Soil Water
Measuring Soil Water Status
Accurate determination of soil-water status is not only important for irrigation and water resources management, it is also a fundamental element of soil-water movement, chemical transport, crop water stress, evapotranspiration, hydrologic and crop modeling, climate change, and other important disciplines. Water content measurement methods can be either direct or indirect. The gravimetric method determines soil moisture directly. Indirect methods do not measure soil water content directly, but they measure some other variable from which the soil water content can be calculated.
Gravimetric Method in Measuring Soil Water Status
A common technique utilized to determine soil moisture content is the gravimetric method. The gravimetric water content is the mass of water per unit mass of dry (or wet) soil. Also known as the drying method, the gravimetric method determines the moisture content of soil using a sample that is weighed prior to and after being oven dried at 221 degrees F (105°C). The moisture content can then be defined by establishing the weight difference between the wet and dry samples. An advantage of gravimetric measurement is that it gives the exact amount of water in the soil and can be used to calibrate indirect measurement devices. It is calculated as: gravimetric Water (%) = (Wet soil weight (g) – Dry soil weight (g)) ÷ Dry soil weight (g) × 100.
Indirect Methods in Measuring Soil Water Status
The indirect methods monitor soil water content by estimating the soil moisture by a calibrated relationship with some other measurable variable using soil sensors. Data from indirect measurements can be recorded and stored in specified time intervals for further analysis if needed. Indirect methods are nondestructive, meaning that the soil profile is not disrupted, and the measurements are available instantaneously. For indirect methods, the sensors need to be calibrated.
Volumetric Soil Water Content (% vol)
The volumetric water content (VWC) is the ratio of the volume of water to the unit volume of soil. Volumetric soil water content indicates the quantity of water in the soil but does not directly indicate the availability of this water to plants. The most common methods for measuring volumetric soil water content include neutron probes and dielectric methods such as (1) Frequency Domain Reflectometry (FDR) or capacitance (C), (2) Time Domain Reflectometry (TDR), (3) Amplitude Domain Reflectometry (ADR)/Impedance, and (4) Time Domain Transmission (TDT). It is calculated as: volumetric soil water content (%) = [volume of water (cm3) ÷ volume of soil (cm3)] × 100.
Soil Matric Potential (kPa)
Soil matric potential (SMP) represents the relative availability of the amount of water held in the soil profile for plant uptake/use. The quantity of soil-water from SMP is determined through soil-water characteristics curves developed for specific soil types. In more practical terms, SMP is a direct indication of how much energy must be exerted (application of pressure) by plants to extract the water molecules from soil particles.
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