A prolonged deficiency or excess of soil moisture can damage or kill plants. Air and water availability to plant roots is affected by factors such as irrigation practices and soil characteristics. Soil properties include
- Texture—the relative proportion of different sizes of soil particles, including sand (large particles), silt (intermediate sizes), and clay (the smallest particles).
- Structure—the arrangement of soil particles into larger aggregates (clumps).
- Pore spaces—voids between the solid particles, which fill with air or water or both.
- Field capacity—the amount of water that can be held in pore spaces after excess water has drained. As plants use water, the remaining water becomes more tightly held in soil.
- Available water—the portion of the water in soil that plants can extract. Because of the attraction between soil and water, plants can extract only the available water.
- Wilting point—when plants have extracted all the available water.
About 1/2 to 2-1/2 inches of water per foot of soil depth is available to plants, varying by soil type. Loam soils generally provide the best combination of available water and adequate oxygen for roots. Loam soils are those with roughly equal proportions of clay, sand, and silt that are not compacted. This provides a wide range of pore sizes, with the smaller pores holding moisture while the larger pores drain and allow air to enter the soil.
Roots require oxygen for growth and the uptake of nutrients and water. Insufficient oxygen in the root zone (aeration deficit, sometimes called waterlogging) results in root asphyxiation and is a serious, often life-threatening problem for plants.
Solutions
Learn the characteristics of your soil to help guide irrigation practices. Sources of this information include the soil survey maps available online from the Natural Resources Conservation Service and SoilWeb by the University of California. Use a soil sampling tube or dig a 1-foot-deep hole and compare your topsoil with survey maps to reveal whether your soil matches the maps. If the topsoil differs from published surveys, for example because it has been disturbed, deeper soils may also differ.
Learn whether irrigation should be modified because soil has poor permeability and infiltration of water (slow drainage). One method to assess infiltration rate of topsoil is to conduct a percolation test.
For more information, see Conserve Water in Landscapes, Estimating Irrigation Needs, Irrigation Methods, Irrigation of Trees and Shrubs, Irrigation Scheduling Using Evapotranspiration (ET), and Water Deficit and Excess. Irrigating Fruit and Shade Trees and Shrubs provides an index to more resources. See also "Water Management" in the California Master Gardener Handbook.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
