What is Organic Media Soiless Culture

By Dr. Tahir Mahood

Various forms of potting mixes are used in the production of different plants in container and greenhouse crop production. The use of organic substances such as sphagnum peat moss, pine bark and straw etc had become extremely popular for soilless plant growing. These organic soilless mixes have the advantage of low cost and ease of use. It is also a common practice to add a mixture of other materials including vermiculite, perlite and sand to the organic substances to provide desired characteristics such as increased porosity, water holding capacity, drainage etc. Another very good source for organic potting mixes being used these days is coconut coir that has excellent properties like aeration, water holding capacity and many other added advantages. It is totally organic, environmentally friendly media, derived exclusively from coconut fiber. It has been used for the last many years in commercial greenhouses of different developed and underdeveloped countries as a growth media, and is a totally renewable resource. There is no biohazard or any disposal problems associated with this, because it is completely natural. The media offers greater moisture retention capabilities than many other Medias commonly used, maintains greater oxygen levels than Rock wool, and is reusable (after being sanitized). It can also be composted into soil gardens. Coconut Coir contains natural anti-fungal properties, offering the gardener protection from many common root diseases.

Organic media have physical and chemical properties that make their use unique as compared to inorganic media. Organic media exhibit to some extent both adsorptive and absorptive properties. So they act more like soil, while these characteristics are not found in inorganic substances like gravel or sand. Moreover organic substances provide buffering capacity which can work to the advantage of the grower, serving as a storage mechanism for the essential elements which both reduces the likelihood of elemental excess and shortages. In addition organic substances used contain some of the essential elements required by plants in sufficient quantity to satisfy the crop requirement. All good organic potting Medias should meet the needs of plant roots for air, water, nutrients and plant support. The balance of these needs will vary, depending upon the plant being grown and the stage of growth. There are a number of organic potting mixes available in the market for use, but, before choosing a mix for any type of plant at any stage following things should be kept in mind.

PHYSICAL AND CHEMICAL PROPERTIES OF A GOOD ORGANIC SOILLESS MEDIUM

1. PHYSICAL PROPERTIES

I. Porosity

Porosity

It is total volume of any mix not occupied by solids. It is further divided into two parts that is air filled porosity (pores occupied by air) and water filled porosity (pore space occupied by water).

Air Filled porosity

The air filled porosity of a mix is the percentage of its volume that is air just after it has stopped draining after being saturated with water. The ideal air filled porosity for a good potting mix is 25-30%.

Water holding capacity

The water holding capacity of any medium is a very important characteristic. Organic soilless mix that hold generous amount of water are less subject to leaching losses of nutrients. This is true because a mix with a limited water holding capacity reaches the saturation point much sooner than a mix with higher water holding capacity. For a good organic mix the desired water holding capacity is 40-50%.

Both aeration and water holding capacity of an organic mix depend on particle size distribution which is an important consideration in organic soil less media. High water holding capacity and humid air spaces in the mix is important for germination, seedling and cutting growth, while good aeration and moderate water holding capacity are essential for long term planting. A fine particle mix is best for seed germination and short term plant production of seedlings and cuttings; the course mixes are best for long term use, such as growing potted flowering and woody ornamental plants.

In general, for container growing long term, the percentage of fine particles less than 0.50 millimeters in size should not exceed 20-30% to minimize water logging. On the other hand, for short term, use of particles equal to and larger than 2.0 millimeters should be completely removed from the mix.

A common component added to organic soil less medium is sand to alter the water holding capacity or increasing weight of mix. It is added to provide porosity in fine mixes or when weight is needed to keep plant containers upright in either fine or course mixes. In either of the case sand should not constitute more than 25 -30 percent of the mix.

CHEMICAL PROPERTIES

1. pH
2. Electrical conductivity (EC)
3. Cation exchange capacity (CEC)
4. Carbon to nitrogen ratio (C: N ratio).

pH

This is one of the most important chemical properties of organic potting mixes because it can affect the nutrient availability to plants. Plants can grow between pH 4 and 8 if micronutrients are available. Ideal pH for any organic potting mixes is between pH 5.0 – 5.5 where nutrient availability to plant is at maximum. Either low or high pH can create problem for the plant in terms of nutrient availability. It is greatly influenced by the materials used for making mix.

Electrical conductivity (EC)

It is an indirect measurement of soluble salts present in any medium. An ideal organic potting mix has EC value below 2 mmhos cm-1. Although some plants can survive 3-4 EC but the best range is around 2 EC. Higher EC means high salt concentration that can cause water to leave the root facing drought problem and secondly it will interfere in the absorption of essential nutrients that will ultimately lead to death of plant.

Cation Exchange Capacity (CEC)

It is the ability of any medium to held nutrients on exchange sites for long term plant availability. There is no limit for this characteristic, higher the better. Generally for a good organic potting mix this should not be lower than 70 cmol (+) liter-1. In organic soilless mixes it increase with the decomposition of organic matter but this would not happen in inorganic mixes.

C: N ratio

It is ratio of carbon to nitrogen present in organic mixes. Higher is the value, lower will be the availability of nitrogen to plants that will cause nitrogen deficiency because of competition between plant and microorganism for nitrogen. Microbes need nitrogen for breakdown of organic matter. The ideal ratio for an organic mix is 30: 1. If the C: N ratio is higher it can be decreased by composting the material used for making mixes or by adding any nitrogen source.

The above said properties should be kept in mind while purchasing any mix or when you want to make your own mix. Secondly, before selecting any soilless mix for your plant, make sure that this going to work better for your plant. Medium used has a great impact on rooting pattern of your plant. For example Tomato plant produces fibrous long and numerous roots while grown in peat Perlite mix, while course, short and few in number in pure pine bark. This suggests that for a fine mix like peat Perlite, one would need big container while for coarse mix (pine bark) the same plant can be grown with out any problem in a small container. Always use organic mixes that will help to make this world a better place. Remember, all the potting mixes available in the market are not organic; make sure that what you are purchasing is certified organic.

Dr. Tahir Mahmood
Director Research and Development
Grotek Manufacturing Inc.