Monitoring Output PPM (EC) and Frequent Fertigation Practices
When considering the target EC for your nutrient solution (inflow), you need to understand that the EC will rise between fertigation events. Water is removed through transpiration and evaporation faster than salts are removed. This causes the EC of the nutrient solution suspended in the coco to rise between fertigation events. Therefore, the target for the EC of the inflow is always lower than the maximum acceptable run-off score.
Plants draw water across the membranes of their roots through the process of osmosis. The force that actually drives the movement of water comes as a result of osmotic pressure that is only partially controlled by the plant. Osmosis always pulls water from a less concentrated solution to a more concentrated solution. Under fertigation, the nutrient solution is on one side of the root membrane and water inside the root is on the other. Plants produce sugars in their roots which raises the concentration of the solution within the root and causes water to enter from the nutrient solution.
The fertilizers that we use are different forms of salt. If you put too much salt in the water then the plant will not be able to pull water in from the nutrient solution. The plant will struggle to get water as the nutrient solution gets saltier. If it is very salty, osmosis can reverse and water can actually be pulled out of your plant. (nutrient burn)
EC is important because it measures the salinity of the water. Managing EC effectively encourages healthy vigorous plant growth. As discussed further below, failure to control EC will slow plants and can cause serious problems with dehydration.
EC stands for “Electrical Conductivity. Both EC and PPM measure the exact same thing, they just use different numbers to express the information.
What Happens to the Water and the Salt?
When you add nutrient solution to the coco, you are adding both water and salts. To understand how EC changes between fertigation events, it is important to consider what happens to each. Plants do not absorb nutrient solution “through a straw” or “like a sponge”. Water and salts (fertilizers) are taken up by the plant separately. Under normal conditions, water is removed about four times faster than salts are absorbed. As a result, the EC of the nutrient solution in the media will rise between fertigation events.
Where Does the Water Go?
The Plant: Plants absorb water through osmosis leaving increased concentrations of salt in the nutrient solution in the coco. Plants also absorb nutrients and thus lower the quantity of salts in the solution. However, they remove the water much faster than they remove the salts. The vast majority of this water moves through the plant and is lost through transpiration. The water that goes to the plant raises the EC of the nutrient solution in the coco.
Evaporation: As water sits in the pot suspended in the coco between fertigation events it is constantly evaporating. The water that is lost to evaporation causes EC in the remaining nutrient solution to increase. Evaporation raises the EC of the nutrient solution in the coco.
Because of transpiration and evaporation, the EC in the coco is lowest immediately following a fertigation and “Peak EC” occurs just prior to the next fertigation event. The fertigation events themselves are responsible for lowering the EC in the coco.
It may seem counter-intuitive to lower the level of salts by adding nutrient solution, but the process occurs through run-off.
The Run-off: As fresh nutrient solution is added to the coco it pushes out old nutrient solution which had been suspended in the coco. The run-off is not the same water that you are adding in the top.
Run-off is higher EC than inflow because it has been sitting in the coco and subjected to the forces of transpiration and evaporation. Run-off carries salts out of the coco. Following each fertigation, the nutrient solution in the coco is closer to the inflow EC.
Run-off lowers EC by flushing excess salts.
Water is removed through transpiration and evaporation, whereas salt is only removed via run-off. As a result, frequent fertigation with adequate run-off is essential to manage EC and get the best results from your coco grow!
High Frequency Fertigation:
It is important to provide nutrient solution that is the correct EC , but the EC rises as the water sits in the pots. As a result, root zone EC is generally higher than inflow EC.
Because EC gradually rises as the nutrient solution sits in the pots, fertigation frequency is one of the main ways that you need to manage the EC of the nutrient solution in the root zone. If the time between fertigation events is long, then the difference between the EC of the inflow and the EC of the nutrient solution in the root zone will be large.
Measuring the EC of the nutrient solution suspended in the root zone is impractical. However, the run-off that is generated at fertigation events is a good indication of the water that had been available to the plants. We measure run-off EC as a proxy for measuring root zone EC.
If the output is high the best solution is to increase fertigation frequency. The most likely cause of high EC run-off is excessive losses to evaporation between fertigation events. Increasing frequency will help to prevent EC from drifting too high between fertigation events.
If increasing frequency is not practical or not helping, then you should increase quantity of nutrient solution applied at each fertigation to ensure a thorough saturation of the media. Increasing the quantity of run-off should help lower the quantity of salts in the root zone. Quantities may be reduced as EC returns to the acceptable range.
Only if run-off EC is dangerously or persistently high, should flushing with a lower EC solution be considered.
When the run-off EC starts to climb, the best course of action is to increase the fertigation frequency. If this is not possible, the second-best option is to increase the quantity of run-off at each event. Only when these methods cannot control the run-off EC within the acceptable limit, should you consider lowering the EC of the inflow or “Flushing to Lower EC”.
Coco For Cannabis EC Run-off Guidelines:
| If Run-off EC is:(Compared to inflow) | Status | Action |
| Lower than inflow EC | Anomaly | Not a problem. Should self-correct. |
| Within +200* | Ideal | Monitor EC twice weekly |
| +200 to +300 | Acceptable | Monitor EC daily, consider increasing fertigation frequency |
| +300 to +400 | Caution | Increase fertigation frequency and/or Increase quantity of run-off. |
| +400 or Higher | Danger | Consider flushing |
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