How Much Water Is Available To Plant?

In this short article, I would like to discuss how different factors determine how much water is available to plants and how they can affect irrigation scheduling. No calculations, I promise. I will leave the math for another time!

Some might think soil has unlimited water holding capacity and they can put as much water as they want in it. They may also assume that all of this water is accessible to plants. This is not true.

Imagine your soil is a bucket. The size of your bucket depends on two things: 1) soil type (fine vs. coarse), and 2) plant root depth (shallow vs. deep). This is illustrated in picture below. The combination of deep root and silty soil provides the largest bucket (water availability), whereas a shallow-rooted plant in sandy soil runs out of water in shorter period of time.

Soil water-holding capacity is primarily controlled by soil texture (amount of sand, silt and clay) and organic matter. Heavier soils (smaller soil particles) tend to have more water holding capacity (also called "more screw up capacity!"). You can go longer between irrigations. A sandy soil has larger particles than a silty soil and therefore less water holding capacity.

Adding more water to sandy soil is not going to help because the soil can’t hold it. Applying more water than a soil can hold simply results in deep percolation. Deep percolation is the water that is lost below the root zone of the plant, along with essential plant nutrients and other soluble compounds. More irrigation just adds to your labor and fertilizer cost.

This makes the sandy part of your field your Achilles heel. You can use a soil moisture sensor and monitor the soil more often. You also need to irrigate a sandy soil more often compared to a silty soil.