Soil Extractants Used in the Southeast

Growers often have questions about the methods used in soil testing, from how an extractant works to the reasons for using a particular type of extractant. Understanding the reasons why a certain extractant is used can be very helpful in understanding the results of a soil test.

What is a soil extractant?

A soil test requires an extractant to determine the amount of plant nutrients in the soil. A soil extractant is a solution made of water and a certain concentration of chemicals. The extractant is added to a fixed amount of soil and the two are shaken together for a prescribed length of time. This mixture is then poured through a filter paper. The soil is retained on the filter paper, and the extractant -- now containing the dissolved plant nutrients -- is caught in a vial.

The plant nutrients in the extractant are analyzed using the appropriate laboratory instruments to see how much was removed from the soil. The amount of the nutrients extracted will represent a fraction of the total nutrients available to the plants throughout the growing season.

Calibration data from previous field experiments serve as a guide in rating the amount of extractable nutrients as low, medium, or high. These ratings are used to determine the amount and type of fertilizer, such as phosphorus (P₂O₅) and potash (K₂O), needed for optimum crop production. (Note:   Soil testing for nitrogen is a special situation which is not included as a routine test in all the Southeastern states due to the mobile nature of nitrate-nitrogen.)

What is the Mehlich-1 extractant?

The Mehlich-1 (use button above to view) extractant (implemented in 1953) was developed by Dr. Adolph Mehlich for the North Carolina Department of Agriculture Soil Testing Division. It is a versatile soil extractant that is used to determine the amounts of phosphorus, potassium, calcium, magnesium, zinc, manganese, and boron. Even though extractable copper is often reported, the Mehlich-1 extractant was not designed as a copper extractant. Because this extractant is made up of a dilute solution of hydrochloric and sulfuric acids, it is often referred to as the "double acid" extractant. It works well for acidic, low cation exchange capacity (CEC) soils, which includes almost all soils in the Southeast. The Mehlich-1 extractant is used by the state labs in Alabama, Florida, Tennessee, Virginia, Georgia, and South Carolina, as well as many private labs.

What is the Mehlich-3 extractant?

In an attempt to make a universal extractant that could be used for any soil in the world, Dr. Mehlich developed the Mehlich-2 extractant which was used for about 2 years. The Mehlich-2 contained four different chemicals, but the chloride in the extractant caused laboratory instrument problems.

By changing the chemical composition of the Mehlich-2 extractant slightly to exclude the chloride component, Dr. Mehlich created the Mehlich-3 (use button above to view) extractant. A chelating agent was added to this formula to increase the amount of micronutrients extracted, particularly copper. The Mehlich-3 extractant was adopted in 1984 and is being used throughout the world. In the Southeast, the Mehlich-3 extractant is used by the state labs in Kentucky and North Carolina and by some private labs.

What is the Bray P-1 extractant?

The Bray P-1 extractant (use button to view) was developed before the Mehlich-1 extractant and was widely used by the Southeastern states. Even though it works well on acid soils, it is only useful for extracting phosphorus. Therefore, an additional extractant must be used for determining other plant nutrients such as potassium, calcium, and magnesium. As the demand for soil testing increased and laboratory instrumentation improved, it became desirable to have an extractant that could be used for a wide range of plant nutrients, including micronutrients. Many Southeastern states switched to the Mehlich-1 since it could be used as a single extractant for many important plant nutrients rather than phosphorus alone.

The only time the Bray P-1 is used on Southeastern soils is when the soil samples are analyzed by a private laboratory which also services farms from the Midwest, where the Bray P-1 soil extractant is still commonly used. Since Southeastern Land Grant Universities have no current information for interpreting soil phosphorus levels using the Bray P-1 extractant, this extractant is not recommended.

Why do some states use the Mehlich-1 extractant and other states use the Mehlich-3 extractant?

Although the Mehlich-1 extractant works very well as a soil extractant, some states in the Southeast have switched to the Mehlich-3 extractant. Mehlich-3 is a better predictor of nutrient levels in soils above pH 6.5 where free calcium carbonates may be present. Soils with a pH value greater than 7.0 are relatively limited in the Southeast, occurring only with over-liming, long-term use of hen manure, or frequent irrigation with high bicarbonate water common to the coastal region.

How does the Mehlich-3 extractant compare to the Mehlich-1 extractant?

The Mehlich-1 and Mehlich-3 will extract about equal amounts of most plant nutrients, but rates vary when it comes to phosphorus. The amount of phosphorus extracted by the Mehlich-3 will be approximately 1.5 times to twice the amount extracted by Mehlich-1. This ratio should not be used to derive Mehlich-1 based recommendations with Mehlich-3 extractant results or vice versa. These are only rough comparisons and do not apply to all soil types.

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