PH is the measurement of Acidity to Alkalinity on a logarithmic scale from 0-14. A PH reading of 7.0 is considered neutral, whereas a PH of 6.0 is 10 times more acidic than 7.0, and a PH reading of 8.0 is 10 times more alkaline than a reading of 7.0 PH. Temperatures and the amount of hydroponic nutrients in a solution affect PH readings in the water reserve. Thus, it is recommended that if you are going to test the PH of your solution, you do it after you add nutrients and not necessarily before as the nutrient content in a reserve directly affects PH. Algae build-up in a reserve can cause PH to go up during the day as it consumes acidic compounds like carbon dioxide. Higher levels of phosphorus and nitrogen decrease PH while higher concentrations of calcium and potassium in a nutrient reserve drives PH up. When the PH level rises above 7.5, it can lead to iron deficiency in plants as the alkalinity of a high PH can block the ability of the plants to absorb iron.
A crash in PH can occur when PH drops below 4.0, resulting in magnesium and calcium deficiencies or copper and iron toxicity in plants. Thus, the ideal PH growing range for most Hydroponic plants is 5.5 to 7.5. A few exceptions to this ideal range in PH would be berries which prefer 4.0 to 5.0 PH and kale, onions, and peas which prefer a more alkaline range between 6.0 to 8.0. Yet, constant adjustment of PH through the use of PH Up or Down disrupts the plant's own ability to correct PH as it changes over time. Thus, it is recommended that if PH up or down is used, that it be added only after the nutrients are added at the very beginning of a grow cycle and not during the grow cycle itself as PH naturally fluctuates as the plants uptake nutrients throughout the grow.
TDS (Total Dissolved Solids) or PPM (Parts per Million) is another measurement in Hydroponics that can be confusing for the beginning gardener. Like PH, TDS fluctuates throughout the grow cycle and is also affected by temperature changes throughout the day. The reason TDS changes as the nutrient reserves heats up is due to the fact that TDS is based upon the EC or Electrical Conductivity reading of a hydroponic reserve. Since most hydroponic nutrients are formulated with different types of salts that conduct electricity, an electrical conductivity reading measures the amount of ions dissolved in a solution and therefore the strength of the hydroponic nutrients found within a water reserve. Since sodium or salt conductivity increases when heated, this causes the TDS readings to naturally fluctuate throughout the day, even in a hydroponic reserve that has nothing growing in it.
When TDS is calculated it is often written in PPM or Parts per Million, so essentially TDS is PPM. The confusion with calculating TDS lies in the ke conversion factor used to calculate TDS/PPM from EC in the following formula: TDS = ke*EC in millisiemens (mS). While most TDS meters use one of two conversions calculations, the ke factor varies between 500 to 700. Thus, a 442 conversion (measuring 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) would calculate an EC reading of 3.0 mS by assigning 700 to the ke factor and multiplying it by the EC reading of 3 millisiemens (mS) providing a reading of 2100 ppm (700 x 3 = 2100ppm). Yet, a meter using the NaCl conversion would assign a reading of 500 to the ke factor, resulting in a reading of 1500 ppm for the same solution (500 x 3 = 1500ppm). That's a discrepancy of 600 ppm between meters! And as all hydroponic gardeners know, these divergences in calculations can raise havoc in trying to properly assess the dosage nutrient levels in a hydroponic reserve. Thus, the safest way to measure TDS is to know the ke conversion factor used by your meter and match that on all subsequent TDS meters you purchase to that conversion, or simply measure only the EC reading of your hydroponic reserve and calculate TDS yourself. The following provides a list of recommended TDS levels for common vegetables:
860 – 1260 TDS/PPM
1120 – 1400
1260 – 3500
1260 – 1540
1280 – 1400
1750 – 2100
- Brussel Sprouts, and Cabbage
1960 – 2450
Remember that for vegetables that take higher levels of TDS, you can also purchased specialized hydroponic nutrients that are formulated to fulfill the specific needs of your particular plant. The dosing recommendations on those calibrated formulas would also help match the recommended TDS levels and adjust the PH naturally without the need to measure and readjust TDS. Keep in mind that a general rule to follow in Hydroponics is to completely change out your nutrient reserve every time you add water and nutrients. Simply adding nutrients without a complete water change can severely alter the PH and ppm content of your reserve, thus a complete change each time you add water is recommended. Thus, my recommendation for whether you need PH and TDS meters is to simply try growing your plants following the dosing recommendations on your hydroponics nutrient packages, usually 1-2 teaspoons per gallon of water. Then, if you have any problems with your plants, you can invest in PH and TDS meters to determine whether you need to adjust PH or provide a different amount of nutrient dose to care for your hydroponic plants.
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