Soil moisture is a fundamental concept governing plant life and agricultural success. The amount of water held within the soil dictates plant health, nutrient uptake, and the potential for water waste. Understanding how soil stores and releases water forms the basis of efficient irrigation practices. A precise threshold exists that represents the maximum amount of water a soil can hold for plant use.
Defining Field Capacity
Field Capacity (FC) is the measure of water remaining in the soil after it has been fully saturated and the excess water has drained away due to gravity. This drainage process typically takes one to three days following heavy rain or thorough irrigation. Once the rate of downward water movement becomes negligible, the soil is considered to be at Field Capacity.
Soil scientists quantify this state using soil moisture tension, also known as matric potential. By convention, Field Capacity is approximated at a tension of -33 kilopascals (kPa), equivalent to one-third of a bar of pressure. This value signifies the maximum water content the soil can retain that is readily available to plants without causing poor soil aeration or waterlogging. The -33 kPa standard provides a reliable upper limit for plant-friendly water storage.
How Soil Holds Water Against Gravity
The mechanism allowing soil to hold water against gravity is capillary action, which relies on two primary forces: adhesion and cohesion. Adhesive forces attract water molecules to the surfaces of soil particles. Cohesive forces attract water molecules to each other, allowing them to form a continuous film.
These forces combine to hold water within the tiny spaces, or pores, between soil particles. When the soil is saturated, the largest pores (macropores) are filled with water, which gravity pulls downward. Once gravity removes this free or “gravitational water,” the smaller pores (micropores) retain water because capillary forces overcome gravity’s pull. Finer-textured soils, such as clay, have a greater proportion of small pores and exhibit stronger capillary action, allowing them to hold more water at Field Capacity than coarse, sandy soils.
Field Capacity and Plant Available Water
Field Capacity is the upper limit of Plant Available Water (PAW), the moisture range plants can successfully extract from the soil. The lower limit of this range is the Permanent Wilting Point (PWP), defined as the soil water content where plants can no longer absorb water and permanently wilt. This lower boundary is scientifically defined by a soil moisture tension of -1500 kPa (15 bars of pressure).
The difference between the soil moisture content at Field Capacity and PWP represents the total Plant Available Water. Efficient irrigation management focuses on keeping the soil moisture level within this PAW range. If soil moisture exceeds Field Capacity, the water drains away, wasting the resource and potentially leaching nutrients. Conversely, allowing the soil to approach the Permanent Wilting Point causes severe plant stress, reducing growth and yield.
Simple Ways to Estimate Field Capacity
For practical purposes, a gardener or small farmer can estimate Field Capacity without specialized laboratory equipment. One common non-technical method is the “feel test,” which involves taking a handful of soil from the root zone a couple of days after a soaking rain or irrigation.
At Field Capacity, a moist soil sample should form a ball when squeezed firmly in the hand, but it should not release any free water or feel muddy. If water drips out when you squeeze it, the soil is still above Field Capacity and has gravitational water present.
A simple drainage test involves filling a container with drainage holes with soil, saturating it completely, and letting it drain for 48 hours before checking the moisture content. This practical estimation is a valuable tool for scheduling irrigation effectively, ensuring water is applied when the soil is depleted but before it reaches the wilting point.