For optimal growth to take place, the nutrient concentration and pH must be consistently balanced over time to insure plants have what they need, when they need it. In any circulating hydroponic system, with every pass the nutrient makes past the root system, an exchange is taking place. As a result, as time goes by, your nutrient solution changes in concentration. Therefore, so does each plant's ability to uptake essential elements. The easiest way to keep on top of your nutrient solution is to take a measurement of PPM or TDS (Parts Per Million and Total Dissolved Solids). This measurement is also commonly referred to as the EC or the "Electrical Conductivity" of a solution, because that is actually what you are measuring. There are a number of methods of measuring PPM. My favorite is the digital PPM meter that is submerged in the nutrient solution for a reading to be taken. Digital PPM meters are calibrated using a solution that has a known PPM value and you must calibrate them every so often. But nothing beats their convenience. Frequent nutrient solution changes will generally keep the concentration where it needs to be. My best advice is to carefully follow the directions that come with the nutrient you plan to use.
All the nutrients in the world will not do a plant any good if it cannot absorb them easily. A major factor in determining a plant's ability to uptake nutrients is the relative acidity, or pH (the negative log of the hydronium ion concentration) of the soil or solution from which they feed. pH is taken by measuring a voltage (potential) in a solution and registering it on a scale of 0-14 that represents the concentration of hydronium ions in solution. Generally, it is used to determine whether a solution is acidic or basic. If your pH reading registers a 1 on the scale, this represents a high hydronium ion concentration (an acid). Pure water is considered neutral at a pH of 7. A 14 on the scale represents the lowest concentration of hydronium ions (basic, alkaline).
When adjusting pH, it is best to give your fresh nutrient mixture several hours to stabilize before attempting to adjust it. You should also be aware that commonly available pH control products are very powerful, and a little bit too much can sacrifice your entire nutrient solution fast. For first timers, I would even suggest mixing up a single gallon of nutrient solution, letting it sit for a day, and then counting how many DROPS of pH adjustment (up or down) it takes to get it to a range of 6.0 to 6.5. You can then multiply your count by the volume of your reservoir as a baseline for rapid, full reservoir adjustments. Some nutrients may become unavailable to the plant if the solution pH drifts from an optimal reading, which for most plants is between 6.0 and 6.5. This condition is called "nutrient lockout". pH can be tested with litmus paper and adjusted with an inexpensive pH control kit. 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 acts as a buffer and maintains pH and concentration better than a reservoir that is "just big enough to do the job." Nutritional requirements vary throughout a plant's life cycle. In addition, light intensity, stage of growth (vegetative or flowering), and the general size of the plants you are growing all play a role in determining 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 following page outlines these signs 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. As a quick reference, the pH of common solutions are as follows;
Battery Acid = 1 Boric Acid = 5 Blood = 7.5 Ammonia = 11.25
Vinegar = 2.75 Milk = 6.75 Sea Water = 7.75 Bleach = 12.5
Orange juice = 4.25 Pure Water = 7.0 Borax = 9.25 Lye (caustic soda) = 13.5
Since pH and PPM generally share an inversely proportional relationship, by measuring pH, you can sometimes infer what's happening to the concentration of your nutrient solution. These charts attempt to illustrate this principle.
In this example a perfect balance exists between plant requirements, solution pH and nutrient concentration. This is exemplified by steady readings in both PPM and pH over time. Naturally the volume of nutrient solution decreases over time, however, that is not indicated here... Your goal is to deliver exactly what the plant requires - no more -no less - temperature and light intensity play a major role in determining this balance.
The crop is consuming more nutrient than water, note the PPM decrease. Since most nutrient solutions have a pH buffer which tends to pull down the pH, the decrease in concentration results in the rise of pH.
Many times what you may observe to be a nutrient deficiency i.e.: yellowing older leaves, red petioles and stems, may actually be caused by an excess of nutrient or unhappy pH - be sure to use that pH and PPM test kit and meter!
Here the plants leave excess nutrient behind. This imbalance causes PPM to increase, effectively decreasing pH, causing nutrient lockout. Possible causes are high heat/intense light which will increase the plant's transpiration of water as the plants "sweat". Diagnosis of these problems is important. Once you get into a routine with a particular crop and growing environment, you will develop a knack for what should and should not be, making this seemingly complex process simple. Keep a log and LEARN!
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