Root zone available water capacity (RZAWC) is commonly misunderstood, misinterpreted, or misused. The calculation of RZAWC begins with the measured Water Retention Difference (WRD), which is the volume of water held in a soil between one-third and 15 MPa, inclusive of rock fragments. Since the Available Water-Holding Capacity (AWC) is the calculated volume of water that is available to plants in a given sample of soil, it differs from WRD because it excludes the amount of water not available to plants due to factors such as incomplete root ramification or osmotic effects. These conditions are associated with certain soil chemical and physical properties, such as high bulk density, excessive soluble salts, or toxic levels of extractable aluminum. To calculate The RZAWC, the AWC for each soil layer is multiplied by the layer thickness. These products are then summed to the expected maximum depth of root penetration, which is commonly either 1 or 1.5m, or to a physical or chemical root limitation, whichever is shallower. RZAWC is sometimes less than AWC, but provides a better assessment of potential soil productivity.