What Is Field Capacity and How Much Water Can Your Soil Hold?
Key concepts in agricultural hydrology and guidelines for optimizing water storage for crops
We frequently encounter a lack of clear answers when asking farmers questions fundamental to their daily management: What is field capacity? How much water can your soil actually retain? In this article, we aim to clarify these concepts in a simple, visual way.
Field Capacity (FC)
Field Capacity (FC) is the maximum amount of water or moisture a specific soil can retain after excess water has drained freely due to gravity (gravitational force), but before the soil profile begins to dry out. It serves as a key biological and physical indicator for determining how much water is available to plants.
Measuring field capacity indicates whether the soil maintains an optimal balance among its three essential components: water, air, and soil solids.
Important: Water that drains from the soil is unavailable to plants because it moves downward via gravity to deeper layers before roots can absorb it. This occurs when the soil is saturated (meaning all its pores are filled with water) and excess water begins to move toward deeper layers.
- High moisture. (Alta humedad).
- Good absorption. (Buena absorción)
- Low absorption. (Baja absorción).
- Salt accumulation. (Acumulación de sales).
- No absorption, drained water. (Sin absorción, acumulación de sales)
Permanent Wilting Point (PWP)
The plant’s environment no longer contains available water. This is the soil moisture level at which plants can no longer extract water and begin to wilt irreversibly. Below this threshold, the plant may suffer irreversible root damage.
Available Water (AW)
This is the water held between Field Capacity and the Permanent Wilting Point. It is measured in millimeters (mm) of water per meter of soil or as a percentage of dry soil weight.
The influence of soil composition and texture
The factors determining field capacity and water storage are directly conditioned by the composition and texture of the subsoil:
- Sandy soils: These exhibit low field capacity, as water drains rapidly through their macropores.
- Clay soils: These show high field capacity, but a significant portion of the water is held so tightly that it is unavailable to the roots.
- Silt-loam soils: These offer excellent field capacity and optimal availability of usable water.
Soil types.
Typical Available Water values by texture – Field capacity percentages
Retention percentages and critical limits vary significantly based on soil texture:
| Soil Texture | Field Capacity (FC) | Wilting Point (WP) | Available Water (AW) |
|---|---|---|---|
| Sandy | 5%–15% | 2%–5% | 3%–10% (30–100 mm/m) |
| Sandy loam | 10%–20% | 5%–10% | 5%–15% (50–150 mm/m) |
| Silt loam | 20%–30% | 10%–15% | 10%–20% (100–200 mm/m) |
| Clayey | 30%–40% | 15%–20% | 15%–25% (150–250 mm/m) |
How to prevent water loss due to deep drainage
To manage water efficiently and optimize soil moisture reserves, it is advisable to implement the following strategic practices:
Irrigation control using moisture probes:
This is the most important pillar. Regardless of soil type, it allows us to avoid water surpluses by applying the exact amount needed for the specific subsoil type and crop root system.
Promoting a deeper root system:
We can use partial drip irrigation by alternating the position of the emitters. Deeper roots access water in lower layers, making much greater use of the total soil profile.
Improving retention capacity:
Regularly adding organic matter through amendments such as compost or manure optimizes soil structural aggregation and increases available water retention levels.
Use of plant cover:
This reduces the rate of direct evaporation from the soil surface and promotes the gradual infiltration of rainfall or irrigation water.
