PLANT AVAILABLE AND UNAVAILABLE WATER IN GREENHOUSE SUBSTRATES: ASSESSMENT AND CONSIDERATIONS

J.S. Fields, W.C. Fonteno, B.E. Jackson
Accurate assessment of available water in substrates usually includes a meas-urement of water unavailable to plants. Plant roots have an ability to pull suctions up to 1.0 to 2.0 MPa, depending on species, with the classic value for unavailability measured at 1.5 MPa. Five samples each of peat moss, pine bark and perlite and a clay soil were placed in a 1.5 MPa porous plate system for 48 hours. The samples were then removed and run in a dewpoint potentiometer then dried for 24 hours at 105°C. The mineral soil potentials averaged 1.39 MPa, but the others were much smaller: peat = 0.38, bark = 0.21 and perlite = 0.28 MPa. Peat and bark were re-tested at 0.3 MPa on the porous plate system then placed in the potentiometer. The peat water potential was 0.33 MPa while the bark was 0.34 MPa, showing good agreement with the porous plate pressures. The samples of highly porous materials of peat, bark and perlite possibly seemed to lose hydraulic continuity between the samples and porous plate above 0.3 MPa of pressure which stopped the flow of water from the samples. This resulted in artificially high values. In a second study, substrate samples (3 peat: 1 perlite: 1 vermiculite, v/v/v) were taken from mature marigold plants in three stages of wilt: Stage 1: light wilt (initial leaf flagging), Stage 2: moderate wilt (leaves wilted to ~ 45°of vertical) and Stage 3: heavy wilt (leaves wilted and curled to main stem). Water potentials were measured at each stage using the potentiometer. After substrate sampling, each plant was re-watered and level of recovery was noted. Plants at Stage 1 wilt had soil potentials of ~ 0.6 MPa. Stage 2 wilt was at ~ 1.55 MPa and Stage 3 wilt was ~ 2.2 MPa. All plants visually recovered from wilt at all stages. The potentiometer may be useful in determining actual soil water potentials under dry conditions, not normally measurable using the traditional porous plate system. Unavailable water content for horticultural substrates may be overly high using the porous plate system as confirmed with the dew point potentiometer. Measuring water potentials during plant wilt may help to refine the nature of permanent wilt and more precisely determine water is truly unavailable to plants.
Fields, J.S., Fonteno, W.C. and Jackson, B.E. (2014). PLANT AVAILABLE AND UNAVAILABLE WATER IN GREENHOUSE SUBSTRATES: ASSESSMENT AND CONSIDERATIONS. Acta Hortic. 1034, 341-346
DOI: 10.17660/ActaHortic.2014.1034.42
https://doi.org/10.17660/ActaHortic.2014.1034.42
15 bar test, dew point potentiometer, permanent wilt
English

Acta Horticulturae