The Science of a Soil Test

MU Soil Labs help find out what is happening in your soil to help produce profitable crops

With harvests winding down, the farm’s focus might turn away from the fields. According to specialists at the University of Missouri, now is just as good as ever to find out what your ground needs.

One way to start planning for next year’s growing season is to get a soil test and MU’s Soil Fertility Labs are there to help.

David Dunn, Extension Associate Soil Testing Lab & Rice Extension, adds an extracting solution to the soil samples. It is a water-based solution used to simulate plants ability to stabilize phosphate. A blue solution is later added to determine the level of phosphate in the soils. This step is one of three different tests performed on one soil sample to find solutions for the producer/grower.David Dunn, Extension Associate Soil Testing Lab & Rice Extension, adds an extracting solution to the soil samples. It is a water-based solution used to simulate plants ability to stabilize phosphate. A blue solution is later added to determine the level of phosphate in the soils. This step is one of three different tests performed on one soil sample to find solutions for the producer/grower.

A soil test is like taking an inventory of nutrients available to plants — which area is too high, too low or just right. While plant growth and prior yields may offer clues to nutrient availability, a producer won’t precisely know until they test their soil.

“We can tell you what is going on below your feet,” said David Dunn, MU Extension soil testing lab associate. “We are all about giving recommendations for farmers to achieve the yields they want.”

Dunn helps manage the Soil Fertility Lab at the Fisher Delta Research Center, one of the many Agricultural Research Centers operated by the MU College of Agriculture, Food and Natural Resources (CAFNR). A second lab is located on the MU Campus in Columbia.

Each year the lab in Portageville analyzes around 10,000soil samples. With each test, producers get a detailed report on pH levels; available phosphorus, potassium, calcium and magnesium; organic matter; acidity and cation exchange capacity. These basic tests provide the necessary data to develop nitrogen, phosphate, potash and agriculture lime recommendations for intended crops.

In Missouri, the soil organic matter tests are used to estimate nitrogen availability in the soil. A general rule is every 1 percent of soil organic matter will release about 20 pounds of nitrogen per acre for crop availability. Less than half of the samples tested by MU in 2013 had medium levels of soil organic matter of around 2 to 3 percent.

With the results a rating system is applied to give the farmer a guide to future nutrient needs. Also included are recommendations from MU specialists on management strategies for specific crops, yield goal levels to gauge future production and pounds per acre fertilizer suggestions.

“To ensure the best results and recommendations we suggest taking at least about 10 to 15 different sub-samples,” said Dunn. “Look at gathering a sub-sample from at least every acre. Even on small farms the soil change at different spots in the field.”

To get to these results, several steps at taken by lab technicians. Once received the samples are sorted and placed in marked containers, but need to be dried. A special chamber uses heated forced air to dry the samples. During a 16-hour drying session, the room can separate 200 pounds of water from 400 pounds of soil and water.

Next, the soil is ground and sieved to remove unwanted pieces such as rocks, sticks or other organic matter that could flaw results. Each soil sample is then sub-sampled for the individual tests required.

When soil samples are received, they often have rocks, sticks or other scrap that can deter from getting accurate soil testing results. At this station Dunn runs the samples through a seperator that grounds the soil and screens out unwanted pieces.When soil samples are received, they often have rocks, sticks or other scrap that can deter from getting accurate soil testing results. At this station Dunn runs the samples through a seperator that grounds the soil and screens out unwanted pieces.

For most of the tests, extracting solutions are added to the soil samples. These solutions are water-based chemicals that simulate a plant’s ability to obtain the nutrient in question with the soil. They are then analyzed with specialized equipment. To find availability of potassium, calcium and magnesium the samples are tested in an atomic absorption spectrometer that burns the soil and solution in an acetylene-fired flame that changes color based on nutrient levels.

To find phosphorus results, a solution that turns blue in the presence of phosphorus is added. The intensity of this blue color is used to determine the level of phosphorus. The deeper the blue color, the more phosphorus is available to the plants.

To find pH levels, the soil is again mixed with a diluting solution and analyzed with an electrode that measures pH levels and gives a read out on how much lime is needed to be added to reach a healthy pH level for the sample.

To find availability of potassium, calcium and magnesium the samples are tested in an atomic absorption spectrometer that pushes the soil and solution through a propane-fired flame that change color based on nutrient levels.

“It’s very important for people to be looking at what is going on in their soil so they know how to manage it properly,” said Dunn. “It’s really easy to get us the sample and we can turn around results pretty fast. Every county MU Extension office can help get the samples to us.”

Before beginning the process of soil test, Dunn also urges producers to think about how intensive of management you would like to pursue. “You might have to manage and apply fertilizers on a plot of five acres differently from the rest of the farm,” he added.

Follow these simple steps for soil test:

  • Identify manageable area within each field with similar characteristics (i.e. hillside versus bottomland).
  • Using a soil probe or shovel, collect sub-samples of soil 6 inches deep.
  • Collect sub-samples from at least 10 to 15 locations, about one for every acre.
  • Collect sporadically from locations in field.
  • Avoid unusual areas in the field — locations near gravel roads, livestock feeding areas, low spots or gullies.
  • Seal soil in plastic bags or containers.
  • Properly record and label location of each sample.

For more detailed instructions, visit http://soilplantlab.missouri.edu/soil/. For more information and additional services offered through the Fisher Delta Research Center, visit their new website at http://delta.cafnr.org.

To download press-quality photos of MU Soil Labs, visit CAFNR’s Flickr site at http://bit.ly/MUSoilLabs.

An electrode measures the pH levels in soil samples. The instrument gives a read out on how much nutrients need to be added to reach a healthy pH level for the sample. An electrode measures the pH levels in soil samples. The instrument gives a read out on how much nutrients need to be added to reach a healthy pH level for the sample.