Matric potential (kPa) -10 -5 0

Matric potential (kPa) -10 -5 0

Fig. 2.7. Soil drainage over time with depth for two hypothetical soil types. (A) Well-drained soil high in sand content, such as a sandy loam. (B) Soil with higher silty—clay content and slower infiltration, especially at about 60 cm depth. Lines represent 3-day intervals.

Different methods and instruments exist to obtain estimates of soil water and soil solution content. The correct procedure depends on the amount and type of soil to be analysed and whether field values or laboratory values are required. Sources of information for choosing the adequate protocol are provided in Methods of Soil Analysis, Handbook of Soil Science and Standard Soil Methods for Long Term Ecological Research, referenced at the end of this chapter. Field instrumentation to gather data about processes at the scale of an ecosystem or watershed can be inadequate to provide data on smaller study sites. Small plots may need to be measured individually due to microclimate variations in rainfall or temperature.

Electronic devices are commercially available to measure soil water content directly, by time domain reflectometry (TDR). The apparatus uses microwave frequencies to obtain a measure of the apparent dielectric constant of the surrounding soil (see review in Noborio, 2001). These are available as portable probes for spot measurements or permanent probes for long-term continuous data acquisition. The measured values need to be calibrated, in situ and in the laboratory, against soils with known water content. The method is relatively insensitive to mineral soil composition, but is affected by SOM content and can be affected by some clays. Measurements of soil water potential are made with ten-siometers in wet soil (0 to -0.48 MPa) and with resistance blocks in drier soils (-0.01 to -3.0 MPa). Tensiometers are water-filled tubes with a porous ceramic cup at one end. The top end of the tube is sealed with a vacuum valve, used to create a negative pressure (or tension) in the water tube. The tensiometer is equilibrated against water to zero tension. The porous cup is inserted at the required depth in the soil. The soil draws water out of the cylinder, creating a negative tension which is read from the instrument. The device only functions if there is a continuous water contact from the tube to the soil. If soil dries too much, the device will empty and it will have to be reset. In drier conditions, resistant block devices are used. These consist of a gypsum or fibreglass block buried at the required depth. The block is connected to a recording device on the surface by wires. The device measures electrical conductance and is therefore sensitive to changes in salinity and will not function in some soils with high salt content.

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