Going below the surface

LAMBERTON – Though there are now high-tech instruments that measure soil moisture at different depths, the University of Minnesota Southwest Research and Outreach Center in Lamberton still uses the same reliable and economical method it’s used since 1966.

“We take soil moisture measurements twice a month around the 1st and 15th, from April to September, conditions permitting,” said Lee Kossner, senior research fellow at the center.

The tools used are hollow probes dating back to the 1980s, mounted on a tractor built in the 1960s.

Kossner drills the hollow five-foot probe into the ground and pulls out a soil sample. The sample is deposited in a tray, and the top two feet is divided into six-inch sections, the remainder into one-foot sections. Each section of soil is put into sample cans which are weighed and baked in a convection oven at 100 degrees Celsius for a minimum of 48 hours, then weighed again.

The difference between the first and second weighings gives the moisture content by weight. This figure is used to calculate the moisture content by volume.

“To do soil moisture consistently, you have to know the bulk density to do the calculation,” Kossner said. “The bulk density depends on the porosity of the soil. Different soils have different densities.”

Crops depend on adequate soil moisture near the surface at the beginning of the growing season. Though ideally the soil should hold moisture throughout the soil profile, once the root systems are established, crops can get moisture from the deeper layers if the top layers dry out.

“In dry conditions, roots can go down four feet for corn and three feet for soy,” said Jeff Strock, professor of soil science at the center.

According to Strock, it’s not just the water that’s important to crops but vital minerals dissolved in the water.

“Rainwater already has things in it plants need, such as nitrogen and sulfur,” Strock said. “Water plays a key role in biological activity, which is highest in the top 12 inches of soil and is a function of temperature and moisture.”

The center takes samples from four locations around the facility. There are other agencies taking samples at other locations around the state but usually not from as deep as the Lamberton center does.

Though there’s a lot of complex science invested in soil moisture research, it all boils down to one simple question of vital interest for farmers.

“How much water does it take to grow your 200 bushels per acre?” Strock said.