Grower 101: Soil Amendments –Sorting Through the Crowd By Kevin Hattori

A soil amendment is any material added to a soil (orsoilless mix) for the purpose of improving its physical and/or biologicalcharacteristics, including water retention, water permeability, air movement,root penetration, drainage, aggregation, increasing organic content, improvingmicrobial viability and so on. In short, the reason for using amendments is toprovide a better environment to support plant life and development.

Aside from holding the plant upright, the growing mediumalso stores minerals for plant nutrition and water for growth, and it serves asa home for important living organisms. The ideal growing media is one in whichthere are enough large particles to ensure proper movement of oxygen to theroots, as well as enough small particles to retain the nutrients and water neededfor proper plant growth and development.

The benefits provided by soilless growing media arenumerous. There are, however, challenges to deal with. By starting with thesame type of growing media (growing mixes or media) each and every time, anyexisting variables, such as lack of nutrients or an inability to retainnutrients for a long period of time, can be easily adjusted/manipulated ifnecessary. And that’s where soil amendments come in.

Organic amendments

Organic soil amendments are derived from things that are orwere alive at some point (mainly plants and animals). Generally, organics helpseparate soil particles and increase nutrient and water-holding capacities ofthe growing media. They are also a food source for important microbialactivity.

Sphagnum peat moss.The most commonly used component in soilless growing media, sphagnum peat mossis known to have excellent water-holding characteristics, and it works wellwith other components to provide the physical properties necessary for optimumplant growth. There are different grades of peat moss, and they are pricedaccordingly. Peat moss does not add any nutrients to the mix.

Beneficial microbes.These include forms of bacteria and fungi that, under natural growingconditions, perform a number of tasks that are essential for proper plantgrowth development. They are found in soils around the world. Unfortunately,because greenhouse conditions are, for the most part, kept as sterile aspossible (to avoid disease), there is usually a marked absence of microbes(beneficial or otherwise) in growing media. Adding a microbial amendment toyour media will accomplish a number of important things. First, with beneficialmicrobes in the media, there will be less room for any would-be diseasepathogens that might find their way into your greenhouse. That means lesschance for plant disease. Microbial activity can also lead to increased rootdevelopment and is essential for the uptake of nutrients by plants.

Coir. Coir, abyproduct of the coconut industry, is produced mainly in places such as thePhilippines, Indonesia, Mexico, the Caribbean and Costa Rica. It has a highcapacity for retaining air, nutrients and water. In fact, coir can hold 1,100percent of its weight in water. Because of these properties, it can be used ina similar fashion to sphagnum peat moss. It is important to note that coirtypically contains some mineral elements (especially phosphorus and potassium).

Wood residues. Barkand bark ash provide large pores or openings in your growing media to helppromote aeration and drainage. These materials hold a substantial amount ofwater, and they are heavier, which is an advantage for larger plants that mightotherwise become top heavy. Adding bark to media may offer some increasedresistance to root rot diseases. One drawback of using bark is that it has atendency to tie up nutrients ? especially nitrogen ? if the barkhas not been properly composted. It is also known to change the pH of thegrowing media.

Kelp extract. Kelpextract comes from seaweed that has been harvested, dried and ground. It addsvitamins, plant growth hormones and trace minerals that help boost plant vigorand vitality.

Inorganic amendments

Inorganic soil amendments are either mined or man-made. Theyare most often used to physically separate soil particles, allowing forimprovements in things such as aeration.

Vermiculite. Thissterile, lightweight product comes in a variety of sizes. It is created whenmica is heated to approximately 1,800¡ F, at which the mica’s plate-likestructure expands, allowing it to retain large quantities of air, water andnutrients. Vermiculite is most often used to improve the water retention ofgrowing media. It can also add cation exchange capacity and buffering capacity.Finer grades of these lightweight granules are most often used for seedgermination, while coarse grades are good for adding to potting media.

Perlite. Perlite isproduced when volcanic rock is crushed then heated at very high temperatures.This process results in a sterile, lightweight, porous, bead-like material.White in color, it is excellent for increasing aeration and drainage andavoiding compaction. It will not change the nutrient quality or pH of the mediato which it is added. It is available in fine, medium and coarse grades. Onedrawback is that perlite holds little, if any, water. And since perlite isdusty, it can have a tendency to float out of the growing media duringwatering.

Calcined clay. Thisis created by heating clay particles to form large and irregularly shapedparticles that have excellent aeration and drainage properties. They also havemany small pores and a large surface area, resulting in a moderate capacity tohold water.

Rock wool. Similarin appearance to fiberglass building insulation, rock wool is created bymelting a mixture of industrial basalt, coke and limestone at extremely hightemperatures. The resulting liquid is then spun to form fibers (similar to howcotton candy is created). Rock wool comes in a ground form for use as anamendment. It has excellent water-holding capacity and also increases aerationand drainage. It is sterile and chemically inert.

Sand. Sand isusually added to greenhouse media to increase its weight, drainage andaeration. When used as an amendment, sand should have a coarse texture. Aswould be expected, the relatively large size of sand particles means it doesnot hold water and nutrients well.

Wetting agents.Water-absorbing polymers are available in various forms, including liquid,powder or gel. These are used primarily during seed germination to improve theability of your soilless media to absorb water from below. While wetting agentsare generally derived from inorganic sources, there are a few created fromorganic sources (such as Yucca plant extract).

With so many amendments available to today’s grower, it iscritical to do your homework. Coupled with an understanding of your plants’needs, knowing which amendments to utilize to achieve your objectives can havea significant impact on your production and profits.

Note: Please note that the preceding is not meant to be anexhaustive list of all soil amendments. Rather, it is intended to be a rundownof some of the more common types of amendments available for use by growers.

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Kevin Hattori

Kevin Hattori, formally of Growth Products, Ltd., has worked in the industry for five years. He may be reached at [email protected]

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