Soil moisture
Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth.
The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away.
[27] The total amount of water held when field capacity is reached is a function of the specific surface area of the soil particles.
Suction has a positive value and can be regarded as the total force required to pull or push water out of soil.
But as the droplet is drawn down, the forces of adhesion of the water for the soil particles produce increasingly higher suction, finally up to 1500 kPa (pF = 4.2).
At 0 to 33 kPa suction (field capacity), water is pushed through soil from the point of its application under the force of gravity and the pressure gradient created by differences in the pressure of water; this is called saturated flow.
At higher suction, water movement is pulled by capillarity from wetter toward drier soil.
[39][40] Tree roots, whether living or dead, create preferential channels for rainwater flow through soil,[41] magnifying infiltration rates of water up to 27 times.
[42] Flooding temporarily increases soil permeability in river beds, helping to recharge aquifers.
[44][45] 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.
[46][47] In order of decreasing solubility, the leached nutrients are: 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.
[53] The soil moisture velocity equation,[54] which can be solved using the finite water-content vadose zone flow method,[55][56] describes the velocity of flowing water through an unsaturated soil in the vertical direction.
Preferential flow occurs along interconnected macropores, crevices, root and worm channels, which drain water under gravity.
[60] Of equal importance to the storage and movement of water in soil is the means by which plants acquire it and their nutrients.
[61] The upward movement of water and solutes (hydraulic lift) is regulated in the roots by the endodermis[62] and in the plant foliage by stomatal conductance,[63] and can be interrupted in root and shoot xylem vessels by cavitation, also called xylem embolism.
[65] Osmotic absorption becomes more important during times of low water transpiration caused by lower temperatures (for example at night) or high humidity, and the reverse occurs under high temperature or low humidity.
[70] Roots must seek out water as the unsaturated flow of water in soil can move only at a rate of up to 2.5 cm per day; as a result they are constantly dying and growing as they seek out high concentrations of soil moisture.
[72][74] The total water used in an agricultural field includes surface runoff, drainage and consumptive use.
The use of loose mulches will reduce evaporative losses for a period after a field is irrigated, but in the end, the total evaporative loss (plant plus soil) will approach that of uncovered soil, while more water is immediately available for plant growth.
