Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

medium with plenty of fresh water. A bathtub and a shower nozzle on a hose are perfect for washing substrate. Place the substrate in the bathtub, set a screen over the drain, and use the showerhead or a hose to wash down the medium. It may be necessary to fill the tub with fresh water and drain it a couple of times to rinse any residual sterilants from the substrate.
    If you decide to use rockwool or coco a second time, you may have problems with pests and diseases. In general, I recommend reusing a medium only if it does not deteriorate or compact. Examples include: pea gravel, expanded clay, lava rock, sand, etc. Once used indoors, reuse rockwool and biodegradable coco in the outdoor garden.
    To sterilize a hydroponic garden, remove the nutrient solution from the reservoir. Pump the solution into the outdoor garden. Avoid pumping it down household drains, and definitely do not pump it into a septic tank. The nutrients will disrupt the chemistry!
    Flood the growing medium with the sterilizing solution for at least one-half hour, let drain, and flush again. Pump the bleach solution out of the system and down the drain. Do not dump sterilants outdoors; they will defoliate plants where they are dumped. Use lots of fresh water to leach and flush the entire system including beds, connecting hoses, drains, and reservoir. Make sure all residues are gone by flushing entire system twice for a half-hour. Remove all solution from the tank, and scrub rockwool outside on a sheet of black plastic, and cover with black plastic. Let the sun bake the layer of slabs, or flock, for several days. The temperatures in the rockwool will climb to 140°F (60°C) or more, enough to sterilize for most all harmful diseases and pests.
     
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Buy pH Up and pH Down rather than making your own from concentrated acids. Commercial mixes are buffered and safe to use.

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PH
    The pH of the nutrient solution controls the availability of ions that cannabis needs to assimilate. Marijuana grows well hydroponically within a pH range of 5.5-6.5, with 5.8-6.0 being ideal. The pH in hydroponic gardens requires a somewhat vigilant eye. In hydroponics, the nutrients are in solution and more available than when in soil. The pH of the solution can fluctuate a half point and not cause any problems.
    Roots take in nutrients at different rates, which causes the ratios of nutrients in solution to change the pH. When the pH is above 7 or below 5.5, some nutrients are not absorbed as fast as possible. Check the pH every day or two to make sure it is at or near the perfect level.
    Deviations in pH levels often affect element solubility. Values change slightly with different plants, grow mediums, and hydro- Follow manufacturer’s guidelines for pH level, and correct the pH using the manufacturer’s suggested chemicals, because they will react best with their fertilizer.

This controller monitors and controls the pH in the reservoir by dispersing pH Up or pH Down. The separate thermometer/hygrometer on top of the controller in the photo monitors ambient temperature and humidity.

ABOVE LEFT: The probe of this meter is submerged in the nutrient solution to monitor pH 24 hours a day.

ABOVE RIGHT: Constant readout, temperature-compensated EC/ppm meters with a probe immersed in the nutrient tank provide round-the-clock intelligence.
    RIGHT: This quick-dip Thruegeon measures EC quickly and efficiently. It was one of the first accurate, economical EC meters that was easy to use.
     
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EC = Electrical Conductivity
CF = Conductivity Factor
PPM = Parts Per Million
TDS = Total Dissolved Solids
DS = Dissolved Solids
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    The pH can easily fluctuate up and down one full point in hydroponic systems and cause little or no problem with nutrient uptake.
    Follow the directions on the container, and remember to mix adjusters into the reservoir slowly and completely. Fertilizers are normally acidic and lower the pH of the nutrient solution. But nutrient solution is still taken in by plants, and water transpires and evaporates into the air, which causes the pH to climb.
    Stabilize the pH of the water before adding fertilizer.
    Make a correction if readings vary ± one-half point.
EC, TDS, DS, CF, PPM
EC METERS
    Pure distilled water has no resistance and conducts little electrical current. When impurities are added to pure distilled water in the form of fertilizer salts, it conducts more electricity. A water analysis will indicate the impurities or dissolved solids found in