To protect their future harvests, Western Canadian farmers need to manage soil acidity across their entire multi-year rotation rather than crop by crop, long-term research from the University of Alberta suggests.
In a study to address the challenge of how modern agricultural practices affect soil acidity, the consequences for crop production and how farmers can manage those changes before they become a serious problem, researchers found that proactively managing soil pH levels pays off across the entire rotation.
The researchers looked at how decades of fertilizer use, crop rotation and corrective lime applications affect soil acidity and nutrient availability over time. Their most important finding was that liming legume forages not only boosts their growth, but also unlocks vital nitrogen for grain crops planted in future rotations.
"Managing soil acidity is about more than simply maintaining the productivity of individual crops," says soil scientist Dr. Miles Dyck, a professor in the Faculty of Agricultural, Life & Environmental Sciences and a co-author on the study with Dr. Linda Gorim.
"It can also help preserve important biological processes."
Historically, soil acidity hasn't been a major concern for many farmers in Western Canada, but agricultural practices have changed considerably over the past several decades, making it more of an issue, notes Dyck, who recently discussed the problem in a podcast.
Beneficial practices, unintended consequences
"Farmers are growing more diverse and productive crops, applying more fertilizer and removing larger quantities of nutrients from their fields through harvests. These practices have many benefits, but they can also gradually increase soil acidity, often over decades. That means producers may not notice a problem until soil acidity reaches a level that begins to affect their yields. By that point, restoring favourable soil conditions can require considerable time and expense."
As soil becomes more acidic, it can become less favourable for plant growth and crop productivity by affecting the availability of nutrients and the activity of beneficial soil microorganisms.
"The main prairie crops - cereals like wheat, barley and oats, oilseeds such as canola, pulses like peas and lentils, and forages such as grasses and alfalfa - vary in their sensitivity to soil acidity. If one crop in a rotation is affected by soil acidity, the crops that follow will also be affected," Dyck notes.
Drawing on data gathered between 1980 and 2020 from the U of A's Breton Plots, the researchers examined how synthetic fertilizer, manure and lime - a crushed limestone powder used to neutralize soil acidity - affected soil health, yields and nitrogen cycling within a five-year crop rotation of wheat, oats, barley and an alfalfa-brome grass mix.
Results showed that legumes like alfalfa benefit most directly from liming. In fields treated with synthetic fertilizers, applying lime boosted forage yields, biological nitrogen fixation in the alfalfa and the absorption of essential nutrients.
Although cereal crops like wheat and oats were less sensitive to soil acidity and didn't see direct grain yield increases from liming, they did gain indirect benefits by taking up more nitrogen when they followed alfalfa grown in limed soil.
"We believe this occurred because reducing soil acidity improved alfalfa growth and likely increased the amount of nitrogen fixed by beneficial bacteria associated with its roots," Dyck notes. "Some of that nitrogen subsequently became available to the wheat and oats."
Dyck says this shows an important relationship between crops in a rotation, and why producers should evaluate the potential benefits of liming for all crops in their rotation on time scales of two or three rotation cycles, which span five to 15 years.
"Managing soil conditions to benefit one crop can also improve the growing conditions for others planted in subsequent years. And in some cases, the benefits may be indirect and become apparent only over several growing seasons."
A surprising finding
However, the researchers also found a surprising contradiction: alfalfa can contribute to soil acidification. As the plant's roots absorb large quantities of positively charged nutrients like calcium and magnesium, they release acid into the soil. When the crop is harvested, a large amount of those essential nutrients are removed from the field, accelerating pH decline.
"This shows how a crop can improve one aspect of soil fertility while at the same time contributing to changes in another," Dyck notes, adding that it doesn't mean farmers should avoid growing alfalfa or other legumes.
"These crops offer important benefits, but their effects on soil acidity need to be considered when planning rotations and nutrient management."
The experiments also showed that soil acidity was most rapid in plots treated with synthetic fertilizers, even when lime was applied, suggesting that it shouldn't necessarily be viewed as a one-time solution.
At the same time, the study showed that manure slowed soil acidity more than synthetic fertilizers by returning nutrients like calcium and magnesium to the soil, helping maintain the capacity to resist changes in pH levels.
The findings point to the importance of considering long-term consequences of different nutrient management strategies, Dyck says.
"Various types of fertilizers and soil amendments can have quite different long-term effects on soil chemistry."
Regular monitoring and management of soil acidity is becoming increasingly important in cropping systems in Western Canada, he adds.
Soil testing carried out by farmers usually focuses on plant-available nutrient levels in the top six, 12 or 24 inches of soil. Under modern zero- or reduced-tillage systems, soil acidity mostly occurs in the top three inches where seeds and fertilizers are placed every year, Dyck notes. Sampling soil in layers, testing repeatedly at the same locations and depths over time, and liming strategically to protect acid-sensitive crops like alfalfa and pulse legumes can help to catch the issue early, he says.
"To be able to identify and manage soil acidity before it begins limiting crop productivity, it's important to pay closer attention to soil pH and how it changes over time," Dyck notes.
"Soils are continually responding to the way we manage them, and understanding these gradual changes is essential to maintaining productive, sustainable agricultural systems for future generations."
