Getting to Know Your Knolls Part 2: Understanding and Managing Low pH Knolls

Ben Rosser, Corn Specialist, OMAFRA

While high pH knolls tend to be more common in Ontario, often caused by soil erosion leaving higher pH calcareous subsoils, this is not the case for all regions.

How do low pH knolls develop?

Low pH knolls (Figure 1) can develop on undulating landscapes where surface soils formed from deposits of sand, such as where glacial meltwater deposited sand at the entrance to historical glacial lakes (Webber and Hoffman, 1967). Tillage, water, or wind erosion may have also removed topsoil from these knolls. Soils naturally acidify over time through rainfall, organic matter breakdown, and mineral weathering, and through applications of ammoniacal fertilizer. Sandy soils have less ability to buffer against these processes, resulting in lower pH.

Figure 1. Soil cores (4 ft depth) of healthy corn stand off knoll (left) and poor corn stand on knoll (right).


A Low pH Case Study

We visited a field in Kent County in May where a producer was struggling with crops establishing on some sandy knolls (Figure 2). There were questions as to why – soil pests? Nematodes? pH? Fertilizer injury? The first step in diagnosing emergence issues is to dig for clues. Most seeds appeared to have germinated, but many failed to emerge and seedlings appeared off colour with poor vigour. While the odd wireworm was found, most seedlings failing to emerge did not show obvious feeding damage. Un-emerged seedlings were also not showing clear, characteristics fertilizer burn symptoms such as blackened roots, missing or “nubbed” seedling roots or root tips.

Figure 2. Poor corn stand on sandy knoll.

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 – much lower pH and magnesium in the poor stand on the knoll compared to the healthy stand around the knolls (Table 1).

Table 1. pH and magnesium soil test levels on-knoll compared to off-knoll

 On Knoll (poor stand)Off Knoll (healthy stand)Critical Value
pH4.96.95.6*
Magnesium (ppm)1511220
* critical value for coarse and medium textured soils

Low pH knolls and crop productivity

Crop growth suffers if pH is below optimum, which varies by crop (Table 2). A reduction in pH itself does not impair growth, but changes in availability of elements and crop nutrients in the soil can. Some elements such as aluminum can become more plant available to the point of toxicity and impair root growth while plant nutrients (calcium, magnesium) can change to less plant available forms and become deficient (Figure 2). For legumes, low pH can also inhibit nodule formation. Topsoil erosion may also impact moisture and nutrient holding or supplying capacity.

Table 2. Critical soil pH for various crops and soil types (Brown, 2017).

Soil TypeCropsSoil pH Below Which Lime is BeneficialTarget Soil pH*
Coarse- and medium-textured mineral soils (sand, sandy loams, loams, and silt loams)perennial legumes, oats, barley, wheat, triticale, beans, peas, canola, flax, tomatoes, raspberries, strawberries, all other crops not listed below6.16.5
corn, soybeans, rye, grass, hay, pasture, tobacco5.66.0
potatoes5.15.5
Fine-textured mineral soils (clays and clay loams)alfalfa, cole crops, rutabagas6.16.5
other perennial legumes, oats, barley, wheat, triticale, soybeans, beans, peas, canola, flax, tomatoes, raspberries, all other crops not listed above or below5.66.0
corn, rye, grass hay, pasture5.15.5
Organic soils (peats/mucks)all field crops, all vegetable crops5.15.5
*Where a crop is grown in rotation with other crops requiring a higher pH (e.g., corn in rotation with wheat or alfalfa), lime the soil to the higher pH.

Managing in-field variability due to knolls

Soil sampling should be completed to confirm if low pH is an issue. Directed sampling on the knolls is important. For instance, grid sampling had been completed in the case study field above but did not capture the low pH on the sandy knoll.

Whether lime is required depends on soil pH and crops grown. If the soil sample pH is low, a buffer pH test result is included with sample results to indicate how easily soil pH will change. Buffer pH along with a target pH depending on the crop and soil type (Table 2) is used to provide a lime rate. Full liming recommendations are available in the Soil Fertility and Nutrient Use chapter of Pub 811 Agronomy Guide for Field Crops, or the Soil pH, Liming and Acidification chapter of Pub 611 Soil Fertility Handbook.

When pH is low, magnesium can also often be low. Dolomitic lime (calcium and magnesium carbonates) is preferred for providing both carbonates to correct low pH and magnesium. If sourcing dolomitic lime is not feasible, magnesium can be addressed with magnesium fertilizer. Correcting pH may be important to get the most benefit from magnesium fertilizer applications, as in the case-study field, plants growing on the knoll displayed magnesium deficiency symptoms (Figure 3) despite magnesium fertilizer being applied.

Figure 3. Reddening of lower corn leaves consistent with magnesium deficiency on low magnesium soil.

Sand knolls may be variable across the field. Field experience may be enough to locate, sample, and address issues. Other tools – bare soil imagery (Figure 4), soil sensor measurements (e.g. electrical conductivity) or historical yield data (if knolls consistently perform poorly) may assist in locating or delineating these areas. Variable applications of lime or magnesium fertilizer may be considered where low pH areas are well defined and would clearly benefit from these applications, but the remainder of field would likely not.

Figure 4. Bare soil imagery of case study field with issue sandy knolls circled.

Low pH may not be the only issue on sandy knolls but correcting low pH may be foundational to improving productivity. Other issues could also be apparent as a nature of lighter soil textures (fertilizer burn, root feeding pests, nematodes).

Sources: 

Brown, C., editor. 2017. Agronomy Guide for Field Crops Publication 811. Ontario Ministry of Agriculture, Food and Rural Affairs.

Webber and Hoffman. 1967. Origin, Classification and Use of Ontario Soils Publication 51. Ontario Department of Agriculture and Food.

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