Parts Per Million (PPM) is a standard agricultural unit used to quantify the concentration of dissolved mineral salts, or nutrients, within irrigation water or a nutrient solution. It represents the mass of a substance relative to the total mass of the solution. Controlling this concentration is important throughout the plant’s life cycle, especially near the end of the growth phase, as it impacts the final quality of the harvested product.
Understanding PPM and Electrical Conductivity
PPM is an indirect measure of nutrient concentration, typically derived from Electrical Conductivity (EC). EC measures how well a solution conducts electricity, a property directly linked to the presence of charged mineral ions from dissolved nutrient salts. The more dissolved salts in the water, the higher the electrical conductivity. Growers use calibrated meters to measure EC, which is often expressed in milliSiemens per centimeter (mS/cm).
The meter converts the EC measurement into a PPM reading, representing the total dissolved solids (TDS). This conversion is not universal, creating confusion for growers. Different meters use different conversion factors, such as the “500 scale” (based on sodium chloride) or the “700 scale” (based on potassium chloride). Because of these varying standards, EC is considered the universally consistent measurement of nutrient strength, and many professionals rely on it rather than the inconsistent PPM conversion.
The Purpose of Nutrient Reduction Before Harvest
Intentionally lowering the nutrient concentration in the final weeks of growth improves the end-user experience of the harvested crop. Throughout the vegetative and flowering phases, plants are supplied with a full spectrum of mineral salts to support rapid growth. A portion of these excess nutrients, particularly nitrates and other mineral ions, can accumulate and be stored within the plant’s tissues.
Stopping the supply of external nutrients forces the plant to metabolize the reserves of stored compounds present in its leaves and flowers. This metabolic shift is important because residual mineral salts negatively affect the final product’s quality. Harvesting plants without this final nutrient reduction means remaining salts can lead to a harsher taste and a less smooth experience when consumed.
The presence of excess minerals can also interfere with the development of desirable compounds, such as aromatic terpenes and cannabinoids, in certain crops. Reducing the nutrient load promotes a cleaner profile, resulting in a more pleasant flavor, enhanced aroma, and better burning qualities.
Recommended Target PPM Ranges at Harvest
The main goal for the final stage of growth is to achieve a nutrient concentration in the growing medium nearly identical to the pure water being supplied. The target PPM for the input water is effectively zero, meaning the grower feeds only plain, untreated water. This water may still have a baseline PPM reading depending on the source, ranging from 0-10 PPM for reverse osmosis (RO) water to 50-300 PPM for tap water.
For the runoff water that drains from the growing medium, the target PPM should be very low, indicating that stored salts have been depleted. In hydroponic or soilless media, which have little buffering capacity, the goal is for the runoff PPM to drop into the range of 0 to 400 PPM. A professional target is often set to an EC reading below 0.4 mS/cm, equivalent to 200 PPM on the 500 scale or 280 PPM on the 700 scale.
For plants grown in traditional soil, which naturally retains nutrients and buffers chemical changes more effectively, the final runoff PPM may remain slightly higher, but the principle is the same: the runoff PPM should be as close as possible to the input water PPM. Maintaining a low, near-zero nutrient delivery for the final phase is a consistent requirement across all growing methods to ensure a clean finish.
Duration and Monitoring of the Pre-Harvest Flush
The period of nutrient reduction, commonly referred to as flushing, typically lasts between 7 and 14 days, depending on the growing medium used. Inert soilless substrates, such as rockwool or coco coir, flush out excess salts more quickly, often requiring only one week. Traditional soil, which holds onto nutrients, generally needs a longer period, sometimes extending to two weeks or more.
During this time, growers should only irrigate the plants with clean water adjusted to the appropriate pH level for the crop and medium. The primary monitoring method involves regularly measuring the PPM or EC of the water that drains out of the container, known as the runoff. This runoff measurement provides a real-time indication of the remaining salt concentration within the root zone.
The flushing period should continue until the runoff PPM stabilizes at the desired low range, confirming that the majority of stored salts have been leached out. Growers must avoid overwatering during this process, as excessive saturation can stress the roots and potentially lead to other issues. Consistently checking the runoff allows the grower to accurately determine when the plant is ready for harvest, ensuring the nutrient reduction process is complete.