Coco coir is a popular soilless medium that offers excellent aeration and high water retention, making it an ideal environment for plant roots. Because it is an inert substrate, the plant relies entirely on the nutrient solution provided by the grower. The acidity or alkalinity, measured as pH, of the nutrient solution and the root zone environment plays a direct role in how efficiently the plant can absorb essential minerals. Monitoring the pH of the water that drains from the pot, known as runoff, is a simple diagnostic practice that helps gauge the health of the root zone and ensures nutrients are available.
The Optimal Input and Runoff pH Range
The target pH range for the nutrient solution supplied to coco coir is typically between 5.5 and 6.5, which is slightly acidic. Many growers aim for a tighter band, such as 5.8 to 6.2, to maximize the availability of a broad spectrum of nutrients. This narrow range is suitable because coco coir, despite being an organic material, functions more like a hydroponic medium than traditional soil.
The cation exchange capacity (CEC) of coco coir allows it to hold onto positively charged nutrient ions, including calcium, magnesium, and potassium. Maintaining the correct pH within the root zone prevents these ions from binding too tightly to the medium, making them inaccessible to the plant roots. The runoff pH serves as an indicator of the current state of the root zone, reflecting any salt buildup or metabolic changes since the last feeding. Ideally, the runoff pH should be within 0.5 points of the input pH, indicating a stable environment. A significant deviation suggests that the nutrient balance is shifting, requiring immediate attention.
How to Accurately Measure Runoff pH
Accurate measurement requires a digital pH meter calibrated regularly using fresh pH 4.0 and pH 7.0 buffer solutions for a two-point calibration. This ensures the meter is accurate across the relevant range for coco. The probe should be kept clean by rinsing with distilled water and stored in a proper storage solution to prevent the glass bulb from drying out.
To obtain a valid runoff sample, first, feed your plants with the nutrient solution until liquid begins to drain from the bottom of the container. Collect the draining water in a clean, non-reactive vessel. The best sample is collected once the runoff is consistently flowing, ensuring it represents the liquid that has fully passed through the root zone. Aim to collect a volume of runoff that is roughly 10% to 20% of the volume of the nutrient solution applied. The pH reading should be taken immediately after collection, as the chemical composition of the runoff can change over time.
Identifying Nutrient Lockout Caused by pH Fluctuation
Nutrient lockout occurs when the pH in the root zone shifts outside the optimal range, chemically altering nutrient compounds and making them insoluble and unavailable for absorption. This condition commonly results from an accumulation of excess nutrient salts in the coco coir, which drives the pH of the medium too high or too low. The plant will display deficiency symptoms.
If the runoff pH drops below 5.5, the environment becomes too acidic, which can significantly reduce the uptake of phosphorus and calcium. This often manifests as stunted growth or dark, bronze-colored spots on the leaves due to a calcium deficiency. Conversely, if the pH rises above 6.5, essential micronutrients such as iron and manganese become less soluble. The resulting iron deficiency typically causes chlorosis, or the yellowing of new growth while the leaf veins remain green. Monitoring the runoff pH helps identify these fluctuations before the visible symptoms become severe and irreversible.
Strategies for pH Correction and Resetting the Medium
When the runoff pH indicates a significant imbalance, the most direct strategy for correction is to perform a thorough flushing of the medium. The goal of this process is to dissolve and wash away the accumulated salts that are causing the pH shift. This is accomplished by applying a large volume of correctly pH-adjusted water or a very dilute nutrient solution.
A proper flush involves running a solution that is two to three times the volume of the container through the coco coir. It is recommended to use a half-strength nutrient mix, pH-adjusted to the low end of the optimal range (around 5.5), instead of plain water. Using only plain water can disrupt the CEC of the coco, potentially leading to further imbalances. Growers should use commercially available pH Up or pH Down solutions to make precise adjustments to their water before application. The solution should be thoroughly mixed and tested before being poured over the medium until the final runoff pH returns to an acceptable level.