Water in soil

Pinova ltd.

All life forms on Earth rely on water, which is continually absorbed and expelled, playing essential physiological and biochemical roles. The water requirements of plants vary, leading to differences in their structure and functioning. Water is a critical factor in plant growth and development, often being the most important limiting factor because plants require substantial amounts of it.

Soil consists of solid, liquid, and gaseous phases—soil particles, water, and air. Within the solid phase, there are variously sized pores that contain water, air, or other gases. These pores serve as natural reservoirs of water and air, with the size and shape of the pores influenced by the soil's type and physical properties.

Pores are categorized into micropores, which hold water, and macropores, which retain air or water for short periods. The number of pores is crucial for agricultural production, but the balance between micropores and macropores is also essential. An ideal ratio is generally considered to range from 3:2 to 1:1, and total porosity in arable soils typically varies between 50 and 65%.

The total porosity, texture, structure, organic matter content, and soil's chemical characteristics determine the soil's water-holding capacity. For example, sandy soils hold less water compared to clay soils because of the higher surface area of the particles and more pores that can bind water. Organic matter plays a significant role in improving the soil's ability to retain water. Timely information about soil moisture is vital for water management in agriculture. To understand the relationship between plants, soil, and water, it's essential to grasp the different types of soil water, water-soil energy dynamics, soil water constants, and soil water movement.

Types of Soil Water
Soil water is divided into several types: chemical, hygroscopic, membraneous, capillary, and gravitational water.

Chemical Water: This water exists as crystallization water (e.g., CaSO4 x 2H2O) or constitutional water (OH-) and is inaccessible to plants, held by forces equivalent to pressures above 5000 bar (500 MPa).

Hygroscopic Water: A molecular film of water adsorbed on soil particles, it is immobile and unavailable to plants, held by forces equivalent to pressures between 30 bar (3 MPa) and 1000 bar (100 MPa).

Membraneous Water: This water forms a membrane around soil particles, becoming thicker as moisture increases, making it less available to plants. It is held by forces equivalent to pressures between 15 bar (1.5 MPa) and 30 bar (3 MPa).

Capillary Water: Held in micropores through surface tension forces, this water moves through the soil from wetter to drier areas. It is divided into immobile (15 bar), less mobile (15–6.25 bar), and easily mobile (6.25–0.33 bar) water. The latter is most accessible to plants.

Gravitational Water: Occurs when all soil pores are saturated, and water drains out of the macropores under gravity, no longer bound to the soil.

Water-Soil Energy Relationship
The movement of soil water is driven by potential energy differences. Water flows from areas with higher potential (more moisture, lower pressure) to areas with lower potential (less moisture, higher pressure), moving toward equilibrium.

Soil Water Constants
Understanding soil water constants is essential for water management. These constants include:

Maximal Hygroscopic Moisture: The maximum amount of water soil can adsorb, equivalent to 30 bar (30 MPa).

Wilting Point: When accessible water is insufficient for plants, equating to a pressure of 15 bar (1.5 MPa).

Lento Capillary Moisture: The transition between less mobile and easily mobile water, critical for optimal soil moisture, at 6.25 bar (0.625 MPa).

Field Capacity: The state where micropores are fully water-filled, and macropores contain air. It ranges from 0.33 bar (clay-loam) to 0.1 bar (sandy soil).

Maximum Capacity: The maximum amount of water soil can hold without retaining it, equivalent to 0 bar (0 MPa).

Soil Water Movement
Water moves in soil through capillary action, infiltration, and filtration. This movement occurs in both saturated and unsaturated soils and can be vertical, lateral, or upward. The rate and direction of movement depend on water’s aggregate state, soil texture, structure, porosity, organic matter, and forces such as capillary tension, gravity, and hydrostatic pressure.

Capillary Movement: Movement of water from wetter to drier areas in micropores.

Infiltration: Water absorption occurs vertically or laterally due to capillary forces, gravity, and osmotic forces.

Filtration: Excess water percolates through macropores from saturated soil to deeper layers due to gravity and hydrostatic pressure.

Determining the Irrigation Moment
Choosing the right time for irrigation is vital to avoid negative impacts on production. Excessive, untimely irrigation increases costs and degrades soil quality, leading to issues like waterlogging and salinization. On the other hand, insufficient irrigation harms crop quality.

Product quality, including fruits, vegetables, and products like wine or olive oil, depends largely on the plant's water status. Proper water management improves plant quality and helps avoid water stress, which is especially critical in regions prone to drought, like the Mediterranean. For soils with low water retention, the risk of water stress is higher.

Several methods can determine irrigation timing, including visual inspection, plant physiological assessment, soil moisture measurement, and mathematical models that combine soil moisture with evapotranspiration calculations.