Continuous Frozen Columns for Determining Low Tension Moisture Characteristic Curves of Soil-less Substrates.

Mixtures of organic and inorganic materials (soilless substrates) are commonly employed for containerized production of plants.  Conventional methods utilized to quantify the water relations of soilless substrates are typically based on container capacity, providing limited information about the hydrology of soilless substrates.  These methods can be expensive and time consuming.  A new method using continuous columns was developed to measure moisture characteristic curves for soil-less substrates.  The method is rapid, inexpensive, and offers excellent precision at low tensions.  Columns (112 cm x 7.6 cm i.d.) made from Schedule 40 polyvinyl chloride rigid pipe) are hand-packed to uniform bulk density, bottom-saturated, drained, frozen (-21°C), and then cut into ten sections [≈10 cm tall].  Gravimetric water content is then determined for each cut section. Relationships between water content (θ) and column height (x) (i.e., tension in cm of water) are fit to sigmoid functions.  The method has been used to generate reliable and reproducible moisture characteristic curves for bark substrates.  The method has also been used to demonstrate the effects of varying ratios of peat, pumice, and bark on moisture retention.  A precise moisture characteristic curve, 0 to 100 cm tension, can be produced in 7 d.