Soil moisture
Soil moisture is the
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.
Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth.[4] Water is essential to plants for four reasons:
- It constitutes 80–95% of the plant's protoplasm.
- It is essential for photosynthesis.
- It is the solvent in which nutrientsare 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
Water moves in soil under the influence of
Soil water is also important for climate modeling and numerical weather prediction. The Global Climate Observing System specified soil water as one of the 50 Essential Climate Variables (ECVs).[18] Soil water can be measured in situ with soil moisture sensors or can be estimated at various scales and resolution: from local or wifi measures via sensors in the soil to satellite imagery that combines data capture and hydrological models. Each method exhibits pros and cons, and hence, the integration of different techniques may decrease the drawbacks of a single given method.[19]
Moisture level concepts
- Field capacity
- 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 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 retention
Water is retained in a soil when the
The forces with which water is held in soils determine its availability to plants. Forces of
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.
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
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]
Water applied to a soil is pushed by
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.[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
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
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
The total water used in an agricultural field includes
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