The water in the soil which is in excess of the hygroscopic and capillary water and which can move freely downwards when the soil is porous and drainage available is called
Free water
Understanding Soil Water Types
Soil water is crucial for plant growth and various soil processes. Water exists in soil in different forms, depending on how tightly it is held by soil particles and pore spaces. The question describes a specific type of soil water: one that is in excess of hygroscopic and capillary water, moves freely downwards in porous soil, and is available for drainage. Let's break down the common types of soil water.
Types of Soil Water Explained
Soil water can be classified into several categories based on its availability to plants and its movement within the soil profile.
- Hygroscopic Water: This water is held very tightly by soil particles through adsorption forces. It forms a thin film around soil particles. Hygroscopic water is generally unavailable to plants because the forces holding it are stronger than the forces plants can exert to absorb it. It can only be removed by drying the soil in an oven at 105°C.
- Capillary Water: This water is held in the pore spaces between soil particles by capillary forces (cohesion and adhesion). It can move in any direction within the soil, often against gravity, filling the smaller pore spaces. Capillary water is the primary form of water available to plants for uptake through their roots.
- Gravitational Water (Free Water): This water is in excess of the hygroscopic and capillary water. It occupies the larger pore spaces (macro-pores) and moves freely downwards through the soil profile under the influence of gravity. If drainage is available, this water will drain away, leading to field capacity. Gravitational water is generally available to plants briefly while it is draining, but prolonged saturation can be detrimental as it displaces air from pore spaces, leading to poor aeration. The question specifically describes this type of water due to its free downward movement and availability for drainage.
- Bound Water: Sometimes, water that is chemically combined within the soil mineral structure is mentioned, but this is not typically considered part of the soil water pool available for plant or movement studies.
Analyzing the Question's Description
The question describes water that:
- Is in excess of hygroscopic and capillary water.
- Can move freely downwards.
- Moves when the soil is porous.
- Is available for drainage.
Let's compare this description to the options provided:
| Soil Water Type | Meets Description? | Reason |
|---|---|---|
| Hygroscopic water | No | Tightly held, not available for drainage, does not move freely downwards. |
| Free water (Gravitational water) | Yes | Is in excess, moves freely downwards under gravity, available for drainage. This matches the description perfectly. |
| Firing water | No | Not a recognized type of soil water. |
| Capillary water | Partially, but not fully | Held in pores, available to plants, but doesn't necessarily move *freely downwards* in large quantities as the *excess* water that drains away. The description of "excess" and "moves freely downwards" strongly points away from capillary water being the primary fit. |
Based on the analysis, the water in excess of hygroscopic and capillary water that moves freely downwards in porous, well-drained soil is known as gravitational water or free water.
Why "Free Water" is the Correct Answer
The term "Free water" or "Gravitational water" precisely defines the water content in soil that is not held by capillary or hygroscopic forces and drains away under gravity. The characteristics listed in the question - being in excess, moving freely downwards, and being available for drainage - are the defining properties of free water (gravitational water). The other options do not fit this description.
Revision Table: Key Soil Water Forms
| Water Form | Holding Force | Location | Availability to Plants | Movement |
|---|---|---|---|---|
| Hygroscopic Water | Adhesion (strong) | Thin film around particles | Unavailable | Immobile |
| Capillary Water | Cohesion & Adhesion (capillary action) | Pore spaces | Available (primary source) | Moves in any direction |
| Free Water (Gravitational Water) | Gravity (weakest holding) | Large pore spaces (macro-pores) | Available (temporarily) | Moves freely downwards |
Additional Information on Soil Water and Drainage
The amount of each type of water present in the soil depends on factors like soil texture (proportion of sand, silt, clay), soil structure (arrangement of particles), and the presence of a water table or drainage layers.
- Field Capacity: This is the amount of water retained in the soil after excess gravitational water has drained away, typically about 2-3 days after irrigation or rainfall. At field capacity, the soil contains mostly capillary water and hygroscopic water.
- Permanent Wilting Point (PWP): This is the moisture content at which plants can no longer extract enough water from the soil to survive. At PWP, only hygroscopic water and tightly held capillary water remain.
- Available Water: This is the water content between field capacity and the permanent wilting point. It is primarily the capillary water.
Understanding the different forms of soil water is essential for managing irrigation, drainage, and overall soil health for agriculture and environmental purposes. Excess free water can lead to waterlogging, which reduces oxygen availability for roots and beneficial soil organisms.
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