Knowing your soil test results is one thing. Knowing how the lab got those numbers - and which extractants it used - is just as important for making solid fertility decisions.
"At Ward laboratories, we like to use multiple extracts changing as we change the elements we're looking at in the soil," says Nick Ward, PhD, president of Ward Laboratories, Kearney, Neb.
"Given the diverse soils that we work with in our customer base, we try to do these different extracts to best accommodate and make an even playing field for everybody," he adds.
That "even playing field" matters because not all soils - or regions - behave the same way. A number that signals a fertilizer response in one soil type or environment might mean something very different in another, depending on the extractant used.
Phosphorus Is An Important Example
Phosphorus (P) is a prime case where understanding the extractants and where they fit can help you make better fertility decisions.
Ward Laboratories typically uses Mehlich-3 ICP as its standard extractant because of its versatility across various soil textures and organic matter levels.
"When we have a Mehlich-3 value of 18 parts per million of P, the chances for yield response by adding fertilizer is very good," Ward says, noting that decades of university research tie these specific numbers to actual yield outcomes.
1. Olsen P (Bicarbonate P): It is often preferred for high-pH, alkaline, and calcareous soils typical of the Western U.S.
"The Olsen test extracts P using sodium bicarbonate and is the best test to use for situations where soil pH is 7.4 or greater," says Dan Kaiser, a nutrient management specialist with University of Minnesota Extension, in this online article.
2. Bray-P1: It is often used in slightly alkaline to highly acidic soils (pH of 7.4 or less). Kaiser says the Bray-P1 test extracts P with acids and has been a popular test for over 50 years as data continue to show the ability of Bray-P1 to predict crop yield response to P.
Kaiser adds that soil-test labs using the Bray-P1 or Olsen will often run the Olsen test at a certain pH automatically, which makes it easier for farmers "as you do not have to decide which test to use before you submit samples."
Matching Extractants To Nutrients
While Mehlich-3 is sometimes promoted as universal, Ward agrees with other experts that different nutrients are best served by different extractants and tests.
For example, when shifting focus to potassium (K) and other cations like calcium and magnesium, Ward Laboratories moves to ammonium acetate, a neutral-pH solution.
This method is used to determine a soil's Cation Exchange Capacity (CEC).
Ward explains that because ammonium acetate is neutral, it prevents overestimating the nutrients a plant can actually absorb. "It's not a harsh chemical that's going to give us too much of an element that would not otherwise be something the plant would see," he says.
For micronutrients like zinc, iron, and copper, the lab employs DTPA, a chelating agent.
The DTPA process "grabs" micronutrient ions so they can be measured with high precision. Ward notes that he is "very confident" in the results because they are backed by decades of data regarding fertilizer responsiveness.
Ask The Lab Or Your Retailer Questions
For farmers and advisers, the main takeaway is that soil tests results and reports are not all created equal - even when the numbers look similar on paper. Knowing which extractant a lab uses, and why, is key to interpreting results correctly and comparing them across time, fields and regions.
For farmers and crop advisers looking to make the most of their investment in soil sampling, Ward offers three recommendations:
Identify the extractant: Know which method your lab is using for each specific nutrient.
Maintain consistency: Stick with the same method over several years to accurately track trends and compare fields. Don't "mix and match" methods.
Seek regional alignment: Use the extractant that matches the calibrated research performed by your local land-grant university.
For those farmers requiring specialized testing not found on a standard menu, Ward encourages direct communication with your laboratory to check your options.
