Field Capacity

Field capacity simply refers to the water-holding capacity of agricultural soil. As we know that plants are very dependent on both water and soil (soil is the major source of food, minerals, and water for plants) because the roots of the plants have direct contact with soil. As soil is formed from rocks by the rock cycle (it must be full of nutrients and minerals). Plants and microbes digest these minerals from the soil and enter the food chain.

Plants have xylem (for the transportation of water) and phloem (for the transportation of food). Water transportation is always uni-directional (i.e. from roots to shoots). This transportation is made possible by creating pressure within plants which we called the capillary action of plants. If this pressure breaks then the plant is no more able to take water from the soil and ultimately died. So plants need to maintain that pressure. By knowing the field capacity of the soil, we can calculate how much time requires to irrigate a particular area of soil so that the supply of water continues to plant from the soil.

Field capacity is used to measure the soil water holding capacity because it tells about the irrigation period to the soil for better growth of plants in the fields. When we watered a particular area of an agricultural field, the water started to bind with soil particles and retain in the soil due to cohesion (bonding between water molecules) and adhesion (bonding between soil and water) forces. This condition is called saturation capacity because a very healthy amount of water is present in the soil.

The net movement of water is downward due to gravitational force. Most of the water penetrates very deep in the soil because of this gravitational force while some of the water succeeds to retains with the molecules of soil.

Different types of soil have different abilities to retain water inside it and the time or duration to keep water inside it also varies with different types of soil and it is the field capacity of the soil (ability to hold water). The water that binds with soil particles is available for plants, so it is also called capillary water because plant performs their capillary action from this available water.

Plants take this water to fulfill their needs. After some time or days, water in the soil is not enough for the plant’s needs, and a stage is reached where plants faced a serious deficiency of water. This is called the permanent wilting point because of wilting of leaves starts when water is not available to plants.

The low concentration of water that plants are not able to take from the soil is called hygroscopic water. Before this hygroscopic point, it is very important to give water to the soil to maintain its health.

To save the plant from wilting conditions, farmers and researchers usually take 75% of the capillary water. This is called optimum moisture content (OMC).

Optimum moisture content = 75% (field capacity - permanent wilting point)

OMC = 75% (capilary water)

Many researchers take optimum moisture content at 80% but most of them agreed at 75%. The main purpose of finding the optimum moisture content is to avoid wilting conditions for plants. When the soil is reached its optimum moisture content level then we irrigate the soil again and in this way, it can be said as an irrigation point.

The water level above the irrigation point (or 75% of the capillary soil water) is called readily available moisture (RAM). The total time taken by water to move from the field capacity point to the optimum moisture content point is called consumptive use by the soil.

The concept of consumptive soil can be understood by taking a numerical example. Suppose the average consumptive use of water is 4cm from the soil while the dept of water we applied during irrigation time is 24cm. Then the frequency of water that needs to be applied can be calculated as:

Frequency of water = depth of water/consumptive use

Frequency of water = 24/4

= 6

It means that we have to irrigate the soil after 6 days. Field capacity is more than the ability to hold water by the soil. Many factors can affect the ability to hold water by the soil. So the measurement of the field capacity can vary with the type of soil, weather, temperature, environment, etc.

Numerical Equations Related to Field Capacity.

Many researchers were interested in the drainage flux of the soil. Drainage flux is a time-dependent quantity and an important component of soil-water balance. It determines the movement of water within the soil. Water-logged soils or (can also be said as) saturated soils are examples of negligible drainage flux. This can affect the plants very bad because water occupies much space in the soil that the roots of the plants faced difficulties to take oxygen from the soil.

According to Hillel (scientist name) for the estimating field capacity of the water content, the drainage flux from the soil should reach a value of 0.05 cm/day. It means that the water level in the soil should decrease up to 0.5cm in a day. Similarly, the field capacity of the water content is a point when the drainage flux reaches between 0.001 cm/day to 0.1 cm/day. The drainage flux value may vary with the different types of soil. This can be calculated as:

(θfc - θr) / (θs - θr) = (qfc/Ks)1/β

θfc is the field capacity
θs is the Saturated water content
θr is the Residual water content
qfc is the Negligible drainage flux from the soil at field capacity
Ks is the Saturated hydraulic conductivity
β = (2 + 3 λ)/ λ, where λ is the pore size distribution index of the Brooks and Corey model.

Saturated field capacity is the maximum amount of water that soil can retain. To attain the saturation point of any soil, the researcher normally gives water to the soil for about 1-2 days freely. The saturation field capacity (Sfc) of any soil can be calculated as:

Sfc = (θfc - θr) / (θs - θr)

Negligible drainage flux or also known as seepage flux refers to the amount of water when the value of the hydraulic gradient is very low and the availability of oxygen at the roots zone is negligible. The low value of hydraulic flux shows that the movement of water through soil is very slow that’s why it is called negligible drainage flux because water occupies the air spaces in soil and so no room for water to move. The negligible drainage flux of any soil can also be calculated as: