Satellites may help monitor soil conditions

July 2, 2008

By Communications staff, Washington State University

LIND, Wash. – In the six counties of central and eastern Washington where wheat growers can count on only 8 to 12 inches of precipitation annually, they fallow fields for a year between crops to accumulate enough moisture to grow the next crop. There’s another thing growers are trying to save: their soil.

Bill Schillinger is a WSU scientist and extension specialist based at the WSU Lind Dryland Research Station. Photo by Brian Clark, Washington State University.

“The biggest environmental risk growers face is wind erosion and loss of suspended soil particulates from their farms,” said Bill Schillinger, a Washington State University scientist and extension specialist at the WSU Lind Dryland Research Station. He has devoted much of the last 16 years developing cropping systems to control wind erosion.

During the worst windstorm in Schillinger’s memory — one lasting a day in September 1999 — an estimated 50 tons of soil per acre was lost from a summer fallow field near Ritzville. That’s a conservative estimate in the WSU scientist’s opinion.

Wind erosion is most severe during the spring and late summer when wind speeds are highest, soils are being tilled and minimal crop residue covers the ground. In recent years, farmers have started to adopt the undercutter method of summer fallow farming, tested at the research station, that helps keep soil particles from becoming airborne.

Derek Schafer, a dryland wheat farmer, uses an undercutter for primary spring tillage and fertilizer injection in one of his fields near Lind. Crop residue left standing provides protection from wind erosion.

As a primary spring tillage implement, the undercutter’s overlapping V-sweep blades slice below the soil surface to sever capillary pores and channels as required to retain seed-zone moisture in summer fallow. The undercutter’s design minimizes surface soil disturbance, which keeps crop residue in place. Fertilizer is applied with the undercutter during primary spring tillage. Tillage during fallow can be reduced from the traditional six to as few as two operations.

In 2006, the Washington Association of Wheat Growers obtained a $1.8 million grant from the U.S. Department of Agriculture to help farmers implement undercutter farming technology through cost-sharing of the implements.

Forty-seven farmers are participating in the program, according to Schillinger. “They get an undercutter for half price but they have to use it on at least 160 acres for three years in the manner in which we prescribe for best wind erosion control, best agronomic production and best economic return,” he said.

How will progress in adoption of conservation tillage practices be measured?

Currently, measurements must be taken on the ground. In the not-to-distant future, satellite images may play a role.

The challenge in using remote imaging is lack
of detail, according to Bruce Frazier, a WSU soil scientist in Pullman. “To the eye, a lot of (crop) residue is the same color as soil.”

Frazier, who oversees the Remote Sensing and GIS Laboratory in the Crop and Soil Sciences Department, said that satellite images can provide information the eye can’t see by sensing wavelengths beyond the visible portion of the electromagnetic spectrum. Computers convert invisible wavelengths into colors that the eye can see.

“Eyes work well between wavelengths of 400 nanometers (blue) and 700 nanometers (red),” he said, “but plant residues are more easily separated from bare soil at several points between 900 (near infrared) and 2500 nanometers (short-wave infrared).”

At near infrared – 900 nanometers – it is possible to separate the reflectance of bare soil from soil that has 100 percent cover. “Nobody is interested in that,” he said. “What they want is everything in between.”

At 2,300 nanometers, reflectance curves start to separate. “Here’s a space where we can create an index that corresponds with data collected by the ASTER (Advanced Spaceborne Thermal Emissions and Reflectance Radiometer) instrument onboard NASA’s Terra satellite.”

On the ground at the WSU Lind Dryland Research Station, Frazier has collected spectral data of old standing residue, new standing residue, flat residue, smooth bare soil, roughened soil, rough summer fallow, green bunch grass and crusted soil using an instrument that measures reflected light at all wavelengths. “Our approach is to understand the spectra of crop residues and background soils to properly interpret the imagery,” Frazier said.

He is developing spectral indexes that eventually will allow him to map areas utilizing satellite images. “We are looking at the range of index values that might tell us something about how much residue cover is out there,” he said. “We are developing a regression equation from the ground data to relate index values to actual residue cover amounts. Then we can model what’s out in the field based on index values from the satellite images.”

Currently, Frazier is calibrating indexes for spectral data collected on the ground with data from ASTER images of the same area taken over the course of two years. The next step, which will be harder, is to reverse the process.

“I want to look at some more relationships between what we find on the satellite image and what’s on the ground,” he said. “We haven’t taken that step. We haven’t gone from the satellite to the ground. It is very difficult to have a satellite image of the same day you are on the ground but we still have to develop some kind of relationship.”

Frazier plans to complete his research before he retires next June.

“If satellite imagery could accurately show residue levels on each field, it might be a good way to document compliance and determine how effective farmers have been in conserving soil,” said Schillinger. “That would be a big step.”