Submitted by: Cameron Ford, on behalf of Northeastern Ontario SCIA & NOFIA

If soil is either too acidic or too alkaline, it has significant impacts on the availability of nutrients for crops, and, by extension, their growth, yield, and overall health. This is why awareness and maintenance of soil pH is such a crucial aspect of a soil health program.

Soil acidity is measured on the pH scale, which is essentially a count of the number of hydrogen ions in a solution. The more hydrogen ions there are in soil, the lower the pH and the higher the acidity it is. The fewer hydrogen ions there are, the higher the pH and the more alkaline the soil is. pH ranges from zero, which is highly acidic, to 14, which is highly alkaline. Agricultural soils in Ontario can range in pH anywhere from four to eight pH, depending on location, soil type, and the crop and application history of a field.

Different nutrients that plants need to grow become soluble at different pH levels. In general, between the pH levels of six and 7.5, most crops can obtain the nutrients they need to survive. There are some outliers, however. For example, blueberries require higher concentrations of zinc and iron than other crops and prefer highly acidic soils that range from 4.5 and 5.5 pH, in which zinc and iron are more available.

In order to grow and thrive, plants need a combination of macro and micronutrients. Macronutrients are nutrients that crops need in relatively high volumes, including nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium. Micronutrients, on the other hand, are nutrients that plants need in much smaller amounts, including chlorine, iron, boron, manganese, zinc, copper, and molybdenum. Collectively, the availability of these essential nutrients is quantified by soil tests. Ensuring that these nutrients are available for crops is crucial, but if pH is either too high or too low, crops may not be able to efficiently absorb them.

Nutrients can bind with other elements in the soil to form different compounds, some of which are easier for plants to absorb and some of which are harder or even impossible. The pH value of soil affects the rate and type of the chemical reactions that bind different elements together. The reason that most plants’ nutrient needs are met between six and 7.5 pH is because that pH level supports the kind of chemical reactions that result in compounds that plants can absorb through their roots.

For example, in high pH environments of 7.5 or more, phosphorus will react with calcium and magnesium to form compounds that are harder for plants to take up. Similarly, in low pH environments of six or less, phosphorus will react with iron and aluminum to create compounds that are insoluble and cannot be absorbed. Between six and 7.5 pH, however, phosphorus is more able to form compounds that are soluble that plants can absorb. The nutrients that a crop requires may be present in the soil, but if the pH is too high or too low, they might be in a form that plants cannot easily absorb.

The activity of microbes and fungi are other mechanisms that assist plants in absorbing nutrients. Microbes play a role in breaking down organic matter into nutrients that plants can take advantage of, while fungi act to expand and improve plant root systems to allow them to find more nutrients. At extreme pH values, microbial and fungal activity is reduced, meaning that organic material breaks down more slowly and releases fewer nutrients, and the fungal networks that can develop and aid roots in finding those nutrients are also stunted, further reducing nutrient uptake.

Graphic of Mineral versus Organic soil pH

Graph adapted from OMAFRA publication 611, Soil Fertility Handbook, © Queens Printer for Ontario, 2018

Another factor that affects the relationship between pH and nutrient availability is soil type. In general, organic soils like peats or mucks have more nutrients available, while sandy or loamy mineral soils have fewer nutrients that are available to plants. That is why it is even more important to keep an eye on pH in mineral soils, since a difference in pH can further reduce nutrient availability in an environment that has fewer available nutrients to start with.

Not all nutrients become more difficult for plants to acquire as pH increases or decreases, and this can become an issue. Manganese, for example, increases in solubility as soil gets more acidic and can reach toxic levels below five pH. In general, all micronutrients decrease in availability as pH increases, which is one of the reasons why a slightly below neutral soil pH is ideal. At high pH, some crops can experience deficiencies in micronutrients like manganese or zinc, but these can be rectified by fertilizer applications.

The relationship between nutrient availability and soil pH can be difficult to balance. Most crops are still able to grow at extreme pH levels, but they won’t be as productive as they would be at a more moderate pH. The improvement in crop yield and overall health makes it worthwhile to do regular soil testing and to keep an eye on pH values.

One Response

  1. Reply
    Eric Jelinski
    Nov 23, 2020 - 07:24 AM

    Thank you. In the graph of nutrient availability vs pH, how do you define organic soil? I think you need a third bar/axis (triangle graph) on this graph that indicates organic mater % as we would observe in a soil test report, or via a pH probe, this for better accuracy.

    The other question I have is pH you guys talk about is just a general random pH with a probe stuck anywhere in the soil or from any soil sample. IMO, the pH that matters is root zone pH as this is where the action of bacteria, fungi, and the uptake of minerals actually occurs. Of course root zone pH and other mineral density can be adjusted by banding. Also the pH measured with a probe depends very much on moisture which is actually the only source of hydrogen ions. (for example, what pH do you read on a probe if the soil is dry? And dry fertilizer does not work unless there is a good rain or irrigation) Please reply, I’d like to hear your comments from you or anybody.

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