The Disadvantages of Passive Sub-Irrigation

Types

• Passive sub-irrigation is the simplest type of hydroponics, where the plants sit in a bottle or pot above a nutrient solution aerated by an aquarium bubbler. Capillary action along a wick delivers water and fertilizer up to the plant roots. This setup reduces labor and provides a continual supply of water. Alternate setups include continuous-flow solutions -- such as the nutrient film technique where a very thin stream of water feeds the root mats of the plants -- and ebb-and-flow systems, which also provide nutrients from below the plant containers but involve active pumps.

Costs

• While sub-irrigation for greenhouse crops offers lower labor costs, the initial cost of the system and the need to retrofit a greenhouse can be drawbacks, notes soil sciences professor Douglas A. Cox of the University of Massachusetts, Amherst. An investment in sub-irrigation may not pay for itself for as long as a decade, he notes, although high productivity and choosing a cheaper system can lead to payback in two to three years. And while labor costs are low, hydroponics setups such as passive sub-irrigation are far less forgiving of neglect than regular outdoor growing. While hydroponically grown plants require only limited attention each day, you cannot skip a day of monitoring as you can with plants growing in soil.

Risks

• Passive sub-irrigation and other forms of hydroponics create high humidity, which can create a favorable environment for the growth of salmonella, note Florida teachers John and Hope Chybion, who teach hydroponics to students in the Brevard County Schools. Salmonella is a rare eventuality for a home or school gardener, they add. Over-watering may also create conditions that lead to verticillium wilt. This problem can be averted by good water management of the sub-irrigation system and sterilizing contaminated growth media.

Techniques

• Without a supportive root system, tomatoes and peppers may require supports such as trellises to hold up their heavy fruits, notes Home Harvest Supply, a supplier of hydroponics gear in East Lansing, Michigan. Root crops such as carrots and potatoes can also be tricky in a soilless medium. And spinach is prone to eventually succumbing to a fungal disease called Pythium aphanerdamatum when grown hydroponically, notes professor Louis Albright of Cornell University’s Biological and Environmental Engineering Department.