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It is usual to express soil water content (SWC), measured gravimetrically as a percentage relative to the dry weight (DW) of soil: where WL is the loss of weight on drying the soil sample (initial weight - dry weight). A relationship between water content of Metromix 360 and water potential was determined with the Decagon thermocouple psychrometer:  It seems that there is a reasonably linear relationship between log10 (water content) and water potential over the range from 50 to 300% water content:  So we can use the equation from this line to derive estimates of water potential (WP) from water content: This equation indicates that water potential is zero when the medium contains 279.5% of its dry weight as water so it should not be applied above this level. 
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