Soil pH and Implications for Management: An Introduction

Soil pH and Implications for Management: An Introduction

FS170E
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Carol McFarland, Crop and Soil Sciences, Washington State University, David Huggins, USDA-ARS, Richard Koenig, Associate Dean and Director of Extension, Washington State University
Decreasing soil pH, also called soil acidification, is a growing concern in eastern Washington and northern Idaho. Researchers and farmers have measured soil pH values below 5.0 throughout the Palouse region. Decreasing soil pH has serious implications for the cropping systems of the Palouse. This introduction is the first in a series of fact sheets on soil acidification and introduces the fundamentals of soil pH and acidification. Other fact sheets in the series will cover more specific information on topics such as the influence of pH on pathogens and microbes, herbicide activity, crop nutrition, liming, and variety selection.
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What is pH?

Acidic, neutral and alkaline/basic are terms commonly used to describe pH. A measure of the concentration of hydrogen ions (H+) in a system, pH is described using a negative-logarithmic scale. This means that for each whole number step down on the pH scale there are ten times more hydrogen ions, or “acidity,” than the previous step. Neutral pH is 7, while the neutral pH range is generally thought to be between 6 and 8. Alkaline or high pH values are above 8, while acidic or low pH conditions are typically considered to be below 6. Figure 1 shows pH values for common substances from battery acid to lye.

Figure 1. The pH of common items. Each pH unit is a ten-fold increase or decrease of acidity (H+).

pH in the Soil

In soil, pH is known as a master variable because it influences almost every process in the soil system. The health of crops and other soil life, the availability of nutrients, and the activity of pesticides are all affected by pH . Generally, soil pH below 5 is considered to be very low and extremely acidic for many agricultural crops.

A pH too far from neutral, either above (alkaline) or below (acidic), will make essential nutrients less available to plants. Figure 2 shows how pH affects nutrient availability in soil.

For many nutrients, optimum availability occurs near neutral pH (7.0) and availability decreases as pH values become more extreme. This can lead to nutrient deficiencies in crop plants. Figure 3 shows yellowing between the veins (interveinal chlorosis) of a corn plant due to iron deficiency in a field where the soil pH is too high.
Figure 2: General pH effects on nutrient availability and soil biology. Wider parts of the bar indicate the element is relatively more available to plants, while narrow parts indicate decreased availability of the element at that pH. Deviation from neutral pH decreases availability of many essential plant nutrients as well as bacteria populations. (Adapted from Truog 1947).
Figure 3: Yellowing between veins in corn resulting from iron deficiency in high pH soil (from Koenig- used with permission).

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Copyright 2015 Washington State University

Published September, 2015

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