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

household tap water. These impurities conduct electricity.
    Nutrient (salt) concentrations are measured by their ability to conduct electricity through a solution. Dissolved ionic salts create electrical current in solution. The main constituent of hydroponic solutions is ionic salts. Several scales are currently used to measure how much electricity is conducted by nutrients including: Electrical Conductivity (EC), Conductivity Factor (CF), Parts Per Million (ppm), Total Dissolved Solids (TDS), and Dissolved Solids (DS). Most American growers use ppm to measure overall fertilizer concentration. European, Australian, and New Zealand growers use EC, however they still use CF in parts of Australia and New Zealand. Parts per million is not as accurate or consistent as EC to measure nutrient solution strength.
    The difference between EC, CF, ppm, TDS, and DS is more complex than originally meets the eye.Different measurement systems all use the same base, but interpret the information differently. Let’s start with EC, the most accurate and consistent scale.
    Electrical conductivity is measured in milliSiemens per centimeter (mS/cm) or microSiemens per centimeter (μS/cm). One microSiemen/cm = 1000 milliSiemens/cm.
    Parts per million testers actually measure in EC and convert to ppm. Unfortunately, the two scales (EC and ppm) are not directly related. Each nutrient or salt gives a different electronic discharge reading. To overcome this obstacle, an arbitrary standard was implemented which assumes “a specific EC equates to a specific amount of nutrient solution.” Consequently, the ppm reading is not precise; it is only an approximation, a ball park figure! It gets more complex! Nutrient tester manufacturers use different standards to convert from EC to the ppm reading.
    Every salt in a multi-element solution has a different conductivity factor. Pure water will not conduct electrical current, but when elemental salts/metals are added, electrical conductivity increases proportionately. Simple electronic meters measure this value and interpret it as total dissolved solids (TDS). Nutrient solutions used to grow marijuana generally range between 500 and 2000 ppm. If the solution concentration is too high, the internal osmotic systems can reverse and actually dehydrate the plant. In general, try to maintain a moderate value of approximately 800 to 1200 ppm.
    Nutrient solution concentration levels are affected by nutrient absorption by roots and by water evaporation. The solution weakens as plants use nutrients, but water also evaporates from the solution, which increases the nutrient concentration. Adjust the concentration of the solution by adding fertilizer or diluting with more water.
    Many factors can alter the EC balance of a solution. For example, if under-watered or allowed to dry completely, the EC reading will rise. In fact, the EC may increase to two- or three-times as high as the input solution when too little water is applied to rockwool. This increase in slab EC causes some nutrients to build up faster than others. When the EC doubles, the amount of sodium can increase as much as four- to six-fold under the right conditions! There should not be any sodium present in your garden unless it is in the water supply, and it should not be in excess of 50 ppm.
    Let 10-20 percent of the nutrient solution drain from the growing medium after each irrigation cycle to help maintain EC stability. The runoff carries away any excess fertilizer salt buildup in the growing medium.
    If the EC level of a solution is too high, increase the amount of runoff you create with each flush. Instead of a 10-20 percent runoff, flush so 20-30 percent of the solution runs off. To raise the EC, add more fertilizer to the solution, or change the nutrient solution.
    A dissolved solids (DS) measurement indicates how many parts per million (ppm) of dissolved solids exist in a solution. A reading of 1800 ppm means there are 1800 parts of nutrient in one million parts solution, or 1800/1,000,000.
    An EC meter measures the overall volume or strength of elements in water or solution. A digital LCD screen displays the reading or the EC of the electrical current flowing between the two electrades. Pure rainwater has an EC close to zero. Check the pH and EC of rainwater to find out if it is acidic (acid rain) before using it.
     
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    Parts per million meters measure the overall level of dissolved solids or fertilizer salts. Each fertilizer salt