GET CULTURED: Managing media to optimize water use

by Don Merhaut

Media is only second to irrigation management in optimizing water use efficiency in containerized plants. With the proper selection of substrates or prepared media, irrigation frequency and water availability to plants can be optimized. This article will summarize the major physical and chemical characteristics of substrates to take into consideration when selecting or preparing media. Best Management Practices (BMPs) of container media to minimize irrigation frequency and optimize water use efficiency are highlighted.

Hydrophobic Vs. Hydrophilic

Hydrophobic roughly means water hating: any substrate or planting medium that is hydrophobic will repel water. Hydrophilic means water loving: any substrate or medium that is hydrophilic readily absorbs water. In container production, dry peat moss is the most notorious for being hydrophobic. On the other hand, moist peat moss is hydrophilic; therefore, it is critical that moist peat moss never be allowed to dry out. Other substrates such as perlite are hydrophobic; however, hydrophobic substrates such as perlite are often used to increase drainage and porosity of media.

BMP: Never allow peat moss and similar organic materials that are likely to become hydrophobic when dry to completely dry out. Rewetting will be difficult if not impossible.

BMP: Consider the use of wetting agents to rewet media that has become hydrophobic.

BMP: If preparing a medium, consider substrates that are hydrophilic instead of hydrophobic.

Water Movement in Media

When irrigation is applied to containers, the water will either remain on the surface, channel down the sides (especially if the medium is hydrophobic), or soak into the medium.

Water-holding capacity. The ability of container media to hold water is referred to as the water-holding capacity. It is expressed as the volume of the root medium that is filled with water at container capacity. Container capacity is the amount of water remaining in the container after runoff. Water-holding capacity is an important indicator of the required irrigation frequency. If water-holding capacity of a container media is low, then water must be added frequently.

Keep in mind that the container shape and size will also influence water holding capacity. As the container height increases, the water-holding capacity decreases. This phenomenon is demonstrated with the classic rectangular sponge: a water-saturated sponge lying flat on its side will retain more water than the same sponge when it is placed in an upright vertical position.

Infiltration rate. The rate at which water soaks into the medium is called the infiltration rate. The infiltration rate of a medium will decrease as the medium becomes wetter. Infiltration rates can be improved with substrates that have larger particle sizes which are porous. Substrates such as perlite and pumice will increase infiltration rate, but will not increase water-holding capacity since these substrates do not absorb water.

The Sodium Adsorption Ratio (SAR) of water is the relative ratio of sodium (Na) to calcium (Ca) and magnesium (Mg) in the irrigation water. As the amount of sodium increases in irrigation water, the amount of sodium adsorbed onto clay surfaces increases, which causes clay particles to disperse or “run together.” This, in turn reduces water infiltration into soil. This chemical-physical problem only occurs on field soils. SAR problems do not occur in organic soils or containerized media.

Channeling water in containers. No, we are not summoning up the water spirits! Water channeling in containers occurs when water takes the path of least resistance when flowing through the pot or container. Usually, when a part of the container medium becomes hydrophobic, the water will flow around this hydrophobic portion, which often results in water channeling down the inside edge of the container or soaking into the portion of the medium that is hydrophilic.

Water channeling may occur if liners planted in a substrate that dries out quickly and becomes hydrophobic are then planted into a larger container with a different media that is moist. This can be critical in new plantings where the liner medium can dry out quickly, since this is where water uptake is occurring (fig. 1). In full sun, the southwest side of containers will dry out faster than the northeast side of the containers, making it difficult to uniformly rewet the container media.

BMP: Select a medium that has a higher water-holding capacity to reduce the frequency of irrigations required, but do not reduce aeration of the medium to the point of impairing root growth of select plant species. For most crops, the recommended water-holding capacity is approximately 50%. Crops such as orchids and other epiphytes require media with more air spaces, and therefore, the ideal water-holding capacity is much lower.

BMP: Where suitable for the plant type, consider “squat” pots to increase water-holding capacity.

BMP: Use substrates with larger particle sizes to improve infiltration rate. Use substrates such as coarse particles of peat moss and coconut husks to increase infiltration rate while simultaneously increasing water-holding capacity.

BMP: To prevent water channeling, use frequent but brief irrigation cycles on newly planted liners in larger containers until the roots grow out of the liners and into the larger container.

BMP: Maintain uniform moisture within the container media by using cultural practices to reduce direct sun exposure to container sides, such as pot-in-pot production, shade cloth around containers on the southwest side of production beds, or “can-tight” placement of containers. Alternatively, in larger containers, place drippers on the southwest side of containers to reduce this problem.

Media Shrinkage

Media “shrinkage” occurs when organic particles break down. This happens to media containing compost that isn’t fully mature or substrates such as sawdust, which easily break down. When nitrogen fertilizer is added to these materials, microorganisms break down the carbon and the container medium will shrink. Media shrinkage may also seem to occur if fine particles such as clay drain or run out of the container drainage holes.

BMP: To prevent shrinkage, use barks, which break down slowly, rather than sawdust when preparing container media.

BMP: Use only mature compost in media.

BMP: Drainage from containers is required, but make sure drainage hole size is appropriate for media particle sizes.

Don Merhaut is a UC Cooperative Extension Specialist for Nursery and Floriculture Crops, Department of Botany and Plant Sciences, UC Riverside.