By Kathy Wythe
With an increased emphasis on precision agriculture and watershed planning, scientists need higher resolution maps of soil properties for improved modeling results and management decisions. One way to obtain these higher-resolution maps is to develop sensors that can rapidly and reliably quantify soil properties in-situ (in the field).
Dr. Cristine Morgan, assistant professor, Texas A&M University Soil and Crop Sciences Department, and her research team have found a way to measure the soil clay content, therefore map soil horizons quickly and accurately in the field.
Through a Texas Agricultural Experiment Station (TAES)-funded Water Resources Research Grant, Morgan was able to use diffuse reflectance spectroscopy to predict the clay content with soil depth in fields in the North Bosque River watershed.
Heterogeneity or diversity of soil affects the quality and quantity of water in streams, reservoirs and groundwater. Knowing the soil properties and their spatial distribution at a higher resolution will help scientists improve modeling and management of water on landscapes, Morgan said.
“If researchers can more accurately quantify the soil properties across the field, then farmers can better manage their crops. Knowing the soil properties can also help in decreasing erosion and non-point source pollution,” she said.
Soil heterogeneity is not sufficiently captured by the current scale of soil surveys, Morgan said. These surveys map one type of soil within a larger area, too course of scale for precision agriculture and field-scale modeling, she said.
Diffuse reflectance spectroscopy uses optical sensors to measure soil reflectance of visible and near-infrared light, which is correlated to soil physical and chemical characteristics. By using the diffuse reflectance spectroscopy in the field, researchers get a higher resolution of soil information in a shorter time.
Sampling three representative landscapes in the North Bosque River watershed, team members collected 72 soil cores in plastic sleeves and then cut the cores in half vertically. Using, the spectrometer, they measured soil reflectance in situ at the field moisture content, in situ as the core dried and of dried and crushed samples passed through a sieve. After making reflectance measurements, they measured the soil clay content in the lab to validate the prediction model.
The results showed that the in situ reflectance measurements could measure the clay content of soil very rapidly and the accuracy was within 6 percent clay content, Morgan said.
Travis Waiser, a Soil and Crop Sciences Department master’s student and recipient of the 2004-05 Mills Scholarship given by the Texas Water Resources Institute, worked with Morgan on the project.
Morgan said her team hopes to build models for organic and inorganic carbon that can further quantify the soil.
“We think we can improve these measurements by adding auxiliary information,” she said.
She also plans to try additional statistical methods to create models that are more accurate and mount a spectrometer onto a soil probe for field mapping.
