Runoff pH is the measurement of the acidity or alkalinity of the water that drains from a plant’s container after irrigation. This diagnostic tool provides insight into the chemical environment of the root zone, where the plant absorbs water and nutrients. When the measured runoff pH is significantly lower than the input water, it signals that the growing medium has become overly acidic, a common problem requiring immediate attention. A low reading indicates an imbalance that can quickly lead to plant health issues. Monitoring this number is important because the pH of the medium influences the availability of essential elements to the roots.
Understanding the pH Scale and Nutrient Availability
The pH scale ranges from 0 to 14, with 7 being neutral; values below 7 indicate acidity, and values above 7 represent alkalinity. For most plants grown in soilless media like peat or coco coir, the ideal root zone pH range is between 5.8 and 6.5. This slightly acidic range ensures that mineral nutrients are chemically soluble and available for the plant to absorb.
When the pH of the root zone drops too low, typically below 5.5, “nutrient lockout” can occur. This happens because the low pH alters the chemical form of nutrients, making them insoluble and inaccessible to the plant roots. A highly acidic environment severely limits the availability of macronutrients like Phosphorus (P) and secondary nutrients such as Calcium (Ca) and Magnesium (Mg). The plant may display deficiency symptoms, such as yellowing leaves or stunted growth. Conversely, certain micronutrients like Iron (Fe) and Manganese (Mn) can become excessively soluble, potentially leading to toxicity.
Primary Causes of the pH Drop
The shift toward a lower pH in the growing medium is primarily driven by three factors: the nutrient formulation used, the natural breakdown of the media, and inadequate watering practices.
Nutrient Formulation
Many commercial nutrient solutions are acidic, especially those with a high percentage of nitrogen derived from the ammoniacal (\(\text{NH}_4^+\)) form. When plant roots absorb the ammonium ion, they release a hydrogen ion (\(\text{H}^+\)) to maintain electrical neutrality, which increases the acidity of the surrounding medium. Furthermore, soil bacteria convert ammonium to nitrate (\(\text{NO}_3^-\)) through nitrification, and this chemical reaction also releases \(\text{H}^+\) ions, accelerating the pH drop over time.
Media Breakdown
Growing media that contain organic materials, such as peat moss or coco coir, are naturally acidic and possess a limited buffering capacity. Over time, the organic components decompose or are metabolized by microbes, which releases organic acids as byproducts, gradually lowering the pH of the root zone. If the medium was initially buffered with an alkaline material like dolomite lime, this buffering agent will eventually be consumed, allowing the medium’s natural acidity to reassert itself.
Salt Accumulation
A third major cause is the accumulation of nutrient salts, often resulting from insufficient runoff during irrigation. When plants absorb water, the mineral salts left behind concentrate in the medium. Since most fertilizers are acidic mineral salts, this concentration increases the overall acidity in the root zone, leading to pH volatility and a sharp drop in the runoff reading. This salt buildup is exacerbated when growers use hard water with high alkalinity without appropriate pH-balancing fertilizers.
Immediate Corrective Actions
When a low runoff pH is detected, the most effective immediate action is to perform a thorough flush of the growing medium. Before starting, check the pH of the input water or nutrient solution to ensure its initial value is within the acceptable range for flushing, typically 6.5 to 7.0.
The goal of the flush is to use a high volume of buffered water to dissolve and wash away the accumulated acidic salts and reset the pH of the root zone. Use a pH-adjusted solution, often plain water or a very light nutrient mix, at approximately three times the volume of the container size to ensure complete saturation. For example, a five-gallon pot should be flushed with about fifteen gallons of solution.
Apply the solution slowly, measuring the pH of the runoff periodically as you flush. Continue this process until the runoff pH stabilizes within the desired target range, typically between 6.0 and 6.5. After the flush, finish with one light feeding of a balanced, pH-adjusted nutrient solution to prevent a sudden nutrient deficit.
Long-Term Preventative Strategies
To prevent the recurrence of a low runoff pH, growers should implement routine monitoring and adjust their feeding practices. Regularly monitor both the input solution pH and the runoff pH to catch small deviations early. A consistent difference of more than 0.5 pH units between the input and output is an early warning sign that should prompt a corrective adjustment.
Alternating nutrient feeds with a watering of plain, pH-adjusted water is an effective practice to prevent the buildup of acidic nutrient salts. This regular plain water irrigation helps leach out excess accumulated salts from the medium, maintaining a more stable chemical environment. Choosing a growing medium with a higher buffering capacity, or amending the medium with materials like dolomite lime, can also help resist sharp pH drops over a longer period.