Different Mediums for Growing

Organic Mediums (Mineral Soil)

A plant grown in regular soil in the back garden needs its particular patch of
I soil to be good. The plant will then surely grow well, but only if :
(a) the soil patch has enough nutrients and trace elements to meet the plant's needs;
(b) it rains often enough to provide enough moisture for the plant to grow;
(c) the Sun falls on the plant.

Oxygen in the air and in the soil will allow the plant leaves and roots to respire and produce energy for living and growing. There is also enough CO2 (about 300-400 PPM) in the air for the plant to make sugars. The plant will slowly seek out nutrients and trace elements in the patch, causing
it to grow bigger all the time and spreading its roots further and further in I search of more oxygen and nutrients.

In a good back yard in one growing season, a plant can get massive. Science can hardly better this. The plant would be happy to grow as big as a tree if it did not have light sensitive hormones that tell its cells to degenerate when the days get shorter. This hormone build-up signals to the plant that "its time is up."

Of course this wonderful backyard with its great soil and elements is not everywhere. When the Sun is replaced with an electric light and the backyard is placed into a planter, everything changes. Even a simple plant is not easily fooled by this set-up, so a grower has to use all the knowledge of plant needs to make the plant think it is still in plant heaven.

Genuine backyard soil, unfortunately, is just not good enough when it is taken out of the ground. Better mediums are super artificial soils (non-mineral soils), and these are called growing mediums.

Non-OrGanic Mediums or Non-Mineral Soils

Start with a good growing medium.

• Moss
• perlite
• sawdust
• lava rock
• pop clay or HEC
• vermiculite
• rock wool
• neoprene discs.

Each of the above growing mediums has both good and bad points. Moss is for amateur growers because it is the most forgiving. Rockwool is mainly for graduated growers since it is the most temperamental. Pop clay or HEC is for advanced growers and needs nutrient sprinklers or drips.

Moss mix is a soilless mixture that has the best buffering (means providing protection against nutrient burn) when blended with other growing mediums. It supports the roots well and holds some oxygen. The medium is slightly acidic and does reasonable pH buffering.

Sawdust, wood, and wood shavings are used in most commercial greenhouses, and they need expert supervision.

Lava rock is not forgiving and can contain toxic minerals from its volcanic origin. It has sharp edges that are not kind to the roots. Gravel is used instead.

Pop clay or Hardened Expanded Clay (HEC) looks like small i brown clay marbles. It is cheap, sterile and re-usable, but has low buffering or low protection against nutrient burn. It is used mostly in drip systems.
Note: Because drip systems tend to leave dry patches in the pop clay,
moisture and nutrient needs for the roots are not completely satisfied. To gain maximum advantage from this type of system, sprinklers and root zone
enclosures are necessary for the humidity control. (Refer also to Hardened Expanded Clay .)

Rockwool is the least understood and the most expensive of all the mediums. It is really a cousin to fibreglass insulation but made from limestone instead of silica sand. It is very popular but is far from simple to use. Rockwool is definitely not for beginners. It has certain differences from other mediums that have to be understood.

Neoprene disc is not a growing medium, but just a piece of sponge that holds the stem of the plant and allows the roots to dangle freely. It is used in aeroponics or nutrient flow systems.

Why Use Artificial Soils?

The subtle difference between soil and non-soil mediums is their ability to hold water and oxygen, and their ability to hold onto nutrient ions and supply them to the plant roots as the plant calls for them (see Cation Exchange Capadtyon). Real soil is not used indoors anymore.
The nutrients, the water, and the gases all have electrical charges. Growing mediums also have electrical charges that control how much nutrient the roots can absorb. pH is a direct measurement of the electrical charges acting in the nutrient, and it is this relationship which determines whether a plant will eat well or starve to death. (Read about pH and Water.)

In mineral soil, the humus and organic particles have high electrical charges (CECs) that do not want to give up their nutrients and water to the plant. A great electrical battle takes place as the roots, the nutrients, and the soil particles each attract the ions. As well, mineral soils with too many clay particles and other small grains in them are packed so tightly that they exclude oxygen and nutrientbearing humus. When a soil particle does finally lose all its moisture, it becomes sterile sand. This is why as the plant gets drier and needs more water from the soil, the soil hangs on to the water more tightly. The plant must then expend enormous energy to get water.

The soilless mediums have a totally different and mostly zero electrical charge (CEC) on their fibres. These synthetic growing mediums tend to have a low CEC or be electrically neutral, so they do not hold nutrients, water, and gases as tightly. The plant can damage itself by being able to attract a strong nutrient directly into its roots. This easy passage into the roots is called "loss of buffering," and things can go wrong very quickly. Nutrient burn from a highly concentrated solution happens almost immediately, so it is best to experiment with low nutrient concentrations for your indoor garden before you switch to using rockwool, clay pellets, or low CEC mediums.

Hardened Expanded Clay

One of the very popular water culture growing mediums is called Hardened Expanded Clay (HEC). Its common brand name is Hydroton. It is made up of small red clay marbles. Another variety is not rounded marbles but irregular-shaped small red clay pebbles.

Most artificial mediums retain positive ion charges, called cations (see Cation Exchange Capacity on). These positive charges change the electrical balance in the nutrient solution, affecting the entire workings of nutrients on plants, and the effect is not easy for a grower to detect until production starts to drop off. Experiments have shown that after 4 weeks in a nutrient solution, the HEC absorbs 12-15% calcium cations (Ca++) and a proportional amount of potassium cations (K+). It takes about 4 weeks in pure water to wash out the Ca ++ and a lot longer to flush out the K+.

Rockwool

There are three varieties of rockwool :
(1) bulk rockwool - used in pots
(2) rockwool cubes and blocks - used in high density layouts
(3) rooting rockwool plugs - 144 plugs to a tray

Consider the following before using rockwool. Rockwool fluff has no buffering. It has an easily raised pH which can get brutal. Always pre-soak the bulk rockwool and set just below pH 6.0 before use. Monitor the pH regularly throughout the plant's life because its pH level will surely climb.

NEVER use rockwool in a pot with a catching saucer, or the pH in the pot will climb 2.0 to 3.0 points in a few days as the run-off is re-absorbed. The salts will build up and start deforming the leaves in a few days.

Salt build-up in rockwool is a constant problem and regular flushing is necessary, or use about 15 - 20% run through during every watering to stop evaporative salt build-up. Always water the rockwool pot so that there is a lot of run-off, and then either discard the run-off or add the pH down before re-using it. Rockwool set around pH 5.5 - 6.0 is best for most plants. Be careful with pH levels for soilless mediums.

As well, rockwool is difficult to use in flood tables because it is hard to judge when the rockwool is too wet or getting too dry. If it is left in standing water, rockwool can hold incredible amounts of liquid and drown the roots.

Some brands of rockwool cubes have already been pH balanced at the factory and are neutral. Check a cube by placing it in a cup of lukewarm water for a few minutes, then measure its pH to be sure that the particular brand you use has been neutralized. Do this same experiment with the rockwool flock or fluff and see the result. Read very carefully the section on pH and Water.

The plastic and paper around the individually-wrapped rockwool cubes should be taken off before using in rooting trays because the cellulose in the paper and the moisture on the plastic grow algae and encourage mould and fungus. Light growths of algae are not a problem. When the outer plastic cover is kept in place on the larger rockwool cubes as a moisture control, heavy growths of algae can take oxygen from the root zone.

Because rockwool can hold so much water for so long, there is no easy solution to the green algae, mould, and fungus on the cubes. Putting black plastic over the top of rockwool cubes is one way of stopping alga growth. Another way is to sprinkle a layer of perlite on top of the rockwool in the pots. These methods keep green algae down by keeping the top dry and the light off it.

Two final points about rockwool are that they are the most expensive medium and the most difficult to get rid of. Because rockwool can handle any amount of heat, many gardeners re-use the cubes by putting them in an oven to kill all the living things in them.

Commercial greenhouses drip the nutrient mix into rockwool and sawdust at a set rate, which gives a similar run-off each day to keep the nutrient salts constant for the plants' needs so they do not build up in the medium. The PPM of the mix going in and the PPM of the run-off are measured. Because the plants are using nutrients, the run-off PPM (EC) must be less than the nutrient "in" PPM (EC). The pH "in" and the pH of the run-off should also be tested, and both must be close or there is a problem.

If you are not using a constant flow through method, it is best to start below 1,000 PPM and adjust as necessary. PPM above 1,500 is really getting high. Flush if you have to, but it is better to use lower PPM and good run-off.

If you have let the salts build up, flush with two or more watering. Stop flushing after the run-off goes below about 1,000 PPM, but be vigilant. As soon as the run-off gets over pH 6.0, make the next watering at a lower pH.

Rockwool does not replenish oxygen quickly. Do not overwater the rockwool because it retains enormous amounts of water, and the roots do not breathe well in excess water.

Straight rockwool does not hold water evenly from top to bottom around the roots. Because the medium has no electrical attraction to water and nutrients, gravity pulls the water down to the bottom of the rockwool very quickly. Peat and vermiculite mixes hold the moisture far more efficiently. Loose rockwool can be mixed with other mediums as needed. A peatrockwool mix is a very good porous medium that retains air and yet keeps the water where it is needed.

All the soilless mediums and mixes yield fairly similar results because they all function to distribute nutrients and oxygen to the plant roots. Thus, nutrients and oxygen now become important concerns.