Jonathan Dahl, Soil Test Expert


Jonathan G. Dahl has an MS from Michigan State University in soil fertility. His current position is soil testing specialist and manager of the MSU Soil and Plant Nutrient Lab, which he has managed for the past 25 years. Jonathan was kind enough to answer our questions about soil testing via e-mail correspondence.

How did you get started in soil testing?
As an undergraduate student at Michigan State University, I did an independent study course on soil testing and then worked in the lab for one spring season. I applied for the position when I finished my master’s degree.

Why is it important for a farmer or gardener to have their soil tested?
Soil testing should serve as the basis for all fertilizer and lime applications. Without soil testing, you are only guessing what needs to be added to the soil for it to be productive. Applying a complete fertilizer when all you need is nitrogen is a waste of money and resources. Also, if you apply only what is called for on the bag, you may be under-applying some nutrients and over-applying others.

Consumers may purchase an at-home soil testing kit, or they may contact a local soil testing laboratory to have an analysis performed. Which method do you most recommend? What are the advantages/disadvantages to both methods?
The home kits are adequate for testing the pH of the soil but are not useful for doing a nutrient analysis. The pH test kits are accurate to within a half of a pH unit. This is normally accurate enough to give the individual a good idea what their soil pH is. They are particularly useful if you are trying to grow an acid-loving plant in an alkaline (high pH) soil. You can acidify the soil with sulfur and then use the kit to periodically check the pH to see if it is in the range that the acid-loving plant requires. The at-home test kits for nitrogen, phosphorus and potassium have not shown to be very reliable. For reliable fertilizer recommendations, it is important to have a reputable lab perform the soil analysis. The results are much more accurate and provide a sound basis for making the proper fertilizer applications.

What kind of elements do you test for when determining whether soil is fertile or not?
The routine analysis includes phosphorus, potassium, calcium and magnesium along with the soil pH. These are the elements that plants require the largest quantities of, and thus must be replenished annually. Labs do not routinely do a nitrogen analysis because the nitrogen level in the soil will vary widely throughout the year. The nitrogen recommendation is based on the type of plant being grown and then can be adjusted for soil organic matter content or application of manures and composts. For farmers on high pH soils, it is often recommended that the soil be tested for the micronutrients, zinc and manganese. These elements become much less available as the pH increases above seven, and therefore should be tested before growing crops that have high requirements.

What are the most important pieces of equipment and supplies that soil testing laboratories use?
pH is measured using a pH meter and electrode to get results that are accurate to within 1/10 of a pH unit. For potassium, calcium and magnesium, an automated flame photometer measures the nutrients extracted from the soil. For phosphorus, a dip probe colorimeter is used for measurement. The micronutrients are analyzed using an atomic adsorption spectrophotometer. The advantage of using several instruments to measure the nutrients is that you can then use a different extractant for each element. This allows you to choose the best extractant for each element. Many of the larger labs are using ICPs (Inductively Coupled Plasma Emission Spectrophotometers) to measure the whole range of soil elements. The advantage is faster analysis times and a wider range of elements that can easily be tested. The disadvantage of using the ICP is that, because you are measuring a wide range of elements using one extractant, the extractant does not work equally well for all the elements.

What degree of education and training is necessary to do soil testing?
In soil test labs, you have employees with all levels of education. Many of the entry-level positions could be performed with a high school education along with training. For individuals involved in actual soil analysis, a bachelor’s degree in one of the sciences is desirable. This gives the individual some training in basic chemistry. There would also need to be a great deal of training. For some of the more complex instruments and methods of analysis, master’s or even PhDs are necessary. Most lab managers have PhDs, but some can get into the smaller labs with a master’s.

Do you have any advice or suggestions for people who wish to get their soil tested?
Before you have your soil tested you need to find a reputable lab. Some basic questions you can ask a lab are:

  • Do you participate in any proficiency testing programs? They should be participating in some outside program to insure their methods and results are reliable.
  • What type of quality control measures do you have in place to ensure accuracy? They should be using internal check samples on a regular (like every 10 or 20 samples) interval. They should also be using methods that are approved for your soils. Check with your state agricultural university to see which methods are recommended for your area. They can also provide you with names of reputable labs.
  • What are your fertilizer recommendations based upon? They should be based on research done in your state.

Do you have any advice or suggestions for people who wish to get a job in the soil testing field?
A bachelor’s degree in crop and soil sciences is helpful, but you can also get an entry-level position and work your way up the ladder with training as opportunities present. Find out when the busiest time in the lab is and then try to get experience in the lab at that time. Most labs need to hire seasonal help to make it through these periods. Work hard and take an interest in other positions in the lab to see what it takes to perform them. When a position comes open, they are more likely to hire someone whose work ethic they are familiar with and they know is genuinely interested in learning the position.

To learn more about soil testing, please read our Soil Testing Guide. To find a test kit or organization, please visit our Soil Test Directory.


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