from the point of its application where it is saturated locally, to less saturated areas, such as the vadose zone. Once soil is completely wetted, any more water will move downward, or percolate out of the range of plant roots, carrying with it clay, humus, nutrients, primarily cations, and various contaminants, including pesticides, pollutants, viruses and bacteria, potentially causing groundwater contamination. In order of decreasing solubility, the leached nutrients are: Calcium Magnesium, Sulfur, Potassium; depending upon soil composition Nitrogen; usually little, unless nitrate fertiliser was applied recently Phosphorus; very little as its forms in soil are of low solubility. In the United States percolation water due to rainfall ranges from almost zero centimeters just east of the Rocky Mountains to fifty or more centimeters per day in the Appalachian Mountains and the north coast of the Gulf of Mexico. Water is pulled by capillary action due to the adhesion force of water to the soil solids, producing a suction gradient from wet towards drier soil and from macropores to micropores. The so-called Richards equation allows calculation of the time rate of change of moisture content in soils due to the movement of water in unsaturated soils. Interestingly, this equation attributed to Richards was originally published by Richardson in 1922. The soil moisture velocity equation, which can be solved using the finite water-content vadose zone flow method, describes the velocity of flowing water through an unsaturated soil in the vertical direction. The numerical solution of the Richardson/Richards equation allows calculation of unsaturated water flow and solute transport using software such as Hydrus, by giving soil hydraulic parameters of hydraulic functions (water retention function and unsaturated hydraulic conductivity function) and initial and boundary conditions. Preferential flow occurs along interconnected macropores, crevices, root and worm channels, which drain water under gravity. Many models based on soil physics now allow for some representation of preferential flow as a dual continuum, dual porosity or dual permeability options, but these have generally been "bolted on" to the Richards solution without any rigorous physical underpinning. Water uptake by plants Of equal importance to the storage and movement of water in soil is the means by which plants acquire it and their nutrients. Most soil water is taken up by plants as passive absorption caused by the pulling force of water evaporating (transpiring) from the long column of water (xylem sap flow) that leads from the plant's roots