depths and neither the significance or the cause are clear. It could be that tilling moves more organic matter to lower levels, or it might be due to plants being able to grow deeper roots in tilled soil.
Does tilling add air to soil which then causes a loss of organic matter? This is true for a short period after the tilling event, but over a period of months or years this effect is insignificant compared to other processes.
How Does No-Till Affect Climate Change?
An in-depth review of the science was done in 2014 and found that “The claims made for climate change mitigation through conversion to no-till agriculture are overstated for the global
potential for soil C sequestration”. Moving to no-till can reduce greenhouse gases through things like reduced fuel usage but there is little reduction from sequestering more carbon in soil. Moving to no-till will have limited effects on climate change.
How Does Tilling Affect Soil Aggregation?
Aggregate size decreases as the amount of tillage increases. No-till produces the highest degree of aggregation, reduced till is not as good and conventional till results in the least amount of aggregation. Aggregation improves air flow into the soil, increases water holding capacity, and results in better growth of both roots and microbes.
How Does Tilling Affect Microbes?
Most studies looking at microbe activity look at no-till combined with other techniques like improved cover crops and find an increase in microbe activity which may be due mostly to the increased vegetation. One study looking only at no-till in Mediterranean climates found a 71% increase in microbial biomass.
A meta analysis of 62 studies found that microbial biomass was greater under no-till compared to tillage, with the exception that chisel tillage (a form of reduced tillage) was the same as no-till. In the short term, tilling increased CO2 production from microbe activity, but over a longer term both systems have similar activity.
The increase in microbial biomass was less pronounced at lower soil levels.
What about the ratio of fungus to bacteria (F:B ratio)? It is believed that tilling is harmful to fungal growth and results in a decreased F:B ratio. A study looking at prairie soil found that although biomass of both fungi and bacteria increase in no-till, the F:B ratio did not change. A global meta-analysis of 60 studies came to the same conclusion.
Does Tilling Harm Fungi?
The traditional belief is that tilling is harmful to fungi because it “chops up the long mycelium filaments”. Tilling does disrupt mycorrhizal networks but the effects are not strong because the networks are able to re-form. Hyphal fragments can colonize roots and regrow. Dr. Maherali explained “a common method of propagation is to culture AM fungi using a host plant in a ‘trap culture’, and then to use the cut-up root fragments to inoculate other plants”. Root fragments containing pieces of fungi are called propagules and they are counted as living fungi on commercial product labels.
A single tilling event such as hoeing in the spring or even rototilling once a year will have limited long-term consequences on fungi. This also agrees with the observation that the F/B ratio does not change.
How Does Tilling Affect Soil Erosion?
“Over the last 80 years, tillage and other forms of erosion may have removed about 15 cm of topsoil from 20 million acres of cropland in the state (North Dakota). “I think that’s conservative,” said Dave Franzen, an NDSU soil fertility specialist, in January 2017.” “That 15 cm of lost topsoil contained a massive amount of nutrients, he added. We would have lost an additional 12.5 million tons of phosphate and 40 million tons of nitrogen. That is the equivalent of 75 years of N and P application at present rates.”
A global analysis found that on average soil losses were 60% lower under no-till than conventional tillage. The difference was greatest in low-clay soil and in temperate regions. There was less erosion in tropical clay soils which tend to be better aggregated and less erodible.
How Does Tilling Affect Yield?
The effect of no-till on crop yields is variable. A global meta-analysis looking at 50 crops and 63 countries and found that no-till matched conventional tillage yields for oilseed, cotton, and legume crop categories. No-till had some negative impact on wheat (−2.6%), rice (−7.5%) and maize (−7.6%). No-till reduced yields, on average, by 5.1% across 50 crops but yields dropped by as much as 20% for miscellaneous crops and root crops.
Yields tended to be lower in the first few years after starting no-till but increased after three years. “Overall, no-till yields were reduced by 12% without N fertilizer addition and 4% with inorganic N addition”. No-till performed best under rainfed conditions in dry climates.
No-till reduced yields the most in tropical latitudes (−15.1%) and the least in temperate latitudes (−3.4%). One downside of no-till in colder climates is that the soil stays colder in spring which delays planting. Tilling opens up the soil allowing it to dry and warm up.
Yields for no-till agriculture improve over time and are best in drier conditions, but even in ideal conditions, no-till has not been shown to provide higher yields than conventional tilling.
Factors Other Than Tilling May Be More Important
Agriculture is a much larger system than just tilling and it is important to look at it more holistically. Dr. Louis Schipper, University of Waikato, explained it well. All organisms produce CO2 through respiration but that is only one-half of the equation. The other half is plants. They take CO2 from the air through photosynthesis and add it to soil through their roots and as dead organic matter. When these two systems are in equilibrium, the soil organic matter level does not change.
Organic levels go down when the organisms in soil continue to live and plants stop adding organic matter. This is the case in bare soil. Keeping soil covered all year long with either crops or cover crops is more import to the OM in soil than the method of tilling. The type of crop rotation used also plays an important role.
Reasons For Tilling
There are some good reasons for tilling. In cold climates it opens the soil allowing it to warm up and dry out. This allows earlier planting.
If you have to change pH, either with lime or sulfur, it is important to incorporate this into the soil, or it can take a long time to change pH.
Phosphate moves very slowly through soil. Farmers tend to apply it very closely to the seeds as they plant them. Adding phosphate on top of the soil, won’t affect plants at the root level for years.
Best Practices For Gardeners
“Minimizing tillage is good for soils but eliminating tillage is not necessary for healthy soil“. There was a significant increase of no-till agriculture in Ontario a number of years ago, but over the last seven years there has been a reduction because it was not producing the expected higher yields. Adding winter wheat to their soybean-corn rotation had a more positive impact on soil health than the type of tillage.
All of the above studies are based on agriculture and it can be difficult to apply the findings to gardens. For example, many gardeners don’t till, but use a shovel. That may be less destructive? Gardeners also tend to amend soil more with organic matter and use mulch.
Organic faming has few tools for weed control and therefore rely heavily on tilling. Gardeners can easily reduce the weed problem with mulching, which is much less practicable on larger farms.
No-till may not increase sequestered carbon and may even reduce yields slightly in the short term, but it is still a good choice for improving the quality of soil. It increases soil aggregation, reduces erosion and increases its microbiology.
Gardeners should look at cover crops to increase the organic matter in soil and keep the soil covered at all times. The downside is that perennial cover crops may need to be tilled in before planting, but that is probably better than leaving the soil bare. Cover crops are also of less value in cold climates that have shorter growing periods.
The bottom line is that both tilling and no-till can produce good crops. Tilling may produce higher yields, but no-till may be better for long term soil health. If you have a normal small garden, skip the tilling and use mulch. If you have a larger market garden there are pros and cons for both options.