Hydroponics Growing Medium - Getting Down to the Nitty Gritty

by Dr. Chris Kline

Growing medium is anything that a plant can grow in, and dozens, if not hundreds of forms exist. This article will focus on various popular forms of growing medium that are suitable for use in hydroponics or plant starting and will introduce a very promising relatively new medium. It is important to understand that it is difficult to select one best medium for all purposes. Commonly used growth mediums each have advantages and disadvantages and may even be combined to form a more perfect growing environment.

Functions of Growing Medium

Growing mediums provide support both physically and biologically. Physical support is provided by allowing the plant to grow its roots through the medium to hold itself in place. Biological support is provided by the medium’s ability to hold nutrients, water and air that the plant needs and its ability to release carbon dioxide. In soil based gardens, the medium (soil) also supports other types of life. Microorganisms and insects live in the soil and they in turn aid plant life by helping to decay organic material, adding structure and nutrients to the soil. Each commonly used growing medium has advantages and disadvantages and may be more suitable for use in one hydroponics system than another.

Soiless Mix

Most of the time products sold as “potting soil” are really not soil at all. They are soilless mixes containing a varied assortment of ingredients. The most commonly found components are Sphagnum moss, Perlite and Vermiculite. Unlike most hydroponics growing medium, soilless mixes are usually considered organic and are frequently used for container gardening, wick systems and non-recovery drip systems. Most of these mixes have some very fine particles that can clog pumps and drip emitters making them unsuitable for recovery systems. The best soilless mixes have great wicking action and retain water and air well, making them suitable for semi-hydroponics methods and container gardening.

Coconut Fiber

Coconut fiber has become a very popular growing medium, and for good reason. For those looking for a totally “organic” growing medium that offers top performance in hydroponics systems, coconut fiber may be the best choice. Compared to rockwool (discussed below) it maintains a larger oxygen capacity and has superior water holding ability which is a real advantage for hydroponics systems that have intermittent watering cycles. Unlike most synthetic mediums, coconut fiber is believed to be high in root stimulating hormones and offers some protection against root diseases including fungus infestation. A common formula popularized by Dutch growers consists of a mixture of 50% coconut fiber and 50% expanded clay pellets. This mixture is particularly suitable for continuous drip systems where water holding ability is not as big a concern and more oxygen holding capacity is desired. There is some concern that all coconut fiber products are not created equally, so care should be taken to purchase a premium blend.

Perlite and Vermiculite

Perlite is often used as an additive to soilless mixes or soil to increase aeration and drainage and may be used by itself in continuous flow hydroponics systems. Because it does not retain water well, it is often combined in a 50/50 mixture with vermiculite. Unlike perlite, vermiculite retains water well, perhaps too well to be used by itself. Both vermiculite and perlite are inexpensive, and combining the two makes a very popular and effective medium for drip and ebb and flow systems. Vermiculite and perlite must be rinsed thoroughly before use and care should be taken not to inhale the dust as it is hazardous to ones health. It is important to buy horticultural grade products, as those produced for other purposes often do not have a stable PH and will make maintaining proper pH in the system very difficult.

Expanded Clay

This product has several nick names including: hydro corns and grow rocks and is an extremely good growing medium. The pellets are great for ebb & flow systems or other systems that have frequent watering cycles. Expanded clay dos not retain much water so it works best in continuous flow systems or mixed with other mediums to increase water retention. One drawback is that expanded clay pellets are rather expensive, but fortunately they are one of the few kinds of growing medium that are easily reusable. Between crops they can be recycled by removing all the old roots and then sterilizing with a 10% bleach and water mix.

Rockwool

Rockwool is one of the most popular growing mediums. It was developed for gardening in Denmark and is used extensively for continuous drip hydroponics systems. Rockwool comes in many shapes from one inch starter cubes to large three foot slabs which make it quite a very versatile growing medium. Rockwool holds water well and unless it is sitting directly in water it holds enough air to supply roots with plenty of oxygen. Rockwool has a few notable disadvantages. Like perlite and vermiculite the dust is hazardous if inhaled. It also must be pre-soaked for 24 hours prior to use. Perhaps the most notable problem is that rockwool has a high pH which requires that the nutrient solution pH be lowered more often to maintain correct growing pH levels.

Sure To Grow MediaTM

Sure to grow media is a relative newcomer that was patented in the late 90’s. It resembles some kind of cloth or foam material that could be used for quilting or crafting, but make no mistake about it; this is a serious growing medium. One of the greatest advantages of Sure to Grow Media is that it comes in a variety of shapes and sizes and if needed it can be cut to fit any application. It was designed for ease of use for hobbyists, but is also finding favor among commercial growers seeking the efficiencies that it provides, because in the plant business, a more efficient grower is a more profitable grower. There is virtually no setup time. It wets instantly, has great aeration and water retention, is pH neutral and sterile, clean and hygienic. For larger growing operations this product offers a marked advantage over most other mediums as it can be rolled out into any size gutter system and rolled back up for change out between crops. After using this medium along with others it is hard not to come back for more, as it just makes setting up a hydroponics system so effortless compared to most other growing mediums, and this same advantage is enjoyed again when it comes time for crop changeover. For information on Sure To Grow products visit www.suretogrow.com.

Dr. Christopher J. Kline is a master gardener and writer living in Paradise Valley Arizona. Chris is a member of the Garden Writers Association and a columnist for Garden and Greenhouse Magazine http://www.gardenandgreenhouse.net. You can contact him at CKline72@Cox.net