In order to measure the amount of nutrients in solution, a measurement of PPM or TDS (Parts Per Million and Total Dissolved Solids) is performed. This measurement is also commonly referred to as the EC or 'Electrical Conductivity' of a solution as that is actually what you are measuring. There are a number of methods of measuring PPM, my favorite is the digital PPM gauge which is simply submerged in the nutrient solution for a reading to be taken. Digital PPM meters are calibrated with a solution that has a PPM of 1000 - you do need to calibrate them every so often but nothing beats the convenience. Frequent changes of your nutrient solution will generally keep the concentrations where they need to be. My best advice is to follow the directions that come with the nutrient you plan to use. In any case, plan to replace nutrient solution on a bi-weekly basis for best results.
All the nutrients in the world will do a plant no good if it cannot absorb them easily. A major factor in determining a plants ability to uptake nutrients is the relative acidity, or pH (potential Hydrogen) of the soil or solution from which they feed. pH is measured on a scale of 1-14 and represents the concentration of hydrogen ions in solution. Generally, it is used to determine whether a solution is acidic or basic. A 1 on the scale represents a low ion concentration (an acid), pure water is considered neutral at a pH of 7. A 14 on the scale represents the highest concentration of ions (basic, alkaline). Some nutrients may become unavailable to the plant if the solution pH drifts from an optimal reading, which for most plants is between 5.5 and 6.5. This condition is called "nutrient lockout". pH can be tested with litmus paper and adjusted with an inexpensive pH control kit as shown below. Follow directions on product packaging.
Replacing your nutrient solution every 2 weeks is the best insurance against crop damage as frequent changes will provide your crop with all the nutrients it needs. Under ideal conditions, pH and PPM will drift only slightly as the nutrient solution is used by the crop. Another great way to keep your nutrients in the "green" is by using a larger reservoir - the extra capacity helps act as a buffer and maintains pH and concentration better than a "just enough to do the job" approach to reservoir capacity. Nutritional requirements vary throughout a plant's life cycle; light intensity, stage of growth (vegetative or flowering) and the general size of plant all play a role in determining its nutritional requirements. By regularly monitoring pH and PPM, you will have the ability to make corrections to your nutrient solution before your crop suffers. There are certain signs to look for when testing the PPM and pH of your nutrient solution. The opposite page outlines them for you. An unusually high pH will decrease the availability of Iron, Manganese, Boron, Copper, Zinc and Phosphorous. A pH that is too low will reduce availability of Potassium, Sulphur, Calcium, Magnesium and Phosphorous. The pH of common solutions are as follows;
Battery Acid = 1 Orange juice = 4.25 Milk = 6.75 Blood = 7.5 Borax = 9.25 Bleach = 12.5
Vinegar = 2.75 Boric Acid = 5 Pure Water = 7.0 Sea Water = 7.75 Ammonia = 11.25 Lye (caustic soda)
Since pH and PPM generally share an inversely proportional relationship, by measuring pH, you can sometimes infer what is happening to the concentration of your nutrient solution. The charts below are over exaggerated for illustration of these principles.
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