Water Content In Soilless Substrates: Spatial and Temporal Dynamics

Increasing the efficiency of greenhouse irrigation requires a better understand of the dynamics of water movement in soilless substrates.  Changes in the distribution of water in soilless substrates occur continuously due to such processes of drainage, capillary action, evaporation, and water uptake by plants.  Using soil moisture sensors, we determined the spatial distribution of water in soilless substrates, and quantified these changes in water content over time.  These data were used to quantify the rate of water movement through the substrate following overhead or sub-irrigation and changes in water distribution within the substrate as a result of plant water uptake.  By measuring the depletion of water from different substrate layers within container, we determined which layer provided most of the water to the plants.  Typical stratification of substrate water content was seen in subirrigated containers, with the highest water content in the bottom layer, and increasingly lower water levels in higher substrate layers.  Water from the lower substrate layers gradually moved up through the substrate through capillary action.  In overhead-irrigated containers we did not always see similar stratification.  In relatively dry substrate, overhead irrigation results in the highest substrate water content in the top layer of the substrate, and there was little capillary movement downward.