Corn Response to Western Corn Belt High pH Soils

• Soil pH is a critical component to understanding soil nutrient availability.
• Corn hybrid response to soil pH varies by the actual pH level and genetic tolerance.

What is Soil pH?

Soil pH is measured using a scale of 0 to 14, with pH less than 7 considered acidic and pH greater than 7 considered alkaline or basic. pH is a measurement of the concentration of hydrogen ions.^1,2 Soil pH is affected by several factors. Environmental factors, such as precipitation, temperature and soil composition, both physically and chemically, play a role in soil pH. Rain, specifically, is naturally slightly acidic due to atmospheric CO₂. The soil composition or the parent material will determine subsoil pH based on chemical composition. Other factors related to crop management also directly impact soil pH. Nitrogen fertilizers may form ammonium in the soil, which, if not absorbed by a plant, will cause soil acidification. Legumes like soybeans and alfalfa will uptake more positive-charged cations than negative-charged anions, which leads to soil acidification. The application of lime (calcium carbonate) to soil will cause a chemical reaction forming a strong base (calcium hydroxide) and a weak acid (carbonic acid), making the soil more alkaline, or raising the pH.

Why is Soil pH Important?

In agriculture, soil pH plays a major role in crop production. Plants obtain 14 of their 17 essential nutrients exclusively from the soil. Soil pH influences those nutrients’ solubility, and thus availability, in the soil (Figure 1), leading to plant stress from deficiencies (Figure 2) or toxicities. Basic soils (pH > 7) lead to toxicity of aluminum while acidic soils lead to toxicity of manganese where these elements are present in sufficient amounts. Slightly acidic soils quickly begin to hold on more tightly to essential elements like phosphorus, calcium and magnesium, which makes them less available to the plant.

Soil pH can also impact potential plant pests and pathogens, such as certain fungi and soybean cyst nematode (SCN). Many fungi (Pythium spp. in particular) seem to perform well in slightly acidic soils.³ According to Michigan State University studies, basic soils have been shown to harbor higher populations of SCN than slightly acidic and neutral soils.⁴ Low pH in soils causes many plant nutrients to be less accessible but can also interfere with the breakdown of certain pesticides, leading to carryover issues and reduced efficacy. Low pH in soils can be managed by applying lime.

The optimum soil pH range for corn is 5.6 to 7.5. Soil pH levels of 7.8 or greater can limit corn growth and yield potential. The severity of corn response to soil pH higher than 7.8 is greatly influenced by the amount of available calcium (also expressed as excess lime and/or percent carbonate) and sodium in the soil solution. Greater amounts of one or both of these elements are typically more detrimental to the crop. If soil pH is high enough to influence corn development, plants often appear stunted and chlorotic (yellowing leaves) and yields can be reduced. High pH tolerance due to genetic variation among corn hybrids can result in stark visual differences (Figure 3). Hybrids that are not tolerant to high pH will appear stunted and pale to bleached in color.

Hybrid selection for high pH soils requires consideration of management factors:

1. Document soil pH

   • Utilize yield maps, aerial imagery and/or plant symptoms to identify potential high pH areas of a field.
   • Use soil sample results to evaluate pH, excess lime rating and sodium levels. Understanding the relationship between calcium, sodium and salt in the soil is important to properly classifying a soil as saline (high salt), sodic (high sodium) or saline-sodic, with each classification carrying different management implications. Saline soils make water uptake more difficult and are best managed by selecting a hybrid with an optimal drought tolerance rating.
   • Create a soil map from results to visualize pH distribution in the field.

2. Match hybrid to field

   • Hybrid selection should be based on pH severity profile of the field (Table 1).

Consider hybrid performance, not just for pH, but also for ear and plant height. In droughty conditions, a taller plant with higher ear placement may perform better and have more harvestable ears than a shorter hybrid or a hybrid with ears too low to the ground which can be exacerbated by soil pH.

• References
• ¹Bickelhaupt, D. Soil pH: what it means. State University of New York. College of Environmental Science and Forestry. https://www.esf.edu/pubprog/brochure/soilph/soilph.htm
• ²Goldy, R. 2011. Understanding soil pH, Part I. Michigan State University Extension. https://www.canr.msu.edu/news/understanding_soil_ph_part_i
• ³Paulitz, T. and K. Schroeder. 2016. Acid soils: how do they interact with root diseases? Washington State University Extension. http://pubs.cahnrs.wsu.edu/publications/wp-content/uploads/sites/2/publications/fs195e.pdf
• ⁴Stanton. M. 2012. Managing soil pH for optimal soybean production. Michigan State University Extension. https://www.canr.msu.edu/news/managing_soil_ph_for_optimal_soybean_production