Soil Water Holding Capacity
What is soil water holding capacity (SWHC)?
To put it simply, soil water holding capacity is a measurement that tells you how well the soil can act like a sponge. Does your soil soak up water, or does it water simply run right through it? Maybe it's somewhere in between.
If you don't know how much water your soil is capable of holding, you probably aren't irrigating it correctly.
For example, if your soil is capable of holding a lot of water but you irrigate your farm more than it can hold, then you're wasting water, energy, and money.
The amount of water soil can hold differs based on two variables:
- 1. Soil texture, which indicates the content of particles of various sizes--such as sand, silt, and clay--in the soil
- 2. Soil organic matter, which is decayed material that originated from a living organism, plant or animal based
Why is SWHC important?
Knowing how much water different parts of your field can absorb and use will help you apply the correct amount of irrigation. In other words, by knowing your soil water holding capacity, you'll be able to provide the exact amount of water your field needs.
These practices help to eliminate run-off, optimize crop production, and save on irrigation costs.
How to measure SWHC?
Soils are made of sand, silt, and clay; the proportion of each determines the soil texture.
Figure 1. Soil texture triangle.
The Natural Resources Conservation Service is a great resource for determining the soil textures in your field. The NRCS's Web Soil Survey allows you to type in an address or GPS coordinates and define the outline of a field for a percentage breakdown of the different soil textures present in your field.
Some experts like to use the "texture by feel" method to help determine soil texture:
- Sand - will feel gritty and therefore not hold a lot of water as a result because pores are so large. Water can rapidly move through it but not enough water available to plants.
- Silt - will feel smooth and is the ideal soil texture because of its small pores, which can hold the maximum amount of water with the highest accessibility to plants.
- Clay - will feel sticky. It can hold a lot of water but will do so tightly in pores so a lower % is available to plants.
Each soil texture is capable of holding a certain amount of water:
- Sand: 0.8”/ft
- Loamy Sand: 1.2”/ft
- Clay: 1.35”/ft
- Silty Clay: 1.60”/ft
- Fine Sandy Loam: 1.9”/ft
- Silt Loam: 2.4”/ft
If, for example, your field is made up of mostly sandy soil, it can only hold 0.8" per foot.
How can you improve SWHC?
There are a variety of ways to improve a field's soil water holding capacity. Implementing any of the following will help increase soil organic matter and therefore water holding capacity:
- Use cover crops
- Use conservational tillage
- Add manure
- Add compost
These methods help add nutrients back into the soil so it can come back replenished for the following season.