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Soil test extractants are chemical solutions used to dissolve or exchange nutrients in soil samples. They are used when performing soil analyses to determine nutrient recommendations. The nutrient concentrations obtained from using soil test extractants are commonly termed available nutrients, exchangeable nutrients, or extractable nutrients. These terms indicate that the nutrient concentration reported for the soil samples is not equal to the total nutrient content.

Common examples of soil test extractants are Mehlich-1 (pronounced Mel-lick; rhymes with [Tom] Selleck), Mehlich-3, Morgan-Wolf, and Olsen. Mehlich-1 is the recommended soil test extractant by University of Georgia Cooperative Extension. However, some Georgia growers are showing interest in the Mehlich-3 soil test extractant, and some growers already have switched from using Mehlich-1 to Mehlich-3.

Common reasons provided by the growers for this change, based on interactions with Extension agents and specialists, include: “I have heard the Mehlich-3 is a better test,” “My grower friends in other states use Mehlich-3 so I decided to switch to Mehlich-3,” “I like the Mehlich-3 test because it generally provides higher nutrient levels, allowing me to reduce fertilizer applications,” just to name a few. Some of these explanations reflect a lack of understanding of the principles of soil test extractants.

This publication should increase your understanding by providing a general overview of soil test extractants and considerations for choosing specific ones. After reading this publication, you should be able to explain the significance of using soil test extractants; differentiate between extractable soil nutrients and total soil nutrients; determine how soil test extractants are correlated and applied; and describe single-nutrient and multi-nutrient extractants along with their advantages and limitations.

Why Test Your Soil?

Crops require adequate and balanced nutrient levels to complete their life cycles and produce optimal yields and quality. Soil testing gives growers a snapshot of the crop-nutrient requirements that can be supplied from the soil. Growers can then supplement the shortfall of nutrients, if applicable, with fertilizers or other nutrient sources. In the absence of soil testing, growers may not supply enough nutrients, which could lead to poor plant health and crop failure. In contrast, growers applying too many nutrients could increase production costs, reduce profit margins, and negatively affect the environment.

It is important to note that routine soil test analyses do not measure the total amount of nutrients in the soil but instead provide an index of the nutrient-supplying capacity of the soil. In other words, a soil test predicts the amount of nutrients that will be made available from the soil. This prediction is achieved by using soil test extractants. Typically, soils have higher amounts of nutrients than reported in routine soil test analyses. Total nutrient concentrations can be obtained through sample digestion, but there are poor correlations between total nutrient concentration and plant uptake—which is why total nutrients are not used to indicate plant-available nutrients.

Table 1 shows the nutrient values of different soil samples for Mehlich-1 extraction compared to nitric acid-hydrogen peroxide digestion. Nutrient values after soil digestion are consistently greater than Mehlich-1 for all of the soil samples. In fact, a grower may assume that their soil does not need an application of fertilizer based on the nutrient values produced when the test is performed with soil digestion. However, some of these nutrients may be held too tightly by the soil or are associated with organic materials and microbial biomass and cannot be accessed by plants. This is why soil test extractants are used to predict the amount of nutrients that will be made available from the soil and will be accessible to plants.