What Are Soil Measurements?

Soil measurements are the set of techniques and instruments used to characterize the physical, chemical, and biological properties of soil for purposes including agricultural management, geotechnical engineering, environmental assessment, and infrastructure siting. Measured properties include moisture content, texture, bulk density, thermal resistivity, electrical conductivity, pH, nutrient concentrations, and organic matter content. The discipline draws from soil science, geophysics, electrical engineering, and remote sensing, and it spans both laboratory methods applied to extracted samples and in-situ techniques that assess soil properties without disturbance.

Accurate soil measurements are foundational to precision agriculture, where spatial variability in soil properties governs the variable application of water, fertilizers, and pesticides across a field. They are equally important in geotechnical site investigation, where soil strength and compressibility data determine foundation and slope designs.

In-Situ and Geophysical Measurement Methods

In-situ measurement techniques assess soil properties at the point of interest without removing material, preserving the natural structure that laboratory samples often disturb. The cone penetration test (CPT) advances an instrumented probe through the soil profile and measures tip resistance, sleeve friction, and pore water pressure, providing continuous data on soil strength and stratigraphy. Ground-penetrating radar (GPR), electromagnetic induction (EMI), and electrical resistivity tomography (ERT) are geophysical methods that sense soil properties over large volumes. USDA Agricultural Research Service studies on geophysical methods for soil characterization document the application of EMI and GPR to mapping soil texture and salinity distributions across agricultural fields. These methods produce spatial datasets that link point sample results to broader field-scale patterns.

Moisture Measurement

Soil moisture content, defined as the mass or volume of water per unit mass or volume of soil, is among the most frequently measured soil properties. Time domain reflectometry (TDR) and frequency domain reflectometry (FDR) sensors infer volumetric water content from the dielectric permittivity of the soil, which changes strongly with water content because water has a much higher permittivity than mineral particles or air. Neutron probes measure hydrogen atom density as a proxy for water content at depth. Tensiometers measure the matric potential (suction) of soil water, which determines whether plants can extract it. MDPI research on soil moisture sensing technologies surveys the principles, calibration requirements, and field performance of these methods across agricultural applications. Satellite and airborne remote sensing platforms, particularly those using microwave radar and passive microwave radiometry, extend moisture measurement from point and field scales to regional and continental scales.

Laboratory and Chemical Analysis

Laboratory measurements on extracted soil samples complement in-situ methods for properties that require controlled conditions or destructive analysis. Particle size distribution is determined by hydrometer analysis or laser diffraction. Organic matter content is measured by loss on ignition or the Walkley-Black wet oxidation method. Soil pH is measured in a water or salt suspension with a glass electrode. Nutrient concentrations, including plant-available nitrogen, phosphorus, and potassium, are extracted with standardized chemical solutions and analyzed by spectroscopy or chromatography. The USDA's Crop Condition and Soil Moisture Analytics tool integrates laboratory reference datasets with remote sensing products to provide national-scale soil moisture and crop condition assessments supporting drought monitoring.

Applications

Soil measurements have applications in a wide range of disciplines, including:

• Precision agriculture for variable-rate fertilization, irrigation, and seeding
• Geotechnical site investigation for foundation and embankment design
• Environmental site assessment for contamination delineation and remediation monitoring
• Buried cable and pipeline routing dependent on thermal and electrical soil properties
• Drought monitoring and agricultural water resource management