Using a pH Meter for In-Field Soil pH Sampling

Using a pH Meter for In-Field Soil pH Sampling

FS205E
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Paul Carter, Associate Professor, Extension Agronomist, and Precision Agriculture Specialist, WSU Department of Extension Ag and Natural Resources (ANR)
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 publication is part of a series of fact sheets on soil acidification and discusses how to select a pH meter for soil testing. 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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Introduction

Soil pH continues to decline across much of the Palouse and the Pacific Northwest, primarily as a result of the application of nitrogen (N) for the production of wheat and other crops. Mahler and Harder (1984) and Veseth (1987) tried to sound the alarm that an acidic soil problem in the Pacific Northwest was developing, but little attention was given them since crop yields continued to be good.

Soil Acidity

Soil acidity is a major limitation to soil productivity in much of the world and is considered the master variable by soil chemists (McBride 1994). This acidity has a direct impact on many of the chemical and biological processes causing yield reductions in many crops (Koenig et al. 2011).

Soil pH (power of hydrogen) is the result of the buildup of hydrogen (H+) ions in the soil solution (acid soils) or hydroxide (OH) ions (alkaline soils), and determines the acidity or alkalinity of soil (Horneck et al. 2007). The breadth and depth of the concentration of the H+ ions in these layers is due to large amounts of N applications, plant nutrient uptake, and rainfall (Mahler et al.1985).

The presence of ammonia NH4 or ammonium NH3 in the soil contributes to the H+ ions from the result of the conversion to nitrate by the bacteria in the soil. This process releases H+ ions into the soil solution. If the H+ ions are not neutralized or bound to soil particles they remain in the soil solution creating the active acid soil environment.

Sampling Soils for Acidification

Standard soil sampling methodologies tend to overlook the problem of stratified acid soil layers that have developed in once neutral pH soils. Reduced tillage intensifies and narrows the layer of stratification compared to inversion tillage systems where more soil mixing occurs.

Inversion tillage systems experience the same rate of acidification, but the soil is mixed in a larger volume and is not as noticeable. New methods of sampling the layers or increments have been adopted to try to identify possible stratification of acidity and nutrients that may have developed.

Traditionally, determining the pH of soil samples has been a laborious and time-consuming process requiring laboratory equipment and technicians. McLean (1982) stated that soil pH is usually determined potentiometrically in a slurry system using an electronic pH meter.

Recently, the development of new technology pH meters has reduced or eliminated much of the time and expense of collecting samples and sending them to a lab to be tested. Some meters have been tested previously for accuracy in checking for soil pH, as well as some other nutrient values (Davenport and Jabro 2001), and reported that pH values were quite accurate for the soils tested. Although most of these meters are limited to pH values only, they make it possible to quickly assess the soil pH condition in the field using a pH meter probe or sensor.

Soil Testing with a pH Meter

There are many “sensors” on the market today, however most of the devices perform the same functions with few differences between them and the technology each uses. Although, there are differences in how the units are used to get good results.

When selecting a pH meter to test soil samples directly in the field, it is important to read the meter description and ask questions of the manufacturer to determine if the meter will work directly on soils in the solid state (paste-like soil samples) from a soil probe.

Only one meter was found to be suitable for direct measurement on the soil core samples; others require making a liquid solution using distilled water with the soil using equal amounts of each by volume (a ratio of 1 to 1, water and soil).

Selecting and Using a Soil Core Sample pH Meter

The price range for pH meters starts at around $20 and can go to over $1,000. Most meters have been developed for greenhouse use in monitoring the acidity of potted plants, where irrigation can be applied. Under these conditions managers have the potential to quickly change the acidity to maximize plant growth and development. Other low cost units are for homeowner and gardener use and are often found to not be very reliable.

A handheld sampling meter (Soil Stik) has been utilized for in-field checking of the soil pH with good results. Within a few minutes of arriving at a field location it is possible to collect a soil sample, test the soil, and record the pH values. An additional value of the Soil Stik is that samples can be checked at various depths without mixing or stirring the soil. This allows for the possible identification of layers within the soil column.

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Copyright 2016 Washington State University

Published March, 2016

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