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Robin Hackett
August 14, 2019

Most growers know that understanding the composition of their soil is essential to making informed decisions. It can affect actions such as what crops to grow and what amendments to apply to the soil. Thankfully, soil testing can help farmers understand the physical attributes of their soil (such as percent composition of sand or clay) as well as the quantity of nutrients present.

Although testing to determine the physical attributes of the soil is straight-forward, testing nutrient levels in the soil can be more complex. This is partly because different kinds of tests yield substantially different results. Today I’ll help you understand the various kinds of tests. The knowledge applies choosing the right soil test for your farm as well as understanding the results.

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Here’s a brief overview of the different kinds of soil nutrient analyses on the market today.


The Basics of Nutrient Testing

In short, nutrient tests work by using some sort of acid to dissolve (or extract) nutrients from the soil. All basic nutrient analyses reveal the quantities of extractable macronutrients in the soil. These include nitrogen, phosphorus, potassium, magnesium, calcium and sulfur. Most tests also provide information about the quantities of available micronutrients, including iron, zinc, copper, manganese and boron.

Strong Acid Extracts

As the name implies, strong acid extracts use strong acids to dissolve the nutrients in the soil sample. The most widely used strong acid test is called the “strong acid Mehlich 3 extract.” Harkening back to high school chemistry, the stronger the acid used to extract nutrients from the soil, the more nutrients it can to dissolve. The Mehlich 3 is the test most often used by land grant universities across the Midwest and Southeast. It was specifically developed to work well on especially acidic soils.

Weak Acid Extracts

Weak acid extracts, on the other hand, use a much weaker acid to dissolve the nutrients from the soil sample. Today, the most commonly used weak acid test is the “weak acid modified Morgan extract.” This is the standard test used by most universities across the Northeast. Comparing the (modified) Morgan and Mehlich 3 tests with one another, the Mehlich 3 typically extracts three to 30 times as much phosphorus from the same sample as the Morgan test does.

The Pros and Cons

Although the debate continues, many growers who use the modified Morgan test believe that a weak acid extract does a better job of identifying the amount of nutrients in the soil that are available to crops. Out in the field, the argument goes, the soil is exposed to a variety of weak acids from rainfall and root secretions (or exudates) from crops, but no strong acids. Although there might be substantial additional nutrients in the soil, without a strong acid to extract them, they remain tied up and unavailable to crops.

Strong acid extracts can, however, be especially important in revealing other things about soil. The Mehlich 3, for instance, is typically used to determine a soil’s Cation Exchange Capacity, commonly called the CEC. This is a measure of the soil’s ability to hold nutrients. More specifically, a soil’s CEC speaks to its ability to hold positively charged nutrients (cations) including calcium, magnesium, potassium and ammonium.

Choosing the Right Test for Your Farm

There is no right answer in choosing between weak and strong acid extractions. Both kinds of tests can have their advantages in different situations. Now that you know the basics, call the lab at your nearest land grant university or consult with your local extension agent to help you  make a final decision on which test is best for your farm.

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