Water that enters a field is removed from a field by
runoff,
drainage,
evaporation or
transpiration.[3] Runoff is the water that flows on the surface to the edge of the field; drainage is the water that flows through the soil downward or toward the edge of the field underground; evaporative water loss from a field is that part of the water that evaporates into the atmosphere directly from the field's surface; transpiration is the loss of water from the field by its evaporation from the plant itself.
It is the solvent in which
nutrients are carried to, into and throughout the plant.
It provides the
turgidity by which the plant keeps itself in proper position.[5]
In addition, water alters the soil profile by dissolving and re-depositing mineral and organic
solutes and
colloids, often at lower levels, a process called
leaching. In a
loam soil, solids constitute half the volume, gas one-quarter of the volume, and water one-quarter of the volume of which only half will be available to most plants, with a strong variation according to
matric potential.[6]
Water moves in soil under the influence of
gravity,
osmosis and
capillarity.[7] When water enters the soil, it displaces air from interconnected
macropores by
buoyancy, and breaks
aggregates into which air is entrapped, a process called
slaking.[8]
The rate at which a soil can absorb water depends on the soil and its other conditions. As a plant grows, its roots remove water from the largest pores (macropores) first. Soon the larger pores hold only air, and the remaining water is found only in the intermediate- and smallest-sized pores (
micropores). The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away. Consequently, not all soil water is available to plants, with a strong dependence on
texture.[9] When saturated, the soil may lose nutrients as the water drains.[10] Water moves in a draining field under the influence of
pressure where the soil is locally saturated and by
capillarity pull to drier parts of the soil.[11] Most plant water needs are supplied from the suction caused by evaporation from plant leaves (
transpiration) and a lower fraction is supplied by suction created by
osmotic pressure differences between the plant interior and the soil solution.[12][13] Plant roots must seek out water and grow preferentially in moister soil microsites,[14] but some parts of the root system are also able to remoisten dry parts of the soil.[15] Insufficient water will damage the yield of a crop.[16] Most of the available water is used in transpiration to pull nutrients into the plant.[17]
A flooded field will drain the gravitational water under the influence of
gravity until water's
adhesive and
cohesive forces resist further drainage at which point it is said to have reached
field capacity.[20] At that point, plants must apply
suction to draw water from a soil. By convention it is defined at 0.33 bar suction.[20][21]
Available water and unavailable water
The water that plants may draw from the soil is called the
available water.[20][22] Once the available water is used up the remaining moisture is called unavailable water as the plant cannot produce sufficient suction to draw that water in.
Wilting point
The
wilting point is the minimum amount of water plants need to not wilt and approximates the boundary between available and unavailable water. By convention it is defined as 15 bar suction. At this point, seeds will not germinate,[23][20][24] plants begin to wilt and then die unless they are able to recover after water replenishment thanks to species-specific adaptations.[25]
Water is retained in a soil when the
adhesive force of attraction that water's
hydrogen atoms have for the
oxygen of soil particles is stronger than the
cohesive forces that water's hydrogen feels for water oxygen atoms.[26] When a field is flooded, the soil
pore space is completely filled by water. The field will drain under the
force of gravity until it reaches what is called
field capacity, at which point the smallest pores are filled with water and the largest with water and gases.[27] The total amount of water held when field capacity is reached is a function of the
specific surface area of the soil particles.[28] As a result, high clay and high organic soils have higher field capacities.[29] The potential energy of water per unit volume relative to pure water in reference conditions is called
water potential. Total water potential is a sum of
matric potential which results from
capillary action,
osmotic potential for
saline soil, and
gravitational potential when dealing with downward water movement. Water potential in soil usually has negative values, and therefore it is also expressed in
suction, which is defined as the minus of water potential. Suction has a positive value and can be regarded as the total force required to pull or push water out of soil. Water potential or suction is expressed in units of kPa (103pascal),
bar (100 kPa), or
cm H2O (approximately 0.098 kPa).
Common logarithm of suction in cm H2O is called pF.[30] Therefore, pF 3 = 1000 cm = 98 kPa = 0.98 bar.
The forces with which water is held in soils determine its availability to plants. Forces of
adhesion hold water strongly to
mineral and
humus surfaces and less strongly to itself by cohesive forces. A plant's root may penetrate a very small volume of water that is adhering to soil and be initially able to draw in water that is only lightly held by the cohesive forces. 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).[31] At 1500 kPa suction, the soil water amount is called
wilting point. At that suction the plant cannot sustain its water needs as water is still being lost from the plant by
transpiration, the plant's
turgidity is lost, and it wilts, although
stomatal closure may decrease transpiration and thus may retard wilting below the wilting point, in particular under
adaptation or
acclimatization to
drought.[32] The next level, called air-dry, occurs at 100,000 kPa suction (pF = 6). Finally the oven dry condition is reached at 1,000,000 kPa suction (pF = 7). All water below wilting point is called unavailable water.[33]
When the soil moisture content is optimal for plant growth, the water in the large and intermediate size pores can move about in the soil and be easily used by plants.[9] The amount of water remaining in a soil drained to field capacity and the amount that is available are functions of the soil type. Sandy soil will retain very little water, while clay will hold the maximum amount.[29] The available water for the silt loam might be 20% whereas for the sand it might be only 6% by volume, as shown in this table.
Wilting point, field capacity, and available water of various soil textures (unit: % by volume)[34]
Soil Texture
Wilting Point
Field Capacity
Available water
Sand
3.3
9.1
5.8
Sandy loam
9.5
20.7
11.2
Loam
11.7
27.0
15.3
Silt loam
13.3
33.0
19.7
Clay loam
19.7
31.8
12.1
Clay
27.2
39.6
12.4
The above are average values for the soil textures.
Water flow
Water moves through soil due to the force of
gravity,
osmosis and
capillarity. 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. This is caused by water's
adhesion to soil solids, and is called unsaturated flow.[35][36]
Water infiltration and movement in soil are controlled by six factors:
Soil texture
Soil structure. Fine-textured soils with granular structure are most favourable to infiltration of water.
The amount of organic matter. Coarse matter is best and if on the surface helps prevent the destruction of soil structure and the creation of
soil crusts.
Depth of soil to impervious layers such as
hardpans or
bedrock
The amount of water already in the soil
Soil temperature. Warm soils take in water faster while frozen soils such as
permafrost may not be able to absorb depending on the type of freezing.[37]
Water infiltration rates range from 0.25 cm per hour for high clay soils to 2.5 cm per hour for sand and well stabilized and aggregated soil structures.[38] Water flows through the ground unevenly, in the form of so-called gravity fingers, because of the
surface tension between water particles.[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]
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.[48]
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.[49]
Water is pulled by
capillary action due to the
adhesion force of water to the soil solids, producing a
suctiongradient from wet towards drier soil[50] and from
macropores to
micropores.[51] 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.[52] Interestingly, this equation attributed to Richards was originally published by Richardson in 1922.[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. The numerical solution of the Richardson/Richards equation allows calculation of unsaturated water flow and solute transport using software such as
Hydrus,[57] 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.[58][59]
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.[60]
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 to its leaves, according to the
cohesion-tension theory.[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.[64] In addition, the high concentration of salts within plant roots creates an
osmotic pressure gradient that pushes soil water into the roots.[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. It is these processes that cause
guttation and
wilting, respectively.[66][67]
Root extension is vital for plant survival. A study of a single winter
rye plant grown for four months in one cubic foot (0.0283 cubic meters) of loam soil showed that the plant developed 13,800,000 roots, a total of 620 km in length with 237 square meters in
surface area; and 14 billion
root hairs of 10,620 km total length and 400 square meters total area; for a total surface area of 638 square meters. The total surface area of the loam soil was estimated to be 52,000 square meters.[68] In other words, the roots were in contact with only 1.2% of the soil volume. However, root extension should be viewed as a dynamic process, allowing new roots to explore a new volume of soil each day, increasing dramatically the total volume of soil explored over a given growth period, and thus the volume of water taken up by the root system over this period.[69] Root architecture, i.e. the spatial configuration of the root system, plays a prominent role in the adaptation of plants to soil water and nutrient availability, and thus in plant productivity.[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.[71] Insufficient soil moisture, to the point of causing
wilting, will cause permanent damage and
crop yields will suffer. When grain
sorghum was exposed to soil suction as low as 1300 kPa during the seed head emergence through bloom and seed set stages of growth, its production was reduced by 34%.[72]
Consumptive use and water use efficiency
Only a small fraction (0.1% to 1%) of the water used by a plant is held within the plant. The majority is ultimately lost via
transpiration, while
evaporation from the soil surface is also substantial, the transpiration:evaporation ratio (T/ET) varying according to vegetation type and climate, peaking in
tropical rainforests and dipping in
steppes and
deserts.[73] Transpiration plus evaporative soil moisture loss is called
evapotranspiration. Evapotranspiration plus water held in the plant totals to
consumptive use, which is nearly identical to evapotranspiration.[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 an uncovered soil, while more water is immediately available for plant growth.[75]Water use efficiency is measured by the
transpiration ratio, which is the ratio of the total water transpired by a plant to the dry weight of the harvested plant. Transpiration ratios for crops range from 300 to 700. For example, alfalfa may have a transpiration ratio of 500 and as a result 500 kilograms of water will produce one kilogram of dry alfalfa.[76]
^Oyewole, Olusegun Ayodeji; Inselsbacher, Erich; Näsholm, Torgny (2014). "Direct estimation of mass flow and diffusion of nitrogen compounds in solution and soil". New Phytologist. 201 (3): 1056–64.
doi:
10.1111/nph.12553.
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^Ghestem, Murielle; Sidle, Roy C.; Stokes, Alexia (2011). "The influence of plant root systems on subsurface flow: implications for slope stability". BioScience. 61 (11): 869–79.
doi:
10.1525/bio.2011.61.11.6.