Balancing the Economy of Soil pH

Managing soil pH is similar to balancing finances, and it’s the key to optimal crop performance. Explore how it affects nutrient availability, soil chemistry, and essentially crop productivity.

April 30, 2024
7 mins
Dr. AJ Foster
Agronomy Technical Services Manager, Southern US

Soil pH is one of the first concepts many learn about in basic soil science because it influences soil chemistry and availability of nutrients – either those nutrients already present in the soil or those added through fertilizers.

What is soil pH?

Soil pH is simply a measure of the hydrogen ion (H+) concentration. A lower pH (<7) indicates a higher concentration of H+ and a high pH (>7) indicates a lower concentration of H+.

Anserd Julius “AJ” Foster, Ph.D., CPAg, and ICL agronomist said he likes to think of the balance of power between hydrogen and hydroxide ions as a tug-of-war between two teams.

“When pH is below 7, it’s like one team (hydrogen ions) is pulling harder. There are more hydrogen ions than hydroxide ions, tipping the balance. When this occurs, the pH is acidic. Conversely, when pH is above 7, it’s like the other team (hydroxide) gains the upper hand,” Foster said.

“pH essentially tells us who’s winning the tug-of-war between hydrogen and hydroxide ions, determining whether it’s acidic, neutral, or alkaline.” – AJ Foster



The Master Regulator — How does soil pH affect nutrient availability?

Soil pH has a direct influence on the availability of nutrients like phosphorus (P), as well as the chemical reactions of other nutrient elements that react with P and make it less plant-available. At pH values greater than pH 7, insoluble calcium phosphates are more likely to form. Under acid conditions, iron phosphates or aluminum phosphates begin to form. With many micronutrients such as Fe, Mn and Zn, availability for plant uptake increases under acidic soil pH conditions.

“In the soil, pH serves as the master regulator akin to a car’s cooling system that regulates the operating environment. The pH regulates the soil environment, orchestrating the availability of nutrients and orchestrating the activities of soil organisms,” Foster said.

Different crops have varying tolerance for soil pH ranges. Many commonly grown crops like corn, soybeans and vegetables do well under slightly acidic to neutral conditions (pH 6 to 7.5), whereas more specialized crops like blueberries require acidic soils (pH 4.5 to 5.5) and will not grow well or grow at all at elevated soil pH levels. Soil pH influences other factors besides nutrient availability. For example, although the cause is not well understood, the prevalence of a common soybean pest—soybean cyst nematode (SCN)—is greater when soil pH values are high (pH 7 to 8) than when pH values are slightly acidic.

Wise pH Management Required

Foster noted that managing soil acidity is similar to managing finances.

“Just as with money, a little acidity is good, but too much can cause problems. In the soil, hydrogen acts like the currency that fuels the soil-plant economy. It’s crucial for the exchange of positively charged ions (cations) that plants need for their growth. Picture it as the money that allows you to buy goods and services,” he said.

Soil testing is critically important – measuring soil pH gives insight into how pH is changing over time in response to crop and soil management decisions. In a business, you manage what you measure, so digging in (literally) early and often empowers you to see differences and adjust to benefit your crop.

It is also important to understand that soil pH reflects distinct climatic and soil formation properties that will have an overwhelming effect on the general range of soil pH values found in a region. The southeastern United States, in general, has more acidic soils due to higher rainfall and more weathered soils, whereas the drier climate and calcareous soils of the western US will cause higher-pH soils to be more common.

Hitting the Sweet Spot for soil pH

Foster said the “sweet spot” for soil pH is usually around 6.5-6.8.  “That is where all the essential plant nutrients are readily available, and there’s enough hydrogen for the exchange process to happen smoothly. It’s like having just the right amount of money in your wallet to buy what you need,” he explained. “So, just like managing your finances wisely, it’s important to maintain the right balance of acidity in the soil to ensure optimal nutrient availability for plant growth.”

Although things can be done from a soil and fertility management perspective to increase or decrease soil pH toward the optimal region of pH 6 to 7, in many cases, these will be temporary fixes requiring constant attention every year or every few years.

“When the pH goes above 7, it’s like running out of money. There’s no hydrogen available for the exchange process, making it difficult for plants to access nutrients from the soil. On the other hand, when the pH drops below 6, it’s like having plenty of money but nothing to spend it on. There’s an abundance of hydrogen ions, but the availability of base cations (the things you want to buy) is limited,” Foster said.

During the exchange reaction, these base cations are either taken up by plants or washed away from the soil through leaching. Leaching leaves the soil saturated with hydrogen ions, resulting in low pH or high acidity, similar to overspending and having less money left in your account.

Soils in the Midwest and eastern US are naturally more acidic, so liming—the application of calcium carbonate or other acid-neutralizing materials—is used to increase soil pH to a target value such as pH 6.5. Liming is often one of the most cost-effective ways to improve nutrient availability without relying on fertilizer application. Other areas of North America – the Northern Great Plains, Pacific Northwest, and Western Canada – are experiencing gradual soil acidification brought about by the long-term effects of nitrogen fertilizer applications and the use of no-till production systems to prevent erosion. Access to liming agents like calcium carbonate is impractical in these areas.

As a N and S source, ammonium sulfate (AMS) is common and widely used across North America. Although it is a proven source of readily plant-available S, a major drawback of AMS is its propensity to acidify the soil.  An alternative is ICL’s Polysulphate (0-0-14-12.2Ca-3.6Mg-19.2S) is a mineral-based fertilizer approach to deliver a sustained release of sulfate-S that does not acidify the soil.

“When it comes to fertilizers, ICL’s Polysulphate stands out as a unique option. It contains three of the four essential base minerals in sulfate form, which has a neutral effect on pH. This makes it ideal for increasing productivity without disturbing pH balance,” Foster explained.

Although soil acidification will still gradually occur due to the use of other nitrogen fertilizers, a simple change from AMS to Polysulphate can eliminate the acidification due to the contributions of one common part of many fertility programs.

Invigorating the Soil Economy

In many areas of the western US and central Great Plains, high pH soils and related issues of poor-quality irrigation water with high pH and high alkalinity cause other management challenges that require using acidifying agents. Acidification of bulk soil is usually not possible or practical due to the need to apply very large quantities of slowly acid-forming agents like elemental sulfur. However, targeted acidification of soil and irrigation water is more possible through approaches that rely on injection of acid-forming compounds like urea sulfuric acid, ICL’s Nova PeKacid and Agrolution pHLow products, and the use of sulfur burners in irrigated production systems.

“Patented Nova PeKacid technology supplies acid into the soil, much like a stimulus check invigorating the economy. This technology proves valuable in high pH soil systems, helping to restore balance and promote nutrient availability. Whether in dry or liquid form, these acid-supplying fertilizers contribute to a flourishing soil-plant economy, facilitating nutrient exchange and plant uptake,” Foster said.

Nova PeKacid is available either as a dry, water-soluble form (Nova PeKacid 0-60-20), a convenient liquid (Nova PeKacid LQ  0-27-9) or in a variety of formulated NPK water soluble fertilizers (Agrolution pHLow).

As an acid forming P and K fertilizer, Nova PeKacid is a great way to supply nutrition, while also acidifying soil and water to promote compatibility and plant availability of Ca and P as well as other nutrients like micronutrients that benefit from a more acidic environment. These recommendations are an easy choice for high pH soil and water conditions, but many ask about their application in soils that are already neutral or slightly acidic.

Through trials all across the United States, ICL is learning that there is a benefit for these acid-forming fertilizers in a broad range of soil types. It’s important to note that these products, typically used for precision applications in fertigation or banded starter fertilizers, are not acidifying the bulk soil, but rather providing a relatively minor change in pH in a concentrated zone. In a way, this mimics how plants access some nutrients from the soil by excreting organic acids into the rhizosphere to enhance the solubility and availability of nutrients like P.

“Understanding and managing soil pH is vital for optimizing nutrient availability and fostering healthy plant growth. By considering all base cations and utilizing innovative fertilizers, we can cultivate a thriving soil ecosystem conducive to robust plant development.” – AJ Foster