We’ve all seen them. Hilltops. Whitecaps. High spots where the crop struggles year after year. Eroded knolls are common in Ontario agriculture and cost farmers in lost productivity each season. Understanding how they’ve formed, how they differ, and how they can be remediated are important steps in managing them.
How do knolls develop?
The major cause of eroded knolls is tillage erosion. In many cases tillage has re-distributed soil from high to low slope positions with the help of gravity over decades. Once soil is moved partially down-slope by tillage, it becomes vulnerable to surface runoff and can be moved further downhill. Wind can also remove material from the tops of knolls.
Knolls and crop productivity
Why do crops grow so poorly on eroded knolls? When topsoil is lost, so is the organic matter which helps hold water and nutrients. The subsoil – or parent material from which the soil formed – is low in nutrients, provides a poor seedbed and may drain water rapidly, and can have a low or high pH. Low or high pH can reduce plant availability of nutrients. The optimum soil pH range for most crops is 6.0–7.5. The pH scale range is 0 to 14 with 7 being neutral, below 7 being increasingly acidic and above 7 increasingly basic.
Are all knolls alike?
Although the processes that contribute to eroded knolls are similar from field to field, the types of knolls that result aren’t always the same.
The most common scenario for southern Ontario is a high pH eroded knoll. This occurs when the soil’s subsoil has a high carbonate content due to free lime (calcium carbonate). Sandy soils, on the other hand, have a poor ability to buffer against acidity from the environment. On sandy soils, low pH knolls can develop.
For more information
For the full report, including low and high pH knoll examples and remediation strategies, visit the Field Crop News website.
