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Whether researchers measure soil hydraulic properties in the lab or in the field, they’re only getting part of the picture. Laboratory systems are highly accurate due to controlled conditions, but lab measurements don’t take into account site variability such as roots, cracks, or wormholes that might affect soil hydrology. In addition, when researchers take a sample from the field to the lab, they often compress soil macropores during the sampling process, altering the hydraulic properties of the soil.
Field experiments help researchers understand variability and real time conditions, but they have the opposite set of problems. The field is an uncontrolled system. Water moves through the soil profile by evaporation, plant uptake, capillary rise, or deep drainage, requiring many measurements at different depths and locations. Field researchers also have to deal with the unpredictability of the weather. Precipitation may cause a field drydown experiment to take an entire summer, whereas in the lab it takes only a week.
|HYPROP||Water potential (wet range)||Tensiometers (T8, T4)|
|WP4C||Water potential (dry range)||TEROS 21|
|HYPROP, and oven drying method||Water content||Volumetric water content sensors|
|KSAT||Saturated hydraulic conductivity||SATURO (field saturated)|
|HYPROP||Unsaturated hydraulic conductivity||Mini disk infiltrometer|
The big picture—supersized
Researchers who use both lab and field techniques while understanding each method’s strengths and limitations can exponentially increase their understanding of what’s happening in the soil profile. For example, in the laboratory, a researcher might use the PARIO soil texture analyzer to obtain accurate soil texture data, including a complete particle size distribution. They could then combine those data with a HYPROP-generated soil moisture release curve to understand the hydraulic properties of that soil type. If that researcher then adds high-quality field data in order to understand real world field conditions, then suddenly they’re seeing the larger picture.
Below is an exploration of lab versus field instrumentation and how researchers can combine these instruments for an increased understanding of their soil profile. Click the links for more in-depth information about each topic.
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Particle size distribution and why it matters
Soil type and particle size analysis are the first window into the soil and its unique characteristics. Every researcher should identify the type of soil that they’re working with in order to benchmark their data. If researchers don’t understand their soil type, they can’t make assumptions about the state of soil water based on water content (i.e., if they work with plants, they won’t be able to predict whether there will be plant available water). In addition, differing soil types in the soil’s horizons may influence a researcher’s measurement selection, sensor choice, and sensor placement.
Particle size analysis defines the percentage of coarse to fine materials that make up a soil. With this knowledge, a researcher can estimate how strongly a particular soil will hold on to water. Particle size analysis goes beyond the simple definition of soil type. The particle size analysis acts more like a soil fingerprint, showing the unique distribution of particle size across the sand, silt, and clay fractions. This information can help a geotechnical engineer understand how a shrink-swell soil will react over time, or it might influence a grower’s irrigation decisions. Particle size distribution also may provide insights into how the soil formed, or eventually will form structure, and it influences saturated hydraulic conductivity: the more coarse the material is, the more easily water will move.
Historically, researchers identified soil texture using crude methods such as the ribbon test, the pipette method, or the time-consuming hydrometer technique. PARIO now automates the process of soil texture analysis, saving time and increasing accuracy. PARIO gives researchers a complete particle size distribution analysis, including a breakdown of the fraction of fine silt, middle silt, clay, and sand. After obtaining the analysis, the software automatically computes its location on the USDA soil texture triangle to accurately identify the soil type.
Researchers should use PARIO as a first step in understanding their soil, before they decide what other parameters to measure. This will help them determine which lab or field instrumentation will be most effective for their research goals.