Managing eroded knolls

OMAFRA Field Crop Report for September 10

Reading Time: 3 minutes

Field Crop News – 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. This article will explore knolls: how they’ve formed, how they differ, and how they can be managed or even re-mediated.

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 that the soil has formed from 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 type 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.

A high pH knoll example – Wellington County

This field near Rockwood, Ontario, has several high pH knolls. As you can see, the 2020 soybean crop was suffering earlier this season due to poor water and nutrient availability on the knolls.

The agronomist responsible for this field shared his strategy, which included seeding soybeans thicker on knolls, to help fill out the stand, and thinner in valleys, to reduce disease pressure in high-yield zones. Variable tillage – tilling valleys but not knolls – is another site-specific strategy used by these growers.

A low pH knoll example – Kent County

At this corn field north of Ridgetown, a farmer has struggled with poor crop establishment on knolls for several years. In 2020, the corn stand was very poor. There were questions as to why – soil pests? Nematodes? Fertilizer injury? After digging up plants, most seeds appeared to have germinated, but many failed to emerge, and seedlings were off-colour with poor vigour. While the odd wireworm was found, most seedlings failing to emerge did not show obvious feeding damage.

With no obvious symptoms to explain stand loss, soil samples were collected from the poor establishment area on the knoll and from areas off the knoll where the corn stand was healthy. Two major differences jumped out on the knoll – much lower pH and magnesium.

Soil acidity changes the availability of elements and nutrients in the soil – aluminum toxicity can develop and impair root growth, while plant nutrients become less available.  As the season progressed, corn plants on the knoll remained severely stunted and magnesium deficiency symptoms developed.

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. In this example, applying magnesium-containing (dolomitic) lime to the low-pH eroded knoll is warranted.

Remediating eroded knolls

Some Ontario farmers have moved a step beyond simply managing in-field variability and are remediating eroded knolls on their farms. 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.

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’s critical that practices are adopted 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 eroded knolls are more common, but acidic knolls also exist and pose unique challenges
  • Management strategies for eroded knolls include variable rate seeding and variable tillage, as well as lime application for low pH situations. Longer-term remediation approaches have also shown promise.

Visit this fall to read the full, two-part version of this article and watch videos that profile the high pH and low pH knoll case studies.

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