Measuring Soil Water Potentials in Horticultural Substrates at Very Low Water Potentials

The study of water availability in substrates involves knowing the permanent wilting percentage (PWP). This has be traditionally defined as water held at soil water potentials between –1.0 and –2.0 MPa, with –1.5MPa as the norm.  The PWP has been determined with pressure plate systems set at 1.5 MPa without plants.  However, the PWP range has not been thoroughly explored for horticultural substrates.  Soil water potential measurements can now be made directly on soil/root systems under various conditions.  This study determined soil water potentials in substrates as plants dried to five levels of wilt. Hibiscus plants were grown in 3 peat:1 vermiculite:1 perlite substrate in 3.8 L pots for 10 weeks before inducing wilt.  Plant containers were completely submerged, drained and weighed.  Plants were then allowed to dry to one of five stages of wilt: 1) first sign of leaf flagging; 2) leaves and stems flagging near 45° angles to main stem; 3) leaves and stems fully flagged; 4) 24 hours after stage 3; and 5) 48 hours after stage 3.  Each plant was allowed to reach wilt stage individually.  At each stage, pots were weighed and soil samples were taken from each pot, sealed and soil water potentials determined. After sampling, each container was watered and level of plant recovery was noted.  Because much water is generally lost from the pot surface during drying, half of all treatments had covers placed on the substrate surface and half were left bare.  After saturation, stage 1 was reached in approximately 7 days; stage 2 in 8–15 days; stage 3 in 15–20 days. Plants with covered soil surfaces were delayed about 2 days in reaching each stage.  Soil water potential varied among stages of wilt.  Plants showed wilt symptoms between 0.4 and 7.68 MPa.  All plants recovered from stages 1, 2, and 3, with all of them beyond 1.5 MPa.  All plants recovered from wilt up to 2.5 MPa.  However, some plants recovered from wilt at 4.0 MPa.  Covering the soil surface altered the time to wilt, but did not affect the severity of wilt or affect plant recovery.  This work showed a range of soil water potentials much wider than 1.0 to 2.0 MPa for Hibiscus.  This direct method of measuring soil water potential may aid in determining PWP more precisely in plants grown in horticultural substrates.