Soils

John Hart and Mark Mellbye

Soil pH and liming - Part 1

Acidity or pH is the most commonly determined chemical characteristic of soil. Soil pH may be a common measurement and term, but can be a puzzle to growers, gardeners, and suppliers of lime and fertilizer. This article is the first in a series that will explore soil pH and crop growth. We address the question “what is soil pH?”. Future articles will explain the SMP Buffer, frequency of lime application, yield response from lime application, lime movement, how rapidly does soil become acidic, and why soil becomes acidic.

Acidity is a measure of the hydrogen ion concentration in soil solution. Soil solution is water held by the soil particles. Soil pH is a chemical property of soil that controls the over-all condition of suitability for root growth. The importance of soil pH or soil acidity on plant growth can be explained using the following swimming pool analogy.

Think of yourself in a swimming pool. If the water is too cold, too hot, or the chlorine content so high your eyes hurt, you don’t want to be in the pool or you might limit your time in the pool. Now think of roots trying to grow in soil. If the soil pH is too high or too low, plant roots are like you in the pool, they don’t want to be there. Unfortunately, plant roots don’t have a choice about being in the soil or not. If they are not “comfortable” from a soil pH that is too high or too low, the roots don’t grow normally or nutrients are not taken into the plant in sufficient quantity.

Soil acidity is measured and expressed as soil pH. Soil pH is measured on a scale from 0 to 14. Soil pH values below 7 indicate acidic soil, and numbers above 7 indicate basic or alkaline soil. As pH numbers decrease, soil acidity or the hydrogen ion concentration increases. Lime is added to acidic soil to raise the pH. Amendments such as elemental sulfur are added to basic soil to reduce the pH.

The scale used for pH is logarithmic or has a 10-fold difference for each unit change. Most of the measurements familiar to us are not expressed with a logarithmic scale. For example, if you have $60 dollars and a friend has $50, you have $10 more dollars than does your friend. The scale here is not logarithmic, rather normal. Let’s explain soil pH or a logarithmic scale using money.

If your soil pH is 6.0 and your neighbor’s soil pH is 5.0, your neighbor’s soil pH is 10 times more acidic than yours. Using a monetary example where hydrogen ions equal dollars, your soil pH would be represented by $6 and your neighbor’s soil pH would be $60.

Soil acidification is a naturally occurring process in western Oregon. Centuries of leaching the soil with the abundant winter rainfall removed chemical constituents called bases, making the soil acidic. The primary bases removed that results in acid soils are calcium and magnesium. They were either leached into the groundwater and now cause well water to be called "hard" or found their way into surface streams and were carried to the ocean. The natural process of soil acidification is accelerated by the addition of nitrogen fertilizers, crop residue removal, and farming or gardening in general. More on the process of acidification will be presented in the third article in the series.

Soil pH tells us the chemical condition roots will experience. As soil pH decreases, the solubility of iron, zinc, manganese, copper, and aluminum increases. The concentration of manganese and aluminum can reach levels that are toxic or at least inhibit root growth. Crop sensitivity to manganese and aluminum vary. Blueberries and rhododendrons are quite tolerant compared to alfalfa and garlic. The most common approach to crop management is to decide what crop will be planted and how the soil in the area needs to be treated or amended. We do not decide to plant blueberries if the soil pH is low or alfalfa if the soil pH is near neutral. 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.

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