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

 

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

Aside from holding the plant upright, the growing medium also 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 which there are enough large particles to ensure proper movement of oxygen to the roots, as well as enough small particles to retain the nutrients and water needed for proper plant growth and development.

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

 

Organic amendments

Organic soil amendments are derived from things that are or were alive at some point (mainly plants and animals). Generally, organics help separate soil particles and increase nutrient and water-holding capacities of the growing media. They are also a food source for important microbial activity.

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

Beneficial microbes.These include forms of bacteria and fungi that, under natural growing conditions, perform a number of tasks that are essential for proper plant growth development. They are found in soils around the world. Unfortunately,because greenhouse conditions are, for the most part, kept as sterile as possible (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 beneficial microbes in the media, there will be less room for any would-be disease pathogens that might find their way into your greenhouse. That means less chance for plant disease. Microbial activity can also lead to increased root development and is essential for the uptake of nutrients by plants.

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

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

Kelp extract. Kelp extract comes from seaweed that has been harvested, dried and ground. It adds vitamins, plant growth hormones and trace minerals that help boost plant vigor and vitality.

 

Inorganic amendments

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

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

Perlite. Perlite is produced 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 and avoiding compaction. It will not change the nutrient quality or pH of the media to which it is added. It is available in fine, medium and coarse grades. One drawback is that perlite holds little, if any, water. And since perlite isdusty, it can have a tendency to float out of the growing media during watering.

Calcined clay. This is created by heating clay particles to form large and irregularly shaped particles that have excellent aeration and drainage properties. They also have many small pores and a large surface area, resulting in a moderate capacity to hold water.

Rock wool. Similar in appearance to fiberglass building insulation, rock wool is created by melting a mixture of industrial basalt, coke and limestone at extremely high temperatures. The resulting liquid is then spun to form fibers (similar to how cotton candy is created). Rock wool comes in a ground form for use as an amendment. It has excellent water-holding capacity and also increases aeration and drainage. It is sterile and chemically inert.

Sand. Sand is usually added to greenhouse media to increase its weight, drainage and aeration. When used as an amendment, sand should have a coarse texture. As would be expected, the relatively large size of sand particles means it does not 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 the ability of your soilless media to absorb water from below. While wetting agents are generally derived from inorganic sources, there are a few created from organic sources (such as Yucca plant extract).

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

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

 

Kevin Hattori