Agricultural Lime and Liming, Part 2: Laboratory Testing to Determine Lime Requirements

Agricultural Lime and Liming, Part 2: Laboratory Testing to Determine Lime Requirements

FS217E
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Wayne Thompson, Regional Agronomist and Dryland Cropping Systems Specialist, Extension Agriculture and Natural Resources Program Unit and Crop and Soil Sciences, Washington State University, Carol McFarland, Crop and Soil Sciences, Washington State University, Tabatha Brown, Crop and Soil Sciences, Washington State University, David Huggins, USDA-ARS

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, in particular. Decreasing soil pH has serious implications for the cropping systems of the Palouse.

This publication is part of a series on soil acidification and discusses the variations among soil types and why laboratory testing is used to characterize soil pH. The series begins with An Introduction, covering the fundamentals of soil pH and acidification, and continues with other fact sheets on more specific topics such as the influence of pH on pathogens and microbes, recommended varieties of specific crops, herbicide activity, crop nutrition, and liming.

Agricultural Lime and Liming is a three-part publication that introduces the basic principles of how and why calcareous amendments are land applied to elevate the pH of acid soils. Part 2 delves more deeply into variations among soil types and characterizations of soil pH by defining “lime requirement,” discussing common test methods, and providing a worksheet for reference.

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Agricultural Lime and Liming is a three-part publication that introduces the basic principles of how and why calcareous amendments are land-applied to elevate the pH of acid soils. It is also part of the companion series Soil Acidification in the Inland Pacific Northwest.

Part 1 – Introduction to Agricultural Lime and Liming. This section contributes to the discussion on the increasing incidence and severity of acid soils in the Inland Pacific Northwest (IPNW) and emphasizes: i) soil sampling and monitoring concepts to consider when managing acid soil conditions; ii) the role of soil testing; and iii) characterization of liming materials used to elevate pH of acidified soil.

Part 2 – Laboratory Testing to Determine Lime Requirements. This section delves more deeply into variations among soil types and why laboratory testing is used to characterize soil pH. We define the term “lime requirement” and how it relates to soil buffering capacity. We also provide information on lime requirement test methods commonly performed by soil testing laboratories. A companion Lime Requirement Worksheet based on common buffer tests and base saturation is provided for reference.

Part 3 – Aglime Product Selection and Comparison Calculator User Guide. This is a user’s guide for the interactive online Agricultural Liming Material Selection and Comparison Calculator. The publication describes the attributes used to perform calculations to characterize liming material, while the interactive utility provides a platform to compare up to three liming materials for their relative effectiveness, costs, and economic efficiency.

 

Amending Acid Soils

Agricultural liming material applications increase the base saturation of a soil by displacing acid-forming (H+ and Al3+) cations with calcium and magnesium. The associated carbonates (CO32-) and hydroxides (OH) of the liming materials bind with acid-forming cations to form insoluble compounds. The net result is an increase in soil pH, a significant decrease in phytotoxic concentrations of aluminum and manganese ions, and the release of carbon dioxide to the atmosphere (Anderson et al. 2013; Collins 2012; Havlin et al. 2013; Horneck et al. 2007).

Laboratory Testing

Soil testing for lime requirements is performed to determine the amount of pure calcium carbonate (CaCO3) needed to displace hydrogen and other acid-forming cations and elevate soil pH by neutralizing an acid soil condition to a level where base saturation and exchangeable acidity are optimized for crop growth.

 

Laboratory testing quantifies a soil’s buffering capacity along with active and exchangeable acidity, which are the acid-forming cations that occupy cation exchange sites on soil aggregates and organic matter and in the soil solution (Havlin et al. 2013).

Buffering Capacity

The amount of liming material needed to elevate pH is related to a soil’s buffering capacity, which is its ability to resist change in pH. Buffering capacity of soil is proportional to the amount of clay and organic matter in a soil. As the clay and organic matter content of a soil increases, so does the buffering capacity. Higher buffering capacity increases a soil’s resistance or resilience to acidification. Additionally, the amount of agricultural liming material required to elevate soil pH increases as the buffering capacity increases. A soil with high clay and organic matter content can require two to three times the amount of lime to elevate soil pH as a sandy soil with low organic matter content (Havlin et al. 2013).

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Copyright 2016 Washington State University

Published June, 2016

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