PPM and electrical conductivity (EC) measure the concentration of total dissolved solids, primarily nutrient salts, in a liquid solution. PPM quantifies the number of nutrient particles in a million parts of the solution. EC, measured in units like millisiemens per centimeter (mS/cm), is a direct measure of the solution’s ability to transmit an electrical current, which increases with ion concentration. When growing in coco coir, “runoff” refers to the nutrient solution that drains from the bottom of the container after feeding the plant.
Why Runoff Testing is Essential for Coco Coir
Runoff testing is necessary when growing in coco coir because the substrate is chemically inert and offers almost no nutritional buffering. Unlike traditional soil, coco coir functions more like a hydroponic medium. This means the plant’s entire diet must be delivered through the nutrient solution, making precise monitoring of the root zone paramount.
Coco coir has a high Cation Exchange Capacity (CEC), allowing it to hold onto positively charged nutrient ions like calcium, magnesium, and potassium. While CEC provides a slight buffer, it can lead to imbalance if not managed correctly. Mineral salts from the nutrient solution can accumulate rapidly, increasing the root zone’s concentration of dissolved solids, which leads to nutrient lockout or toxicity.
Regularly measuring the runoff ensures salts are not building up to problematic levels in the root zone. Comparing the concentration going in to the concentration coming out determines if the plant is consuming nutrients or if salts are accumulating. Failure to monitor runoff often leads to salt-related issues, manifesting as deficiencies or burn. A consistent runoff regime is required to flush away excess salts and maintain a balanced root environment.
Collecting and Calibrating Your Runoff Measurement
Accurate measurement requires a calibrated PPM or EC meter, a handheld device designed to measure the electrical conductivity of a liquid. Before testing, the meter must be calibrated using a standard solution of known PPM or EC to ensure reliable readings. It is important to know whether your meter uses the 500 scale or the 700 scale for PPM, as this conversion factor affects the numerical reading. The 500 scale is common in North America, while the 700 scale is often used in Europe.
The measurement process begins by feeding the plant until a significant volume of runoff is achieved, typically 10 to 20 percent of the total volume poured in. The runoff sample should be collected from the final portion of the draining solution, as this liquid best represents the salt concentration in the root zone. Waiting 10 to 15 minutes after the initial feed allows the nutrient solution to fully interact with the medium before collection.
When collecting the sample, use a clean, non-porous container to avoid contamination. The goal is to compare the input PPM with the runoff PPM. This comparison reveals whether the plant is actively feeding (lower runoff PPM) or if excess salts are pooling in the medium (higher runoff PPM). Using the more universal EC measurement is often preferred by experienced growers because it avoids the confusion caused by variable PPM conversion scales.
Ideal Runoff PPM Targets by Stage
The ideal runoff PPM is a dynamic range that changes as the plant progresses through its life cycle. The general goal is for the runoff PPM to be slightly higher than the input PPM, indicating a healthy root zone concentration without excessive salt buildup. A difference of no more than 100 to 200 PPM (on the 500 scale) higher than the input is considered the benchmark for a stable system.
During the Seedling and Cloning stage, plants require low nutrient concentration, and the target runoff PPM should be between 200 and 300. In the Early Vegetative Growth phase, demand for nitrogen and macro-nutrients increases, and the runoff PPM should climb to a range of 500 to 700. For Late Vegetative Growth, the robust and rapidly growing plant allows for pushing nutrient strength further, maintaining runoff PPM toward the upper end of the 700 range.
The plant’s needs shift during the transition to flowering. In the Early Flowering or Transition stage, the plant undergoes a significant stretch, and the runoff PPM should be between 700 and 900. Mid-Flowering represents the peak of nutrient demand, and the runoff PPM can be maintained in the highest range, typically between 800 and 1000. In the Late Flowering or Ripening stage, nutrient strength is intentionally reduced to prepare for harvest, so the target runoff PPM should gradually drop to around 500 before the final flush.
Interpreting High and Low Runoff Readings
Interpreting the difference between input and runoff PPM dictates the necessary corrective action. If the runoff PPM is significantly higher than the input PPM—for example, more than 300 PPM higher—it indicates an undesirable buildup of unconsumed nutrient salts within the coco coir. This salt accumulation increases osmotic pressure around the roots, making it difficult for the plant to absorb water and nutrients, which can lead to nutrient burn or lockout.
To correct a high runoff reading, the primary action is to increase the volume of runoff by feeding more solution at each watering event. This increased volume helps flush away excess salts and reset the medium’s concentration. If the PPM remains persistently high, a full flush is required. This involves running a large volume of plain water or a very light nutrient solution through the medium until the runoff PPM drops back into the acceptable range.
Conversely, if the runoff PPM is lower than the input PPM, it indicates the plant is consuming nutrients faster than they are being supplied. A slightly lower runoff is normal, but a significantly lower reading suggests the plant is underfed and rapidly stripping the medium of available nutrients. The corrective action is to gradually increase the strength of the input nutrient solution. This increase should be done in small increments over several feeding cycles to avoid shocking the plant or causing immediate salt buildup.