Aeration is much easier in aggregate culture than in water-culture system. Draining and refilling the tank with nutrient solution causes air to move in and out of the aggregate material, thus supplying adequate oxygen to the roots.
Water Supply. Water requirements for this system are the same as those for the solution-culture methods. The mineral nutrients and the minerals present in the water as impurities accumulate in the aggregate materials as a result of evaporation. To overcome this accumulation of minerals, flood the aggregate material with water every two weeks. Drain off the water to wash out the minerals.
Applying Nutrient Solution. The "slop" or surface method is the simplest for adding the nutrient solution. In this method, the solution is poured over the aggregates by hand. A manual gravity-feed system with buckets or other vats and small growing containers may be used. The vat is attached to the bottom of the tank or container with a flexible hose, and is raised to flood the tank and lowered to drain it. The vat may be lowered and raised by hand or by means of a mechanical device. The vat should be covered to prevent evaporation and filled with new nutrient solution at least once every two weeks.
The gravity drip-feed system also works satisfactorily, and reduces the amount of labor. The vat is higher than the tank in this system, and the solution drips from the vat just fast enough to keep the aggregate moist.
A pump can be used to raise the solution to the desired depth for sub-irrigation. Sub-irrigation is a system of supplying the nutrients by raising and lowering the solution level from the bottom. The solution must be raised to a higher level for younger plants than for older plants. A timer may be arranged on the pumping system so that the nutrient solution can be added whenever necessary. If the pump is not a non-rusting pump, it should be washed carefully after each use to prevent rusting. This mechanical system for adding the nutrient solution is practical only for a large setup.
The nutrient material must be added and drained or raised and lowered in the tanks once or twice a day. When the weather is especially hot and dry, the aggregate material may need more than two drenchings. Examine the aggregate material frequently to be sure that it has not dried out around the roots. After a few examinations, you will know about when the nutrient solution should be added. Remember - frequent drenchings will cause little harm, and permanent injury may result if the plant roots dry out.
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