Soils

John Hart

Soil pH and liming - Part 2

The December 2001 issue of News and Notes contained the first article in a series about soil pH. Soil pH or liming questions and problems have been continuous and prominent during my almost 2 decades as extension soil specialist at Oregon State University. The continuing questions about soil pH and liming are the motivation for this series.

Reinforcement that the decision was correct came from “Soil Test Levels in North America” prepared by Paul Fixen of the Phosphate and Potash Institute and presented at the 2001 American Society of Agronomy meeting. Fixen reported that 34% of the soil samples from Oregon were placed in a “medium or low” category for potassium and 41% of the samples were in the same categories for phosphorus. In contrast, the soil pH for 72% of the samples was below 6.0, or below the optimum as defined for the survey. This data substantiates the idea that soil pH is continuing problem.

The first section of this article finished with idea that soil pH tells us the chemical condition roots will experience. From a management viewpoint, soil pH tells us if the soil is suited to the crop we desire to grow or conversely, if lime is needed to raise the soil pH to a suitable level for the crop.

Let’s explore soil pH and the addition of lime by using some substances familiar to us. We will begin with coffee. The pH of coffee is about 5.5. Coffee can be too acidic for some people. How do they reduce the acidity of coffee? They add cream. The action of cream in coffee is the same as adding lime to soil. If milk is added to coffee to neutralize acidity, you might think the pH of milk to be, quite high. We measured the pH of milk and found it to be 6.7. Similarly, the pH of calcium carbonate, most common agricultural liming material, is slightly above 8.

Maybe the pH of milk or lime is not as high as you expected. Remember, a single pH unit is a ten-fold change in hydrogen ion concentration or that milk has 10 times fewer hydrogen ions in solution than does coffee. The goal of liming or putting cream in your coffee is not to neutralize all hydrogen or raise the pH to 7, rather reduce acidity to tolerable level.

The pH of a liming material is not the criteria for effectiveness. The material must be able to form a compound with the hydrogen, the primary component of soil acidity. Formation a compound with hydrogen, removes hydrogen from soil solution, which in turn reduces acidity. The compound formed by reaction of most agricultural liming materials with hydrogen is water.

If you are still a bit unsure about the idea of milk or cream as a “liming” agent in coffee as a small amount of cream is used compared to the volume of coffee, consider the amount of lime used to change soil pH, generally one to 5 tons per acre. This amount of lime is expected to change the pH in approximately 2 million pounds of soil. The proportion of lime to soil is much smaller than cream to coffee.

Let’s return to familiar substances and their pH to explain how soil becomes acidic. A substance we usually consider to be acidic, vinegar, has a pH of 2.7. The pH of many soft drinks is between 3 and 4. The pH of carbonated beverages is generally fairly acidic. Carbonation or addition of carbon dioxide is part of the natural process by which soil becomes acidic. Rainwater falling through the atmosphere absorbs carbon dioxide. Carbon dioxide and water combine to form carbonic acid. The pH of rainwater is acidic, approximately 5.5. Even thought carbonic acid is weak, repeated washing of the soil with a weak acid will make the soil acidic.

Before continuing the explanation of soil acidification, some basic soil chemistry concepts will be reviewed. Soil particles have a negative charge and attract positively charged ions to the surface of the particles to off set the negative charge. Positively charged ions commonly found in soil are calcium, magnesium, potassium, and hydrogen. As a soil becomes acidic, hydrogen ions from soil solution replace calcium, magnesium, and potassium ions attracted to the surface of soil particles. As the soil is repeatedly washed with dilute carbonic acid from rain, the water in the soil or soil solution is supplied with hydrogen ions. These hydrogen ions replace the potassium, magnesium, and calcium ions on the soil particles and are leached from the soil.

The calcium, magnesium, and potassium ions leached from the soil find their way to groundwater. You observed material dissolved in well water remaining after water evaporates. It can be deposits on shower doors or leave a film on cars that are washed with well water and not dried.

Soil becomes acidic naturally by leaching with rain or weak carbonic acid. Soil acidity can be reversed or the soil pH increased by the addition of lime. Soil pH will tell us if an addition of lime is necessary. Determination of the amount of lime to add is the topic for next month.

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