Water properties

Water Properties

• Water's molecular weight is 18 g/mol.
• Water can dissolve substances due to its polarity and ability to form hydrogen bonds.
• Pure water has a neutral pH of 7.
• Autoionization happens when two water molecules (H2O) form hydronium and hydroxyl/oxidryl ions.
• Water reacts with acidic and basic oxides, metals, non-metals; it also bonds with salts, forming hydrates.
• Water's electrical conductivity refers to its ability to conduct electric current via dissolved ions.
• Water hardness refers to the total dissolved mineral salts, mainly Ca++ and Mg++, measured in equivalent amounts of CaCO3, CO32, HCO3, and SO42-.
• Moderate hardness levels between 100 to 150 mg/L, are ideal for plant growth.
• Cohesion of water is the attraction between water molecules.
• Adhesion of water is the attraction of water molecules to other substances at the water-solid or water-air interface.
• Surface tension is a liquid's property that allows it to resist an external force, because of the cohesive nature of water molecules.

Types of Water in Soil

• The three physical types of water in the soil are hygroscopic, capillary, and gravitational water.
• Gravitational water occupies macropores larger than 10 microns (unsaturated zone). It infiltrates, driven by gravity, into deeper layers.
• According to speed of circulation, types of gravitational water are slow flow and rapid flow gravitational water.
• Slow flow gravitational water circulates through pores between 8 and 30 microns, taking 10 to 30 days to cross the soil, available for plant use during this period.
• Rapid flow gravitational water is useless to plants because it saturates the soil pores, creating an asphyxiating environment where roots can't absorb it.
• The maximum soil capacity is when all soil pores are saturated with water, with no gas phase, and the total soil porosity equals the total water volume.
• Soil retention capacity, equals the maximum amount of water a soil can hold, represents water storage in the soil.
• The laboratory equivalent measurement of retention capacity corresponds to pF=3.
• Field capacity is a practical concept that reflects the water amount a soil retains after losing rapid flow gravitational water, about two days after rainfall.
• Field capacity generally corresponds to a suction force of 1/3 atmosphere or pF=2.5, correlating to pores <30 microns.
• The wilting point occurs when the soil dries to a level where the water is retained with a suction force greater than what plant roots can absorb.
• The pressure at the wilting point is 15 atmospheres or pF=4.2.
• Hygroscopic water is air moisture absorbed directly, arranged on soil particles in a 15-20 molecule thick layer.
• Hygroscopic water is not available to plants, as root suction power can't extract these water particles from the soil.
• The amount of hygroscopic water in soil is reliant on the soil's texture, mineral composition, and organic matter content.
• Factors favoring greater hygroscopic water capture are greater particle dispersion, larger contact surface, high organic matter content, and clay.
• Capillary water occupies the micropores and is held in the soil via forces from the water's surface tension.
• Capillary water matters to plants because they can use it, making it part of the soil's water reserve.
• Non-absorbable capillary water enters the smallest capillary tubes (<0.2 microns), it is very tightly held and not absorbed by plants
• 31-15 atmospheres, equal to a pF of 4.5 to 4.2, corresponds to non-absorbable capillary water.
• Absorbable capillary water resides in 0.2-8 micron capillary tubes, is accessible to plants, and forms water reserves during dry spells.
• The retention force of absorbable capillary water, varies between 15 to 1 atmosphere, extracted at a pF of 4.2 to 3.
• Capillary tubes help trees survive in dry, hot weather. Trees have roots in these capillary conduits, absorbing capillary water with their lateral roots when hot and dry.
• The forces of water cohesion and adhesion between water and soil particles mediates capillarity in soils.

Evapotranspiration and Soil-Water-Plant-Air Relationship

• Evapotranspiration combines water loss through soil surface evaporation and plant transpiration.
• Evapotranspiration is impacted by climate variables, crop traits, management techniques, and environmental conditions.
• Potential evapotranspiration (PET) is independent of crop type/condition. It is the volume of water that could be evaporated or transpired with sufficient water available.
• Actual evapotranspiration (AET) is the amount of water that is really evaporated and transpired from a crop which is determined by the conditions of water availability in the soil.
• Evapotranspiration links to the hydrologic cycle by returning a significant portion of precipitation back to the atmosphere.

Transpiration

• Plant transpiration is water absorbed by roots moves through the plant and evaporates as vapor, mainly through leaf stomata.
• Stomata regulate gas exchange and water loss through transpiration by opening and closing.
• Stomata opening and closing is controlled by light, CO2 concentration, temperature, relative humidity, leaf water potential, and plant hormones such as abscisic acid.
• Water deficit prompts stomata to partially or fully close, decreasing transpiration and affecting photosynthesis.
• Transpiration stream involves continuous water movement from the soil, through the roots, stem, and leaves, to the atmosphere, mostly powered by transpiration.

Photosynthesis

• Photosynthesis is a process where plants, algae, and some bacteria convert light energy into chemical energy, employing CO2 and water to form glucose and release oxygen.
• The equation of photosynthesis is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
• The water input in photosynthesis works as essential reactant that delivers electrons to cut CO2, it is photolyzed in the light phase releasing oxygen, and sustains the necessary cell turgor for chloroplast function.
• Water stress restricts stomata closure, decreasing CO2 entry in plants and reducing photosynthetic rates. It may directly impair biochemical processes and harm the photosynthetic unit as a whole.
• Photosynthesis and transpiration are connected in that stomata must open for CO2 to enter for photosynthesis. This leads to water loss through transpiration, which creates a trade-off between carbon gain and water loss.

Soil-Water-Plant-Air Relationship

• The soil-plant-atmosphere continuous system describes water movement as a continuous system from the soil, through plants, and into the atmosphere, influenced by water potential gradients.
• Water potential measures the free energy of water, expressed in pressure units. Water moves spontaneously from areas with high (less negative) water potential to areas with low (more negative) water potential.
• Soil water potential includes matric potential (related to adhesion and capillarity forces), osmotic potential (due to dissolved solutes), gravitational potential, and pressure potential.
• Plant roots absorb water once their water potential is more negative compared to the surrounding soil. Water may travel across roots apoplastically or symplastically.
• The rhizosphere is the soil zone directly next to the roots where interactions among the roots, microorganisms, as well as soil happen. It proves critical for water and nutrient uptake.
• Root hairs, epidermal cell extensions on roots, greatly enlarge the absorption area, aiding water and nutrient harvesting in the soil.
• Water moves primarily via the xylem, due to a water potential gradient powered by transpiration. This flow follows the cohesion-tension theory.
• The cohesion-tension theory explains how the tension from transpiration pulls continuous stream of water up the xylem, while cohesion forces between water molecules and adhesion forces between water and xylem walls maintain this continuous stream.
• Cavitation is when the water stream from xylems break due to very high pressure, this shapes air bubble interrupts the flow of water.
• Available water for plants represents water accessible to roots, commonly defined as water amount that is held between field capacity and permanent wilting point.
• Sandy soils hold less water, but the majority of water is usable. Clay soils hold a higher amount of total water.

Water Use

• Water use efficiency is the ratio of biomass production or crop yield to water consumed through evapotranspiration, and determines how well a plant utilizes the water in order to generate dry matter.
• C3, C4, and CAM plants vary in water-use efficiency. C4 and CAM plants use water better than C3 plants since they can fix CO2 with less water loss.
• Vapor Pressure Deficit (VPD) is the difference between the amount of moisture in the air and the amount of moisture the air can hold when saturated. High VPD increases evaporative demand, and may lead to water stress in a plant.
• Osmotic adjustments refer to the mechanisms where plants build up solutes in their cells, lowers the osmotic tension, this permits maintaining of cell turgidity including water absorption when available water in the soil is limited.
• Mycorrhizae, symbiotic relationships between fungi and plant roots, significantly increase the absorption area, enhancing water and nutrient uptake, especially during water stress.
• Soil hydraulic conductivity characterizes the ability to carry water movement throughout the soil. Which then relies with the consistence of soil texture, structures, and organic matter with moisture contents within the soil.
• Soil compaction reduces spaces for air, lessens water seepage and penetration , boosts runoff, restricts proliferation of root and causes stagnation or anaerobic situation.
• Organic matter enhances soil by increasing storage of water, encourages infiltration, shrinking the density, builds up the power of cation exchange, acts like reservoir for nutrients storage.
• The soil atmosphere (soil air) affects plant roots. Roots need oxygen to respire, a good soil ventilation allows an adequate gaseous exchange. The deficit due to water saturation or compaction could initiate radicular asphyxia, reducing water absorption and help diseases to develop.