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This article has two sections:
Managing soil acidity to improve crop nutrition
Ideal soil pH for canola is 6.0 to 7.0. Fields with pH below 5.5 may see an economic benefit from lime.
The lime rate required to make a difference can be two tonnes per acre or more and the benefit only lasts five to 10 years. Soils tend to revert back to their natural pH levels, and pH levels have a tendency to fall over time.
This article explains soil acidity and provides liming tips.
What is acidity?
Key point. Canola yield loss often occurs with soil pH less than 5.5 and is likely significant at pH below 5.0.
The simple pH test, short for “potential of hydrogen,” indicates the number of free hydrogen ions in a solution. Acids, which have values between 0 and 7 on the scale, have more hydrogen ions. Alkalines, which have values from 7 to 14, have more hydroxide ions.
Each unit represents a tenfold difference. For example, soil with a pH of 5.0 has ten times more free hydrogen ions than soil with a pH of 6.0.
Soil tests provide a pH rating. Prairie soils have a range of acidity. Manitoba soils tend to be alkaline. Alberta soils tend to be acidic. Saskatchewan is in the middle, and exceptions are everywhere. Variability is often high within fields. (See the image below from SKSIS.)
Parent material is the primary determinant for acidity. Soils from eroded granite are acidic; soils from eroded calcareous shale or limestone are alkaline. Soils from forest areas also tend to be more acidic than soils from grasslands. Decomposition of organic matter releases acidifying ions, so forests – with years and years of falling leaves and needles, and rotting trees and understory plants – have more organic matter in the topsoil area. This makes them more acidic.
For reasons of parent material, soils will eventually revert back to their normal state of acidity. And over time, soils – through natural and human-influenced processes – become more acidic. Liming is not a permanent solution.
Factors that increase soil acidity:
- Slow weathering of granite material releases acidifying cations.
- Nitrogen fertilizer makes soil more acidic. The nitrification process, which converts ammonium to nitrate, results in release of hydrogen ions that increase acidity. Nitrate-only fertilizers, such as potassium nitrate, and manure are the only non-acidifying nitrogen fertilizer sources.
- High levels of rainfall leach alkaline cations, leaving more acidifying cations in the topsoil layer.
- Decomposition of organic matter acidifies soil. No-till can make the top layer of soil more acidic because it concentrates both fertilizer and organic matter at or near the surface. Tillage blends both fertilizer and organic matter through the top few inches, diluting the effect.
- While decomposition of organic matter increases acidification, removal of organic matter increases acidification even more because biomass contains high levels of alkaline cations calcium and magnesium. Removing these macronutrients – through forage harvest and even through grain harvest – shifts the soil balance to acidifying cations.
Why is soil acidity a problem?
Key point: Acidity below 6.5 starts to hamper phosphorus availability, and this gets steadily worse as pH drops. Below 5.0, aluminum and manganese toxicity will poison crops.
Major crops on the Prairies thrive at pH of 6.0 to 7.0. Below that, various soil chemistry factors start to affect crop yield. And those effects continue to get worse as pH drops. Somewhere between pH 5.0 and 5.5, yield will start to drop for the common crops on the Prairies and yield loss can be significant below 5.0 to 5.2.
Research from northern Idaho (Mahler and McDole, 1987) found wheat and barley yields in soil with pH 5.0 were only 60 to 80 per cent of yields in soils with pH of 5.3. For pea and lentil, yield drop started at pH 5.8 and was down to 50 per cent at pH 5.0. The study did not include canola.
The two most yield-damaging effects are:
Phosphorus insolubility. Restrictions to phosphorus availability kick in around pH 6.5 and get steadily worse. Free aluminum and iron cations bind with phosphorus ions, making them insoluble – and unavailable for crop uptake. The lower the pH, the more free aluminum and iron cations and the more phosphorus tied up and essentially lost. As acidity drops, this same effects kicks in for potassium and eventually also nitrogen and sulphur. By pH 5.0, fertilizer availability plummets.
Aluminum toxicity. At soil pH below 5.5, the concentration of hydrogen ions reaches a point where it removes high levels of aluminum and manganese ions from soil particles. Clay particles and soil organic matter contain a lot of aluminum and manganese, and these ions normally bond tightly to clay and organic matter. Once released into the soil solution, aluminium (and manganese to a less extent) interferes with normal root function. Plants become stunted and weak. Plants short of nutrients also have lower resilience to insects, disease or weather. The combination causes a major loss of yield potential.
Acidity can also reduce nitrogen fixation in the root rhizosphere of legumes, and the timely breakdown of some residual herbicides.
How does lime help?
Key point: Lime contributes to a chemical process that removes free hydrogen and also increases calcium, both of which will reduce acidity.
The standard lime product is calcium carbonate (CaCO3). Calcium carbonate dissolves in water in the soil to form Ca2+ ions, bicarbonate (HCO3–) and hydroxide (OH–). These help in two ways:
- Hydroxide binds with hydrogen (H+) ions, the ones causing acidity, to form water – which is neutral.
- Calcium ions are basic, and further improve the ratio of alkaline to acid ions. Calcium is also a key macronutrient for crop growth. Magnesium atoms in dolomitic lime serve a similar function.
Many studies show the benefits of liming, and acid soils around the world get regular lime treatments.
An early 1990s study at Agriculture and Agri-Food Canada’s Beaverlodge centre in northern Alberta looked at lime effects on canola yield and brown girdling root rot. The study site had a soil pH of 5.13 in the top four inches. Researchers applied 7.5 tonnes per hectare (three tonnes per acre) of agricultural grade calcitic lime – primarily calcium carbonate – in May 1991, and roti-tilled it in to a depth of four inches. This increased pH to 6.6 in year one. Canola grain yield increased 37 per cent in tilled soil and 17 per cent in no-till soil. Brown girdling root rot severity went down.
A 1970s Peace region study, led by Alberta researcher Doug Penney, compared lime benefits for canola, barley, alfalfa and red clover. Researchers limed to a target pH of 6.7. The study concluded that at pH below 5.0, all crops will likely have severe yield loss without liming. For soils with an original pH of 5.0 to 5.5, the lime application increased alfalfa yield 80 to 100 per cent, barley 10 to 15 per cent, and canola and red clover five to 10 per cent.
Where to apply lime?
Key point: Use grid soil tests to identify highly acidic areas within a few fields. Target lime to those areas and measure the results to provide an on-farm analysis of lime ROI.
Identifying strong acid areas within fields will require a number of separate soil samples, taken on a grid. However, probes to measure pH only are available as quick indicators of target areas. Full soils samples in those specific areas will identify the right rate, but this does cut down on the grid soil sampling cost.
One idea is to select one area with very low pH. (Avoid areas that drown out or have high salinity.) Use soil tests and follow soil test recommendations for lime rates. Measure the yield improvement relative to historical yield for that area and also yield for the rest of the field. Calculate a timeline for return on investment.
What lime source to use?
Key point: Products that increase soil pH include lime, spent lime and wood ash. Soil analysis labs can test lime sources for purity and grit size. This will affect the rate required.
For soil pH purposes, choose a lime source that contains carbonate, bicarbonate, hydroxide or oxide—all of which react with the hydrogen ions to increase pH. Soil test labs will measure lime samples for purity and grit size. Lime with larger grit takes longer to breakdown and react. It also doesn’t spread as uniformly through the soil. Wood ash and spent lime are other sources.
Labs compare lime sources to pure calcium carbonate, which has a relative neutralizing value (RNV) of 100. Some labs use calcium carbonate equivalent (CCE) or effective neutralizing power (ENP).
Not all lime sources are equal. Not even all calcium carbonate sources are equal. A Kentucky study found some calcium carbonates with an RNV of 24 due to larger grit size and lower purity.
Other sources:
- Dolomitic lime – CaMg(CO3)2 – provides both calcium and magnesium, and can benefit soils short of both macronutrients.
- Hydrated calcitic and dolomitic lime have RNVs over 100.
- Wood ash, a byproduct of forestry mills, is available in the Peace River region, home to millions of acres of acidic soil. Typical RNV for wood ash is 55 to 65, but can be higher. Wood ash also contains phosphorus, potassium and magnesium.
- Spent lime – from water treatment plants or sugar beet processors. Municipal water treatment plants use lime to soften water. Spent lime has high moisture, which adds to the transportation challenge, but it is lower cost and local.
Gypsum is not a lime source.
What lime rate to use?
Key point: Right rates depend on (1) quality of the lime source, (2) target improvement in pH and (3) buffer pH of the soil. Rate usually ranges from two to four tonnes per acre.
Soil “buffer pH” is a big factor in the lime rate. Buffer pH, also called reserve pH or lime index, accounts for the soil’s cation exchange capacity (CEC). Clay and organic matter provide the cation binding sites, so soils with more clay and organic matter have a higher CEC and therefore hold more cations. In acidic soil, hydrogen, aluminum and manganese – the cations that cause acidity – occupy more of the CEC sites. Lime has to neutralize these cations as well as the ones suspended in the soil solution. From a rate perspective, soil with low pH and low buffer pH requires a lot more lime than soil with low pH and higher buffer pH. Soil test labs account for CEC when recommending a lime rate. Table 2 on this factsheet shows how recommended rates vary by buffer pH.
What does lime cost?
Prices are all over the map. The biggest cost with lime is transportation.
How to apply lime?
Lime has almost “no mobility” in the soil, says Doug Penney, which is why small grit size, uniform spread and incorporation will improve results. He recommends farmers apply lime on dry soil, harrow to mix lime with soil, then cultivate to move lime throughout the top 3” to 4” of soil.
Precipitation helps the lime react faster, but application and incorporation in dry soil will improve distribution and uniformity, Penney says. So apply on dry soil and wait for rain.
In a no-till system, pH is usually lowest in the top 2” and a surface application will target that surface area. However, incorporated lime reacts faster.
The ideal lime spreader will be different from the standard fertilizer spreader. A lime-specific spreader box has steep side walls and conveyer chains designed to keep lime from bridging. Spinner settings and exit hatch suit high rates. Various companies make lime specific spreaders.
Manure spreaders work for wood ash or spent lime. Assiniboine Injections in Manitoba uses a JBS manure spreader with Hurricane attachment to apply spent lime. This allows them to apply variable rates based on a prescription map.
Seed-row-placed lime. An Oklahoma three-year study (Lollato et al, 2017) looked at the effectiveness of in-furrow pelletized lime for winter wheat grown in soil with pH just below 5.0. Pelletized lime comes in larger prills, not powder. That makes it easier to handle, but the lime takes longer to become active. The researchers wanted to test the idea that in-furrow placement would allow for lower rates. They applied pelletized lime with the seed at rates of 200 and 400 pounds per acre per year. They compared that to one application (not yearly) of ag lime (RNV 100) broadcast and incorporated at 2,000 pounds and 4,000 pounds per acre. The pelletized lime had almost no effect on soil pH. Only the large one-time application of ag lime made a difference.
Any alternatives to lime?
Lime is the only real way to increase soil pH.
Alternatives:
- Live with lower productivity on strong acid soils.
- Apply higher rates of phosphorus. This will overcome some of the phosphorus no longer plant available because of bonds with aluminum and iron. Required rates will continue to increase as pH falls.
- Some wheat cultivars are more tolerant to low pH. This information may be more readily available in the U.S. Ask seed companies.
- Consider alternative crops for highly acidic areas. Blueberries tolerate strong acid soils. Nova Scotia has strong acidic soils and wild blueberry is the number one export crop in the province. Potatoes, wild rice and commercial grass sod also tolerate more acidity.
- In many fields, the lowest pH is in the top 2″ to 4″ of topsoil, especially in no-till systems. At 6″ soil depth, pH can be significantly higher. Soil tests of problem pH areas will show the difference. In pH is highly stratified, farms could see a short-term benefit from turning over soil with a moldboard plow. Humberto Blanco, tillage expert from University of Nebraska, says moldboard plowing can address stratification of pH (and other properties) but says to limit the practice to once every five or 10 years to avoid or minimize soil degradation. He would avoid plowing soil with low carbon, low organic matter and high potential for erosion.
More
- Canola Encyclopedia – soil chapter on pH
- Canola Encyclopedia – general section on nutrient management
- Nutrien eKonomics – soil pH toolkit
Managing soil acidity to reduce clubroot risk
Lime is one possible treatment for clubroot. Greenhouse trials in Alberta showed that clubroot galls will grow prolifically in solutions with pH of 6.5 but not at all in solutions with pH of 7.3.
The challenge is that this target pH of 7.3 is far beyond what is economical or required from a general nutrient availability, aluminum toxicity and yield benefit. The lime required to reach 7.3 and keep it there can be prohibitively expensive.
For that reason, clubroot researchers conclude that cultivar resistance and crop rotation are the best management practices for clubroot.
Farmers who want to try lime on clubroot patches can set rates based on results from an Alberta study completed in 2023. The study included field trials with three rates of hydrated lime and a clubroot-susceptible canola cultivar. Rates were low (4.7 tonnes per hectare or 1.9 tonnes per acre), medium (8.0 t/ha) and high (11.4 t/ha).
At the first field site, untreated control blocks had a pH of 5.6. The highest lime treatment increased pH to 7.8 at the time of seeding. This rate reduced the clubroot disease severity index by 91 per cent at eight weeks after planting and by 71 per cent at harvest time. Yield increased 13 per cent.
At the second field site, untreated control blocks had a pH of 5.5. The highest lime rate increased soil to a pH of 7.7 at the time of seeding. The high rate reduced the clubroot disease severity index by 45 per cent at eight weeks after planting and 50 per cent at harvest time. Yield increased 343 per cent.
The greenhouse trials compared clubroot resistant and clubroot susceptible cultivars. It concluded that hydrated lime was much more effective than “zero grind” limestone powder. It also concluded that lime to increase pH provided little to no benefit for resistant cultivars.