The pH is a measure of the acidity or alkalinity of the nutrient solution. It directly affects a plant's ability to absorb vital minerals.
The pH is measured on a scale of 0 to 14. A pH of 7 is neutral, above 7 is alkaline and below 7 is acidic. The scale is logrithmic, which means there is a ten-fold difference between whole numbers. For example, a pH of 5 is ten times more acidic than a pH of 6.
Most hydroponic crops thrive in a slightly acidic zone between 5.5 and 6.8, where essential macronutrients and micronutrients remain chemically dissolved and available to the roots. Some crops like blueberries may require very acidic conditions and grow best in a pH range of 4.0 to 5.0.
If the pH drifts too high or too low, minerals like iron or calcium bind together and precipitate out of the solution, causing nutrient lock-out and subsequent plant deficiencies.
Because water chemistry changes as plants consume nutrients and release waste, the pH levels must be monitored using digital meters or test drops and adjusted using specialised pH Up or pH Down solutions.
The Sweet Spot
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Target Range: For almost all hydroponic plants, the ideal pH range is 5.5 to 6.8.
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Plant-Specific: Plants have different pH requirements. See the pH requirements for some common plants.
Why the pH Shifts
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Plant Feeding: As plants drink water and absorb specific nutrients, they excrete ions back into the reservoir, naturally altering the pH.
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Evaporation: As water evaporates from the system, the concentration of minerals increases, which typically drives pH levels up.
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Algae Growth: If light leaks into your reservoir, algae will grow and rapidly fluctuate your pH levels between day and night.
Read more about pH fluctuations.
How to Manage pH
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Test Regularly: Use a digital pH pen, liquid test drops, or paper litmus strips at least 2 to 3 times a week. Daily testing may be needed during adverse weather conditions or for intensive, recirculating systems where changes can happen quickly.
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Make Small Adjustments: adjust up or down as needed using commercial pH Up and pH Down solutions. Allow the solution to mix well and make small adjustments gradually until the target pH has been reached
Read more about adjusting the pH.