Soil Water Types and Their Measurement
**Understanding Soil Water**
Soil water is all the water found in the soil. This includes water in soil pores and water stuck to soil particles. Water is the main source of hydration for plants. It can be found in three forms: vapor, liquid, and solid.
The main sources of soil water are rain and irrigation. Groundwater movement and atmospheric condensation can also add to it.
Water in the soil interacts with different forces. These include gravity, capillary action, molecular forces, and how surfaces stick to soil. These interactions create various types of water. Each type has its own unique properties.
– **Solid water**: Ice crystals form when water in the soil freezes.
– **Vapor water**: Water vapor is present in the air found in soil.
– **Bound water**: Includes water that takes in moisture and water found in membranes.
– **Free water**: Includes capillary water, gravity water, and groundwater
– **Gravity water**: Water that flows down because of gravity.
**Types of Soil Water Explained**
1. **Hygroscopic Water**
Hygroscopic water forms a thin layer around soil particles. It attracts moisture directly from the air. Its amount depends on the humidity in the air. It is almost gone in dry air and is highest in very humid conditions.
Hygroscopic water sticks closely to soil particles. This makes it less mobile and harder for plants to absorb. The water’s content is influenced by soil texture. It is also affected by the presence of solutes and the amount of organic matter.
2. **Film Water**
Once soil particles have taken in all the hygroscopic water they can, they can attract more layers of liquid water. These layers connect to form a continuous film surrounding the hygroscopic water.
This “film water” is affected less by adsorption forces than hygroscopic water. This allows it to move slowly in the soil.
The amount of film water depends on soil features like texture and humus content. Soils that are rich in clay and humus usually hold more film water. Film water is available to plant roots, but it helps with hydration very little because it does not move much.
3. **Capillary Water**
Capillary water fills the small spaces in the soil once it reaches its maximum holding capacity. Water in these small spaces moves easily and can be absorbed well by plants due to capillary forces.
Capillary water is important for plants because it provides hydration. It also carries dissolved chemicals found in the soil. It comes in two forms:
– **Ascending capillary water**: Groundwater moves up through capillary forces when the water table is near the surface. This helps keep the soil moist and helps plants grow.
– **Suspended capillary water**: When groundwater is deep, this type results from rainfall or irrigation. Some of the water drains down because of gravity. The rest stays up due to capillary forces.
4. **Gravitational Water**
Excess water that exceeds what capillaries can hold moves through larger soil pores due to gravity. Gravitational water keeps the traits of liquid water. Plants can absorb it directly.
However, its movement limits how much stays in the root zone. As a result, much of it drains away before crops can use it well.
**Gravitational Water and Its Effects on Plants**
Gravitational water is the water that moves through soil due to gravity. It is important for plants. This kind of water helps plants take in nutrients.
When it rains, water seeps into the ground. Some of this water stays in the soil, while some drains away.
Plants use gravitational water to grow. It fills the spaces in the soil. This water is available for roots to take in.
If there is too much water, it can harm plants. Roots may drown if they sit in water for too long.
In dry times, plants use the water they have stored. They need enough moisture to survive. Gravitational water plays a key role in plant health.
It helps them thrive in different conditions. Knowing about this water can help us take better care of plants.
Soil water is very important for plant growth. This starts even before the seed begins to grow. For example, seeds from poplar and willow need to touch moist soil right after they mature. This helps them grow and take root.
Moisture effects
Moisture helps to soften the seed coat during this process. It changes the protoplasm from a jelly-like form to a gel-like state. This change allows germination to happen.
As plant roots grow in the soil, the water content of the soil greatly affects their growth. In dry areas like grasslands and deserts, plants grow deep roots. This helps them reach water that is deeper underground.
Conversely, in moist soils, roots are typically confined to the upper layers, often just a few inches below the surface.
Plants need water within certain limits to grow well. There is a minimum, maximum, and ideal amount of water. If water levels fall below the minimum, plants wilt and stop growing completely.
Too much water can take away oxygen from plant roots. This can cause choking and lead to root rot.
Plants can keep a healthy water balance for growth when they are in the right range. Different plants need different amounts of water to grow best.
For example, horseshoe grass grows well in sandy dunes with a soil water content of about 60%. Fenugreek grows well in normal soil conditions at about 85% moisture. Water spinach, on the other hand, thrives in wetlands. It can tolerate moisture levels above 110%.
**Understanding Soil Water and Soil Moisture**
“While they are related, ‘soil water’ and ‘soil moisture’ are different ideas. Soil moisture means the amount or percentage of water in the soil.”
Soil moisture is the amount of water in the soil. It is a ratio of water volume to the total soil volume. This ratio is often shown as a percentage, like 25% soil moisture.
We use devices like soil moisture sensors and tensiometers to measure it. Soil moisture changes over time and depends on climate and soil type.
Soil water includes all types of water found in the soil. This means liquid water, solid water on soil particles, and water vapor in pore spaces. We usually measure it by volume or weight, like cubic meters or kilograms, using methods such as weighing.
1. **Gravimetric Technique**
The drying technique is a common way to measure soil moisture. First, soil samples are taken from the field and weighed. Next, these samples are put in an oven at 105°C to dry completely.
After drying, the samples are weighed again. The moisture content is found by comparing the first weight to the weight after drying. The difference shows the water content.
2. **Soil Moisture Sensors**
Using soil moisture sensors is now the most common way to check moisture levels. These sensors send out electromagnetic waves at a certain frequency through their probe. The waves travel through the soil and come back to the sensor.
The sensor notices changes in output voltage, which relate to changes in the soil’s dielectric constant. Since moisture levels affect soil dielectric properties, the sensor figures out the water content based on this link.
3. **Resistance Method**
Some materials, like gypsum, nylon, and fiberglass, have electrical resistance that changes with moisture. Blocks made from these materials have electrodes and are put in wet soil.
They soak up water until they reach balance. Once stable, the electrical resistance of these blocks shows the soil’s moisture level. This method gives accurate measurements for moisture levels between 1 and 15 atmospheres.
4. **Neutron Scattering Method**
The neutron scattering technique measures soil water content using a neutron meter. This device has a fast neutron source and a slow neutron detector. In the soil, fast neutrons from the source hit hydrogen atom nuclei.
They lose energy and turn into slow neutrons. The number of slow neutrons detected directly relates to the soil moisture level.
5. **Tensiometer Method**
The tensiometer measures how much water is pulled from the soil. It has a porous clay head that goes into the soil. Water inside the tube moves through the clay wall and mixes with soil water.
This continues until both water levels are equal. At this point, you can read the suction value from the tensiometer. To find soil moisture content, you use the suction value and soil matrix potential.
