What property of soil is measured by a hydrometer?

A hydrometer is used to measure the density of a liquid, which in the context of soils, allows for the determination of particle size distribution. When soil is mixed with water and placed in a hydrometer, the hydrometer measures how much the particles sink or float, thus helping to differentiate between various sizes of soil particles—clay, silt, and sand. This method is particularly valuable because understanding the distribution of particle sizes is crucial for interpreting soil properties such as drainage, fertility, and susceptibility to erosion.

The other options focus on properties that require different methods for assessment. For instance, soil moisture content is typically measured using techniques such as moisture meters or gravimetric methods. Soil temperature can be gauged using thermometers or temperature sensors, while soil compaction is usually evaluated through tools like penetrometers or by measuring bulk density and structural integrity. Understanding particle size is fundamental to soil classification and behavior, which is precisely what the hydrometer method provides.