A Protocol to Estimate Plant Available Water Thresholds from Empirical Soil Moisture Sensor Data

Plant available moisture is typically defined as the difference in volumetric water content (% VWC) between field capacity (FC) and some proportion of soil moisture that corresponds to temporary crop water stress. This is typically defined as a negative soil matric potential; for some sensitive crops, temporary water stress could be experienced at -20 to -30kPa. However soil matric potential is also influenced by the total water-holding capacity (i.e. FC) of a specific soil or substrate type. For example, this is typically much lower for sandy soils than for silt loams. Furthermore, comparing soil matric potential to soil volumetric water content values is very difficult, especially for growers. We will describe a protocol where field capacity can be determined from soil water content curves after a saturating rainfall or irrigation event; the point of inflection of the curve typically between 4 and 8 hours after irrigation. Readily-available soil moisture (RAW, defined as -8 to -40 kPa) can be estimated for a range of different soil types and used to calculate the proportion of RAW as a function of total soil moisture (FC). These readily-available water ranges can then be confirmed by in situ soil matric potential or leaf water potential measurements throughout the growing season. In this way, threshold values for %VWC can be entered to the graphical software, to provide a quick visual guide for growers to more accurately schedule irrigation events.