Jake Munroe, Soil Management Specialist – Field Crops, OMAFRA
We’ve all seen them. Hilltops. Whitecaps. High spots where the crop struggles year after year. Eroded knolls are common to Ontario agriculture and cost farmers in lost productivity each season. However, not all eroded knolls are alike. In this two-part series, we will explore knolls: how they’ve formed and how they differ, as well as how they can be managed, and in some cases, remediated.
How do knolls develop?
Tillage erosion is the main cause of eroded knolls. In many cases, growers are dealing with legacy effects of decades or centuries of soil movement from tillage, which has re-distributed soil from high to low slope positions with the help of gravity (Figure 1). Water erosion plays an important role, too. Once soil is moved partially down-slope by tillage, it becomes vulnerable to runoff events and can be moved further downhill (Figure 2). As erosion progresses over time, topsoil is moved off the knoll and subsoil or parent material becomes exposed at the surface.
Are all knolls alike?
Although the processes that form knolls are similar from field to field, the type of eroded knoll is not always the same. The most common scenario for southern Ontario is a high pH eroded knoll. This occurs when the soil’s parent material has a high carbonate content.
In other cases, particularly on sandy soils, the eroded knoll can become acidic (low pH). When topsoil is removed, the sand below has very little buffering capacity and becomes acidified. Ben Rosser will discuss such a situation in Part 2.
Soil sampling eroded knolls separately from the rest of the field is the best way to determine whether you are dealing with low or high pH. Another diagnostic method is to use dilute hydrochloric acid – fizzing indicates high pH parent material (Figure 3).
Knolls and crop productivity
Why do crops grow so poorly on eroded knolls? On high pH knolls, the main factors are a lack of available water and nutrients.
When topsoil is lost from a hilltop, so is the organic matter, which is a major storehouse for both water and nutrients. The calcareous parent material is left, although high in calcium and magnesium, is low in macronutrients, provides a poor seedbed, and drains water rapidly. The high pH may also reduce plant-availability of nutrients such as phosphorous. These soybeans on an eroded knoll near Rockwood are struggling from a lack of water and poor nutrition (Figure 4).
Managing in-field variability due to knolls
All fields have variability due to subtle differences in topography, drainage, and nutrient availability. Those with severely eroded knolls, however, can show drastic variability and may warrant site-specific management.
As Dan Breckon, Agronomist with Woodrill Farms, shared with us this summer, one approach for soybeans is to seed thicker on knolls – to help fill out the stand – and thinner in valleys – to reduce disease pressure in high-yield zones.
When it comes to nutrients, Breckon says that knolls don’t always have lower nutrient levels, depending on the history of crop yields and fertilizer application. He recommends sampling according to zones to understand soil test patterns based on your field’s topography.
Remediating eroded knolls
Some Ontario farmers have moved a step beyond simply managing in-field variability and are remediating eroded knolls on their farm. This grower near New Hamburg, for example, used an earth mover after wheat harvest in 2019 to scrape soil from valleys and place it on knolls (Figures 5 and 6).
At least 6 inches of topsoil should be left in the valleys and 4 inches or more of topsoil added to the knolls. Once the soil is moved, it is critical that practices are put into place to ensure it does not move back down-slope (e.g. no-till or minimum till, cover cropping, and a diverse crop rotation). Ontario research, although limited, has found that this practice can improve crop productivity on knolls without affecting yield in the valleys.
Putting it together
- Eroded knolls are common in Ontario and have been caused by tillage erosion, in combination with water and wind erosion, over decades.
- High pH, calcareous eroded knolls are more common, but acidic knolls also exist and pose unique challenges (see Part 2).
- Crop productivity is often poor on high pH knolls due to lack of available water and nutrients.
- High pH knolls can be managed in-field through variable rate seeding and fertilizer application; soil moving is also an option for long-term remediation.