pH
It is important to observe the pH of your leachate, that is the solution that is draining out of your pot after an irrigation event. pH is largely tied to nutrient availability. While we might be feeding the crop with a solution that is on target, the substrate can be a bit of a confounding factor. Looking at the drain that comes out of the pot is a good way to get an idea of what is happening in your growing matrix. A pH that is significantly higher or lower than what you are feeding the plants is an indication that there is something happening to your nutrient availability in the substrate, and that you may be at risk of nutrient deficiency in the crop. Different substrates (pine bark vs coir vs peat) are going to have different baseline levels of acidity, which will impact the interactions between the plant and the irrigation feed.
Target value should be between 5.0-5.8, depending on your substrate and the crop you are looking to grow.
EC
The EC of your leachate solution will give you an indication of the salt level in the substrate. While we know the EC of our feed solution, it is important to directly compare with what we are getting after the plant has been fed. An extremely low EC value in the drain sample indicates that there is very low salt in the substrate, and that the plants likely require a higher EC in the feed solution to make sure that the plants are getting all of the nutrients that they need. A high EC value indicates a high concentration of salts in the substrate, and action should be taken to either flush the pots with straight water (depending on how high this value is) or reduce the EC of the feed solution to bring the growing media back to a more balanced nutrient content.
Target values should be 10-25% higher than your feed solution. A feed EC of 1.5, and a drain EC of 4 indicates there is a problem with salt accumulation in the pot.
Volume
The volume of liquid that drains out of your growing container/bag is a good way to monitor if your plants are getting sufficient water throughout the day. Leachate is a necessary evil for substrate production. Ideally the leachate sits between 10-20% of the total volume that was fed to the crop. If you find that your drain is lower than this percentage, it is recommended to increase the amount of water fed to the crop. Not only is water required for the plant to grow, but it also acts as a cooling mechanism through the heat of day. Plants that are actively transpiring water are much happier than ones that are experiencing a shortage of water, while still keeping up with the demands of fruit production and vegetative growth. When considering higher % drainage values, it is important to consider the time of year. 30% leachate in the spring or in the fall is not necessary. There is a significant amount of water and fertilizer that is being wasted at this point, not to mention the spike in humidity associated with the excess water which could lead to other problems in the crop. 30% leachate during the peak of the summer, with high temperature and radiation levels is a different story. The plant is very actively transpiring at this point, and you don't want to run the risk of under-watering your crop. Providing excess water at this stage in production is a good strategy to reduce the temperature of the plant and allow it to maintain peak productivity.
Photo 1. Here is an example of a very basic leachate collection container. It is important to collect the drainage from a series of pots that are representative of the crop. That means that your monitoring station should not be raised up above the rest of the pots, as your sample is now going to dry out faster than the rest of the plants, and lead to overwatering due to a response to the measured values.
How can I monitor this?
Keeping an eye on your irrigation solution is easy to incorporate into the daily routine. Ideally you are monitoring the volume of water coming out of your emitters every day, in addition to the amount of water draining out of the pot. It is best to set out your collection bins before the first irrigation event of the day, and collect/assess that solution 24 hours later, before the first irrigation event of the following day. This 24 hour period will take fluctuations in your irrigation volume into account - the amount of water draining from your pot after the first irrigation event of the day should be little to none. The amount draining from the pot at 2 pm on a hot sunny day is going to be quite a lot.
Photo 2. A basic example of a drip irrigation monitoring setup. Photo credit: Talia Plaskett
In terms of equipment, basic plastic containers are all you need to collect the drain solution. To set up a drip monitoring station, be sure to install an extra dripper along your irrigation line that can feed directly into your monitoring station. After that, you will need an EC/pH meter, as well as something to measure the volume of liquid. And last but not least, a notebook or form of electronic record database, to record the values you observed.
For more information on irrigation management:
- 2020 - 2021 Getting Into the Weeds Session on Irrigation Management
- Contact Talia Plaskett, our Protected Crop Specialist