What happens when soil gets too much water

How much rain is too much? It’s a question a lot of us soggy Sacramentans have wondered about lately as we see pools forming in our backyards.

Besides flooding and downed trees, this drenching rain has another impact: Saturated soil.

It can happen in almost any landscape and can have major long-term impacts. Some of those effects won’t be clearly visible for weeks or months to come.

Saturated soil is waterlogged with all its air squeezed out. If soil stays saturated too long, it can promote devastating diseases, leading to the death of trees, shrubs and other plants.

Healthy soil has little pockets of air between the molecules of sand, clay, silt, gravel and rock. Those air pockets supply oxygen to soil microbes that break down organic matter into plant-ready nutrients.

Water also moves through those same pockets of underground space, supplying moisture to those microbes as well as plants. In saturated soil, all those pockets are filled with water with nowhere to go, and the microbes drown.

Usually, the ground is a great place to store rainwater. As it travels down through those layers of sand and gravel, water is naturally filtered. That filtration removes pathogens and pollutants. (That’s important for well water.) Tree roots can tap into the underground reserve. (That’s vital during drought.)

According to university research, pathogens in saturated soil can remain viable and can move hundreds of feet, contaminating nearby drinking water wells and infecting plants. Saturated soil also can lead to septic system failure, backing wastewater into the home and (on the other end of the system) contaminating groundwater.

In home gardens as well as commercial orchards, saturated soil can lead to outbreaks of Phytophthora-related root and crown rot.

“Periods of 18 to 24 hours or more of water-saturated soil favor Phytophthora infections,” say the UC IPM agriculture notes.

The result can be deadly but may not show up until months later. “Generally, crown rots advance rapidly and trees collapse and die soon after the first warm weather of spring,” say the UC IPM ag notes. “Chronic infections, usually of the roots, cause reduction in growth and early senescence (deterioration due to aging) and leaf fall. These trees may have decreased yield and vigor for several years before succumbing to the disease.”

Crown rot pathogens can wait dormant in dry soils for years before being reactivated by saturated soil. I know from experience. After the last major drought ended with huge winter storms, I lost several heritage camellias and a huge rhododendron to crown rot.

“Almost all fruit and nut trees, as well as most ornamental trees and shrubs (including many California natives), can develop Phytophthora rot if soil around the base of the plant remains wet for prolonged periods,” say the UC IPM experts. “In trees and shrubs, the pathogen kills plants by growing from the roots up through the root crown and into the lower trunk, where it kills the inner bark and causes a browning of the outer layer of sapwood.”

When crown rot infects a tree or shrub, the plant may actually look like it’s not getting enough water.