Water Holding Capacity of the Soil

Introduction

The water-holding capacity (WHC) of a soil is measured as the total quantity of water that can be absorbed by the soil per gram. This capacity is also known as “the water-binding capacity” or “the water absorption capacity”. This characteristic is closely connected to the affinity that soil molecules have for water and other solutes in the environment.

The capacity of a soil to retain water is an important hydraulic feature that plays a significant role in determining how well a given soil will operate within an ecosystem. This property also has a significant impact on how well soil is managed. The water-holding capacity of the subsoil layer is the most important aspect for plant development, whereas the moisture content of the soil represents the most important component of a buffer system against flooding.

Effect and Importance of WHC

The effects of soil retention depend on

• The amount of organic carbon in the soil.

• The number of different types of materials present in it.

When the carbon content of the soil is raised from a low level, an increase in water retention becomes apparent in coarse-textured soils, whereas a reduction in water retention can be observed in fine-textured soils.

When the amount of carbon is high, the amount of water that can be retained will also be high. Soil organic matter can hold up to 20 times its own weight in water.

When the temperature changes, things like evapotranspiration, soil moisture, and infiltration may also change. This controls the amount of groundwater recharge that occurs by adjusting the ratio of surface run-off to infiltration.

Evapotranspiration

It is the combined processes including evaporation and transpiration to transport water from Earth's surface to the atmosphere. It includes movement of water from the soil, through roots and bodies of vegetation, on leaves and then into the air.

Infiltration

Infiltration is the process through which surface water penetrates the soil.

Surface Run-Off

Surface runoff is the flow of water that occurs on the ground surface when surplus precipitation, stormwater, meltwater, or other sources can no longer enter the soil quickly enough.

Climate change and related projections for the future show a decrease in midsummer soil moisture and an increase in severe rainfall and off-site soil erosion. Maintaining or even improving the ability to hold water is a good way to reduce the effects of both heavy rain and frequent, severe droughts.

Calculations and Units for the Measurement of WHC

Mw = Mt – Ms,

where, Mw is mass of water in grams.

Mt is total mass of container and wet soil in grams.

Ms is total mass of container and dry soil in grams.

Note: Grams of water is equal to Millilitres of water.

Therefore, Vw = Mw

Percentage of Holding capacity (WHC%) = (Vw/Vt) * 100,

Vw is volume of water.

Vt is total volume of saturated soil.

Methods to Measure WHC

For estimating the WHC, a force like pressure (filter paper press method) or suction (filter paper method) must be applied to the soil sample and the amount of water must be measured (WHC).

Filter Paper Press Method

In 1952, Grau and Hamm's method known as the "Filter paper press method," was developed to measure water holding capacity at a cheaper rate, in a very easy manner. This method was quick as well as it doesn't need any special equipment, at least for visual screening.

Drip Loss Technique

In 1987, Honikel gave a simple method that doesn't need any special tools but takes 48 hours to work. In the drip loss method, pressure is created by gravity and the fact that the sample material shrinks as it is stored. The method is time-consuming because it involves cutting, mixing, and weighing.

Factors That Increase Water Holding Capacity

The structure of the soil and the quantity of organic matter that is contained within it are the primary determinants of its capacity to store water. A soil that is composed of tiny particles, such as silt or clay, will have a larger surface area when compared to a soil that is composed of bigger sand particles.

A larger surface area enables a soil to store more water than a soil that is composed of larger sand particles. In other words, the ability of a soil to store water is increased when the soil has a high percentage of silt and clay particles, which is the definition of a fine soil.

Organic matter percentage impacts water retaining capacity. The percentage increase in water holding capacity is a result of organic matter's high attraction for water. After a significant amount of rain, the ground quickly loses water to gravity, which causes the soil to become dry. It's possible that plants will consume this water before it moves beyond the root zone.

When the surplus has been drained out, the water that is available is stored (field capacity to wilting point). The availability of this water is crucial for the cultivation of crops or pasture. When presence is at 50%, it can be used without stress, but when it drops below 50%, it causes stress. Soil holds water so tightly that plants can't use it, and water stays in the soil even when plants are already wilting.

The soil ultimately emerges as the most valuable resource for maintaining plant life, yet water continues to play the most important and fundamental role in this system. The plants growth relies on crop type, plant population, irrigation schedule, nitrogen fertiliser application, and soil moisture during the growing season.

By knowing about the soil's many physical properties, it is possible to assess the strength and vulnerability of the area. The water retention capacity of sandy soil is lower than that of heavier soil textures, however sandy soil may get recharged more quickly than heavier soils since heavier soils have a higher capacity for water retention. Because of the small pore spacing in the tiny soil particles, the wilting point rises as the texture becomes heavier. This is because soil with broader pores would store water more loosely than soil with narrow pore spacing.

Effects of Microbes on WHC

Soil's ability to retain water, as well as its hydraulic conductivity and permeability, are all dependent on its texture (sand, silt, clay), structure (bulk density and porosity), and organic matter content.

Changes in soil structure and organic matter content brought on by mycorrhizae are directly or indirectly useful in moulding hydraulic property.

Mycorrhizae is a symbiotic interaction between fungus and plants, where plants get nutrients and minerals from fungus, and fungus gets steady glucose through this connection. This exchange affects nutrient cycling, ecology, soil erosion, and plant physiology.

Studies about this interaction show that the majority of mycorrhizal mycelia correlates positively with water retention capacity, permeability, and infiltration capacity and negatively with water erodibility.

Direct Mechanisms Include

• Mycorrhizal improvement of soil aggregation and porosity.

• Mycelium carbon enhancement to soil, improving soil organic matter and protecting insoluble soil organic matter inside the aggregates.

• Competitive interactions with other soil microbial guilds.

• Mycorrhizal regulation of soil water repellence.

At the level of the biosphere, indirect mechanisms seem to be more useful than direct ones.

Indirect Mechanisms Include

• Mycorrhiza improves plant growth.

• It improves plant community diversity and productivity.

• It stimulates the build-up of organic matter.

• It also enhances soil porosity and water retention.

FAQs

Q1. What is permanent wilting point?

Ans. Permanent wilting point is the point where there is no minimum amount of water that is required for the plant is available. At the point of permanent wilting, a plant will wilt without ever growing better.

Q2. Which soil has maximum and minimum WHC and why?

Ans. Clayey soil has the maximum WHC, while sandy soil has the minimum WHC. Clay is made up of small closely aggregated particles, that has minimum space within them to percolate water. Whereas, sandy soil consists of large particles which has small surface area to hold the water, thus results into less WHC.

Q3. What is soil quality?

Ans. The definition of soil quality is determined by the soil's application. Soil that is suitable for agricultural cultivation may be unfit for utilizing in constructing homes. There are three distinct types of factors—physical, chemical, and biological—that affect soil quality in relation to agricultural productivity.

Soil structure and water-holding capacity are examples of physical elements. The pH of the soil and the total and usable quantities of plant nutrients are two examples of chemical parameters. Soil organisms' abilities to recycle nutrients and fight plant diseases are mostly determined by biological variables.