Soil as a multifunctional system consists of a solid, liquid and gaseous phase. The solid phase of soil is about 50-60% of its volume (mineral phase and organic matter) and the remainder is air and water (Fig. 1). The three-phase soil system can change as a result of soil-forming processes and agrotechnology. Proper tillage and organic matter application can significantly increase the water holding capacity of the soil.
Water and air fill the soil pores, so there is an antagonistic system between them. When most of the pores are flooded by water, plants suffer from lack of oxygen in the root zone. When most of the pores fill with air, soil drought occurs. It is still important to remember that not all the water in the soil is available to plants.
1 - mineral phase, 2 - organic matter, 3 - soil air, 4 - soil water available to plants,
5 - soil water not available to plants.
Fig. 1. Three-phase soil system.
Water in the soil is important not only as an essential factor for plant growth and organic life, but also affects the movement of mineral salts in the soil. The rate and nature of decomposition of organic matter also depends on the amount of water in the soil.
Water, found in the soil, can be divided into four types:
- Hygroscopic water (not available to plants); it coats soil particles (Fig.2).
- Membranous water (inaccessible to plants): it adheres to hygroscopic water, and also fills the angular spaces between soil particles (Fig.2).
- Capillary (plant-accessible) water; fills capillary spaces, is not subject to gravity.
- Gravitational water (available to plants in a short period of time); it fills soil cavities larger than capillary, under the influence of gravitational forces it flows deep into the soil profile (it has no major agrotechnical significance)
Fig.2. hygroscopic and membrane water
Types of water capacity of soils (Fig.3)
Maximum water capacity (MWC)
This is the maximum amount of water the soil can hold. It occurs after very heavy rainfall or intensive irrigation. In this state, the soil contains very little air, which can be held only in the largest gaps.
Field water capacity (FWC)
Once the gravitational water in the soil has drained away, there is a state of equilibrium between the force of gravity and the force with which the water is attracted to the soil particles. The soil then contains a relatively large amount of water and sufficient air.
Permanent Wilting Point of (PWP)
The amount of water in the soil at which plants permanently wilt. At this level of soil moisture, plants will wither and will no longer be helped by rain or irrigation.
Fig.3 Types of water capacity of soils.
Agronomic categories
Agronomic categories of soils were established based on their granulometric composition (grain size). Soils belonging to different categories have different water capacity and thus different susceptibility to drought.
| Soil quality | |
|---|---|
| I - Very light - very susceptible |
loose sand - pl loose silty sand - plp weakly loamy sand - ps weakly loamy silty sand - psp |
| II - Light - susceptible |
light clayey sand - pgl light dusty clayey sand - pglp strong clayey sand - pgm strong dusty clayey sand - pgm |
| III - Medium - moderately susceptible |
light clay - loam light dusty clay - glp clayey dust - płg ordinary dust - płz sandy dust - płp |
| IV - Heavy - not very susceptible |
medium clay - gs medium silty clay - gsp heavy clay - gc heavy silty clay - gcp silty clay - płi silt - silt loam - ip |
Water availability for plants
The availability of water to plants depends on the magnitude of the forces with which it is bound in the soil (gravitational, capillary, osmotic, electrical and external pressure forces). The strength of water binding in the soil is the resultant of their interaction, but mainly depends on capillary forces. Soils containing the same amount of water, but having different granulometric composition, are characterized by different water binding strength. Heavy soils (fine-grained soils) contain a large number of fine and medium pores, and show greater water binding strength than coarse-grained formations (light soils), which have larger pores. The effect of this condition is that heavy soils have a higher water holding capacity with also more water unavailable to plants. The force with which plants can draw water from the soil is referred to as suction force (capillary potential). We measure the suction force in units of vacuum - cm of water column, atmospheres, mm of mercury column, hPa, MPa. The magnitude of this force depends on the diameters of the soil cavities, filled by water. The smaller the caverns, the greater the force required to draw water from the soil. This relationship is described by the water availability curve (pF). Due to the very wide range of pressures from 0 up to 10000 at. the value of the suction force is expressed by the decimal logarithm of the water column (Fig. 4).
Available water is water that ranges from field water capacity to the level of strong plant growth inhibition. It is retained in soil pores with diameters of 30 -1.5 μm, which corresponds to the soil suction force in the range of 0.1 - 1.6 at, pF 2.0 - 3.2. The total water available to the plant is divided into water:
- very easily available (tied with a force of 0.2 to 0.7 at. pF 2-2.85)
- readily available water (bond strength from > 0.7 to 1.6 at. pF 2.85-3.2)
- difficult available (bond strength from >1.6 to 5.0 at. pF 3.2-3.7)
- very difficult available (bond strength from >5.0 to 15 at. pF 3.7-4.2).
A) weakly loamy sand, B) light clay, C) silt
Fig.4. Soil water binding curve - pF curve (illustration)
At suction forces above 15 atmospheres, plants can no longer take up water from the soil. If our goal is to provide water comfort to plants, the water content of the soil should be kept at the level of very readily available water. When the very easily and readily available water is exhausted, strong inhibition of plant growth begins (fig. 5).
Fig.5. Water retention of soils.