Understanding drainage in horticultural growing media

Ask a room full of growers how they want to improve their substrate (i.e., growing media), and you’ll hear a dozen different answers.

One answer will commonly include, “I want better drainage.” Unfortunately, the term “drainage” is a poor characterization for soilless substrates and entails many factors.

This article is intended to better define factors impacting the common jargon of drainage to help growers better identify and convey problems. Clear communication and understanding are paramount when making new substrate blends, refining existing substrates or altering the irrigation schedule or fertility.

In soil science, drainage can be defined as the rate and extent of water movement through and across the soil. But that definition is inadequate when referring to substrates. This is because different substrate blends are used in containers of a preset volume and height and do not have endless width or depth like soils do.

In simplest terms, drainage of a substrate is how fast and what volume water comes out of the bottom when irrigating a specific container with known dimensions.

To better understand drainage, we must identify the specific phenomena that occur when producing a plant: how water gets into the substrate surface (i.e., infiltration); how water is stored (i.e. physical properties, specifically water holding capacity); and how water moves through the substrate (i.e., hydraulic conductivity).

The answer will differ based on the substrate, the given container’s shape and the container height. This in turn affects how air space and gas exchange change over time — all while ensuring your substrate delivers enough water and mineral nutrients between irrigation events to avoid plant stress.

Each of these phenomena is in part a result of how the substrate components (e.g., sphagnum peat and engineered wood fiber or perlite) interact. Your daily retention of air or water fluctuates due to irrigation scheduling. Root growth and the changing physical properties of growing media constituents or mixtures alter the substrate’s performance over the production period.

Pore size

Pores occur primarily between substrate particles. They differ in size due to the intermingling of particles when mixing substrate components and filling a container.

For example, perlite may provide increased air space relative to fine peat by creating larger pores. However, perlite would fill in larger pores in a bark-based substrate, decreasing air space. If either substrate’s density is increased or compressed during potting, the pores change size and shape, typically resulting in increased water retention and decreased air space.

The ability for the substrate’s void space or total pores (typically 70% to 90% by volume) to be filled with water or air is the result of two opposing forces. The first force is the water matric potential, where water is retained between smaller pores due to capillary action and on the surface through water’s attraction to a particle’s exterior.

This second force is gravitational potential pulling water down to the bottom of the container. The pores that are unable to retain water due to gravity provide the air space needed for roots to respire. This is a delicate balancing act. Water is continually removed from the substrate by transpiration, roots sucking up water and evaporation occurring from open surfaces on the container. Water is also routinely added back through irrigation, refilling pores with water.

We refer to the portion of the substrate holding water after being thoroughly watered and drained as container capacity, which is synonymous with water holding capacity. Horticultural practitioners control the volumetric water — the measure of water in the container throughout production based on volume — and air content through a combination of desirable pore sizes and distribution, container height and intervals between irrigation events.

Infiltration

Infiltration refers to how fast water enters the substrate. Movement of water into the substrate is influenced by the fineness and wettability of the substrate.

A fine or hydrophobic, water-repellent substrate will pool water on the surface. A coarser or hydrophilic, water-loving substrate allows water to readily enter. The use of wetting agents can aid in infiltration to a degree.

Hydraulic conductivity

Hydraulic conductivity is the rate of water movement in the container. The rate of movement can be measured under saturated or unsaturated conditions. Saturated refers to every pore in the pot being filled with water. Unsaturated is a system that can be anywhere between moist from recent watering to dry and needing a watering. The latter, unsaturated hydraulic conductivity is used for all practical applications in a substrate.

Fine particles and fiber substrate components (e.g. coir, peat, wood fiber) increase unsaturated hydraulic conductivity due to particle size and greater pore connectivity.

Coarse substrates have pores that vary greatly in size and are more likely to have preferential flow where water goes through channels and only wets a portion of the substrate instead of the entire profile. This is most pronounced in dry substrates and typically decreases with increasing volumetric water content of the substrate.

Container height

Container height directly impacts gravitational potential. A short container will retain more water than a tall container with the same substrate. This is primarily due to increased gravitational potential overcoming the matric potential as container height increases.

If irrigated containers of varying height are placed side by side and filled with the same substrate, you would find the same volumetric water content value in each container at the same height, and the very bottom of each container will be saturated.

The selection of young plant tray depths has an influence on watering regime when germinating a seed, rooting a cutting and hardening a tissue culture plantlet.

The more recent adoption of open cell liners that are wrapped in paper or mesh allow for more uniform drying and greater air exchange, promoting greater ease of root development. The perforated bottoms of young plant trays and purposeful large moisture swings when growing in closed cell systems are needed to allow for similar air exchange.

The evolution of a mix

Lastly, every substrate is always evolving. Each component starts with known physical properties, maximum water-holding capacity, and minimum air space. However, when growers begin to blend components, they intermingle and alter the static and dynamic physical properties which are further affected by potting method.

The result is a soup, where one ingredient plus another ingredient or component will not equal the total volume of the two components, with each contributing to the whole differently.

As plants grow, the hydraulic properties of the substrate continue to change. Roots alter pore sizes, the substrate ages and breaks apart to fill pores or wash out and agrochemical applications affect wettability. Growers further control water availability, air space and gas exchange at any given time through irrigation management practices.

Thus, horticultural producers should start with a well-drained or airy mix that is forgiving to best address changes as the substrate evolves. This approach provides “breathing room” through the production cycle, but also may be wasteful or cause issues early in production. This is one reason for using different substrate mixes when producing young plants, liners and finished stock.

Balancing air and water in containers can be better understood by paying attention to density at time of filling, pore structure, infiltration of water, irrigation schedule, container height and substrate evolution. Consider these factors when you next make or refine your substrate mixtures.

Jim Owen is technical manager and Daniel Norden is director of technical development at Profile Products. Contact them at jowen@profileproducts.com and dnorden@profileproducts.com.