What is Hydroponics?
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Screened and washed bedding sand ready for use.
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There are many types of growing media ranging from straight nutrient solution to clay pebbles like children's marbles, to expanded minerals such as Perlite, to sawdust, down to common old sand. The job of the medium chosen is to support the plant's roots and be free draining. Free draining is essential as the roots need access to air when the nutrient solution drains away.
Each media has advantages and some disadvantages.
Straight Nutrient Solution:
The main advantage is that it requires no solid growing medium as such. Commercial growers of lettuce in South East Queensland use this method because of its low cost, especially when used on a very large scale. The plants are grown in plastic cups with a hole for the roots to dangle into the nutrient. The cups sit in holes in a lid of a trough shaped somewhat like a rainwater gutter. Earlier models were like the Author's pipe modules with holes for the cups along the top. Access to air is provided by aerating the nutrient solution. The main disadvantage of this method of growing is that it requires pumps to run much of the time circulating nutrient solution; it is quite expensive when used on a small scale such as in one's back yard.
Porous clay pebbles are favoured by some hobbyists. The porosity is supposed to hold water better than solid pebbles. The main advantage of this sytem is that it is clean. The main disadvantage is the cost of the clay pebbles and their availability.
Expanded minerals such as Vermiculite are favoured by some hobbyists. The minerals are expanded by calcining (heating to blow up water trapped in the molecular structure of the mineral); the effect is a mineral version of "puff pastry". The Author has tried Vermiculite and found it was quite light in weight and that when it dried out it tended to blow around quite a lot. It was found to hold too much water. An advantage is its lightness, and its main disadvantage is its cost.
Sawdust is also favoured by hobbyists. A bag of sawdust is laid on its side and a slit put in the top side with a sharp knife. Apparently, not just any old sawdust is suitable for use as a growing medium. This method is useful when you do not wish to recover the used nutrient solution; just let it run to waste. The main advantage in using sawdust is that it is low in cost; one disadvantage is that nutrient solutions are rarely re-used, which will drive up the running costs. Sawdust tends to decompose over time, but it is cheap to replace.
Common Old Sand:
The Author favours the use of sand as it is cheap and easy to obtain. Suitable sand for hydroponic use is sold in South East Queensland as bedding sand. It is a coarse sand used for filling building foundations when they have been dug too deep. To improve its draining ability, the Author sieves out the fines and puts them on low spots in the lawn or mixes it with garden soil. Virtually any small grades of gravel are suitable as well, but when one digs holes to plant seedlings with one's fingers, gravel can be quite abrasive, so it pays to obtain coarse sand composed of rounded pebbles rather than sharp pebbles. The plants apparently do not mind whether the medium is rounded or not, as long as it drains properly. The advantages of sand are its low cost, ready availability and ability to be cleaned when it gets dirty. About the only disadvantage is its weight.
The next page answers the question: "What Foods do Plants Like?"