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Available water capacity in soils and relationship to water stress in plant

Abstract

Available water capacity is an indicator that explains the soil's ability to retain water and make it available for plant use. However, the available water in soil determines the extent of water stress in plant

Available water capacity
Available water capacity also known as available water content (AWC), available soil moisture (AWSC) or total available water (TAW) is the maximum amount of plant available water a soil can provide. It is the water held in soil between its field capacity and permanent wilting point ("Insert diagram of available water capacity"https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/agricultural-land-and-environment/soil-nutrients/600-series/619000-1_soil_water_storage_capacity.pdf). It is an indicator of a soil’s ability to retain water and make it sufficiently available for plant use. The three terms associated with the water budget in soils are field capacity (FC), Permanent wilting point (PWP), and available water (AW)

Field capacity (Ɵfc): is the water remaining in a soil after it has been thoroughly saturated and allowed to drain freely usually for one to two days.

Permanent wilting point (Ɵpwp): is the moisture content of a soil at which plants wilt and fail to recover when supplied with sufficient moisture. Water capacity is usually expressed as a volume fraction or percentage, or as a depth (in or cm).

The concept, put forward by Frank Veihmeyer and Arthur Hendrickson, assumed that the water readily available to plants is the difference between the soil water content at field capacity (θfc) and permanent wilting point (θpwp):

θaw ≡ θfc − θpwp

Maximum Soil Water Deficit (MSWD): is the amount of water stored in the plant’s root zone that is readily available to the plant.

Saturation: occurs when all the voids in the soil are filled with water. Although there is plenty of water available to the crop at saturation, water uptake is seriously curtailed by the lack of oxygen in the soil at soil water contents greater than field capacity.

Importance of available water capacity in soils
Soil is a major storage reservoir for water. Water availability is an important indicator because plant growth and soil biological activity depend on water for hydration and delivery of nutrients in solution. In areas where rain falls daily and supplies the soil with as much or more water than is removed by plants, available water capacity may be of little importance. However, in areas where plants remove more water than is supplied by precipitation, the amount of water held by the soil may be critical. Water held in the soil may be necessary to sustain plants between rainfall or irrigation events. By holding water for future use, soil buffers the plant – root environment against periods of water deficit.

Available water capacity is used to develop water budgets, predict droughtiness, design and operate irrigation systems, design drainage systems, protect water resources, and predict yields.

Water stress in plants
Plants experience water stress either when the water supply to their roots becomes limiting or when the transpiration rate becomes intense. Plant growth is controlled directly by plant water stress and only indirectly by soil water stress. Plant water stress depends on the relative rates of water absorption and water loss rather than on soil water supply alone, hence it is not safe to assume that a given degree of soil water stress always will be accompanied by an equivalent degree of plant water stress. Water stress in plants is primarily caused by the water deficit, i.e. drought or high soil salinity. Soils with high salt concentration tend to have reduced available water capacity because more water is retained at the permanent wilting point. Lack of available water reduces root and plant growth, and it can lead to plant death if enough moisture is not provided before a plant permanently wilts.

== Effects of water stress in plants == Water is a vital component to the existence of plant life. Not only do plants use it to keep their cells from drying out, but they need water to move nutrients and raw materials throughout their systems to areas where photosynthesis and seed production take place. When water stress occurs, whether caused by drought or root impairment, many invisible processes are affected.

Wilting
One of the first obvious effect of water stress is wilting because turgor pressure, which inflates plant cells and keeps them erect, is lost. Without this force, the cells inside plant leaves begin collapsing, giving them a limp appearance. As wilting increases, plant cells fully deflate, causing their deaths. Partially wilted plants that are still green may recover if watering is quickly initiated. The addition of mulch helps to keep soil moisture even.

Reduced Photosynthesis
Photosynthesis is the process through which plants create their own food. The amount of water, sunlight and carbon dioxide available to the plant directly influences the amount of food a plant can produce. When water levels are low due to water stress, photosynthesis can slow or even stop, causing internal food supplies vital to other processes to diminish or disappear -- yellowing may also occur if photosynthesis stops completely.

Reduced Respiration
Respiration is the process through which plants break down their food supply for energy to power system processes. When plants are actively growing, they respirate heavily, using up food stores quickly. With low water levels reducing the plant's ability to photosynthesize, the plant's system processes slow down, causing reduced or delayed growth and discoloration of leaves, as well as flower or fruit drop, since the plant can't support this extra baggage.

Reduced Transpiration
Hearts pump blood through animal bodies, carrying nutrients and removing waste - similarly, plants use osmotic pressure to create circulation in their systems. As water moves through the system, vital minerals and nutrients are delivered to different areas of the plant. Water taken up by a plant's roots is slowly drawn up to openings in its leaves called stomas. The stomas release waste products such as oxygen into the environment and bring in carbon dioxide. This release also helps cool plant tissues. In addition, transpiration maintains turgor in plants, keeping cells evenly filled with water. When transpiration is stopped or slowed, the plant begins to die from lack of nutrients, usually from the top down.

Evolutionary Developments
Plants living under constant cycles of drought stress have evolved specialized metabolisms known as C4 and CAM, or crassulacean acid metabolism. Plants with a C4 metabolism photosynthesize faster deeper inside the plant's tissues, protecting water from loss through the plant's stomas. CAM allows plants such as cacti and agaves, as well as some orchid and bromeliads, to close their stomas completely during the day, opening them only briefly at night to exchange gases with the environment. CAM, along with a thick, waxy coating, keeps evaporation to a minimum in these plants to help prevent water stress.

== Irrigation to prevent water stress in plants == To prevent plant water stress, the manageable allowable depletion (MAD) needs to be calculated using an allowable depletion factor ("Insert diagram to explain the amount of allowable depletion and 50% depletion point" http://www.intermountainfruit.org/orchard-irrigation/swc). This factor varies but is usually around 50% (" Insert table 3 to show availability coefficient" https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/agricultural-land-and-environment/soil-nutrients/600-series/619000-1_soil_water_storage_capacity.pdf). The soil water storage and the maximum soil water deficit needs to be determined, and this can be calculated when the crop rooting depth, available soil moisture (available water storage capacity) and the availability coefficient of the water to the crop is known.

Allowable Depletion (readily available): is the point where plants begin to experience stress. For most fruit trees, the amount of allowable depletion, or the readily available water represents about 50% of the total available water in the soil.

According to water conservation fact sheet of British Columbia, Canada. ("Insert table 1 to show the effective rooting depth of some mature crops for irrigation system design, table 2 shows that the available water storage capacity or soil moisture varies among different soil texture class and table 3 shows the available coefficient for some crops"https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/agricultural-land-and-environment/soil-nutrients/600-series/619000-1_soil_water_storage_capacity.pdf ).

How to determine the soil water storage and the maximum soil water deficit
Step 1 Determine the crop rooting depth, RD (m)

Step 2 Determine the available water storage capacity of the soil, AWSC (mm/m)

Step 3 Calculate the total soil water storage, SWS (mm)

SWS (mm) = RD (m) x AWSC (mm/m)                   (Equation 1)

Step 4 Determine the availability coefficient of the water to the crop, AC (%)

Step 5 Calculate the maximum soil water Deficit, MSWD (mm)

MSWD = SWS (mm) x AC (%)                                (Equation 2)

For Example:

For a mature corn crop in a loamy sand soil.

Rooting depth (Table 1)                                                          = 0.90 m

Soil Water Storage Capacity (Table 2)                                   = 100 mm/m

Availability coefficient (Table 3)                                           = 50%

SWS = 0.90 m x 100 mm/m = 90 mm

MAD = 90 mm x 50% = 45 mm

For the same crop in the early summer the rooting depth may be only 0.3 m, therefore:

SWS = 0.30 m x 100 mm/m = 30 mm

MAD = 30 mm x 50% = 15 mm

When irrigating the mature crop more water is needed to fill the root zone. When the crop is immature the irrigation amount required will be less.

Effect of organic matter on soil available water capacity
It is widely accepted that the available water capacity in soil can be improved by increasing organic matter content. However, the increase in amount of water that is available to plants with an increase in organic matter is still uncertain. According to Minasny, McBratney (2017) it was reported that the increase in organic carbon in soil had a small effect on soil water content. A 1% mass increase in soil organic carbon (or 10 g C kg−1 soil mineral), on average, increases water content at saturation, field capacity, wilting point and available water capacity by: 2.95, 1.61, 0.17 and 1.16-mm H2O 100 mm soil−1, respectively. The increase is larger in sandy soils, followed by loams and is least in clays. Overall the increase in available water capacity was very small.