The correct gravel depth is important for the development of the roots system. Too shallow gravel will not only restrict root development and volume, but will also cause the development of algae on the surface of the gravel. The algae attract fungus gnats which feed on the roots of plants. The reason why the gravel should be at least 50 mm in depth is to prevent the formation of the algae layer. Just above the nutrient solution layer, which should be approximately 5 mm thick, the air is saturated with moisture and oxygen. This layer moistens the gravel on which the root hairs of the plants absorb nutrients. If the gravel is to shallow, algae feed on the nutrient rich moisture. If however the gravel is at the correct depth, light does not penetrate to where the gravel is moist and algae cannot grow. Over long periods a thin white powder might form just below the top layer of gravel. This is a salt layer and does not influence the plants. It can be washed with clean water.
I studied agriculture at the University of Stellenbosch in South Africa in 1984 and completing my M.Sc Agric. in 1998. . My love for "Controlled Environmental Agriculture" (CEA), started in my third year when I was exposed to the Welgevallen Research Station. There Prof. P.C.Maree showed us what hydroponics consisted of. It was awesome. There were no large tractors involved, no dusty fields, no uncontrollable storms to destroy your crop (well that is what I thought). Since then I put hydroponics to much better use, not just farming. We solved pollution problems by cleaning mines effluent with hydroponics and aquaculture. They were used to remove toxic metals to produce clean water (which we sold and make more money of than the produce). What I learned from 1987 I tried to compile in this website and I hope it is from some value to the serious commercial farmer that wants to take the journey into Commercial Hydroponic Farming.
Lettuce was cultivated in 4500 B.C. in the Mediterranean basin. Only in 1543 was the