Fertigation is adding fertilizers to the irrigation water.
It is the way that we deliver both water and nutrients to the plants. A tremendous amount of confusion exists in the cannabis growing community about how plants interact with water and nutrition. It is important to understand that although you mix the nutrients with the water, the plant takes them up separately. Water is pulled across root membranes through osmosis and nutrients enter the plant separately, through diffusion, passive transport, and active transport.
Effective fertigation is the key to maximizing the speed of growth and the size of harvests. In order to fertigate effectively there are 5 key principles to follow.
Principle #1: Fertigation is a Balance
Osmosis and nutrition are two separate processes and they must be balanced.
Osmosis and Electrical Conductivity (EC)
Osmosis is controlled by the salinity of the nutrient solution. Electrical Conductivity (EC), is a measure of how saline (salty) the water is. The fertilizers that we add to the water are different forms of salts. If you put a lot of nutrients into the water, it becomes very salty (High EC) and the plants will struggle to pull in water through osmosis. If the nutrient solution is very strong, osmosis can reverse, and water can actually be pulled out of the plant. This is the main reason we must be careful with the total dose of nutrients applied. We limit the amount of nutrients, not because of nutrition, but because of osmosis!
Nutrient Element Ratio (NER)
On the other side of the equation, if you don’t put enough nutrients in the water then the plant cannot absorb nutrition, because it isn’t there. Cannabis plants do best when nutrients are provided at the correct Nutrient Element Ratio (NER) for each stage of growth. All cannabis feed charts alter the ratio of nutrient elements over the course of the life cycle. It is important to follow these feed charts to provide the correct NER for your plants. However, the recipes in the feed charts usually produce nutrient solutions that are way too strong in EC.
Balance Osmosis and Nutrition
To maintain the balance, nutrients must be added to all irrigation water at the correct ratios and doses. The feed chart deter mines the ratio, and EC determines the dose.
When the balance is struck, fertigation fulfills the plant’s water needs and delivers nutrients to the plant in their most easily absorbable form. The plant has access to everything it needs, all the time, at the correct ratios – and growth explodes!
Principle #2: Always Fertigate: Provide Fertilizer at Every Irrigation
Many guides recommend alternating nutrient solution with “plain”, “pHed”, or “CalMag” water. Although this may help prevent over-ambitious growers from burning their plants, it is not the correct practice to follow in a carefully managed grow. For a variety of reasons, it is much better to fertigate with the appropriate ratio and strength of nutrition at each and every irrigation event.
In Coco All Irrigations Should Be Fertigations!
Plants Need Nutrition:
In a soil-less media like coco, the plant takes nutrients directly from the water. Nutrients must be present in the water for plant nutrition to take place. Buffered coco is wonderful because it does not interact with the Nutrient Element Ratio (NER) and you, the grower, can precisely control the nutrients available to the plant. This, of course, is done by providing the proper ratio of nutrients at each and every irrigation (fertigation).
Consistent EC = Fastest Growth:
In order to achieve the best growth rates in any media, the EC of the nutrient solution available to the plant must be consistent. As discussed above, plants draw in water through osmosis. In order to do so, plants must adjust the concentration of sugars in their roots so that it is higher than the EC of the nutrient solution. When roots are exposed to alternating irrigations with nutrient solution and “plain” water, they deal with a constantly fluctuating EC. As a result, the plants are forced to adjust constantly. Those adjustments cost the plant energy that would otherwise have gone into plant growth. One of the key benefits of coco is that it is relatively easy to maintain a consistent root zone EC by applying fresh nutrient solution at each irrigation (fertigation).
Principle #3: Always Fertigate with 10-20% Run-Off
When you add nutrient solution, you are adding both salts and water. As nutrient solution sits in the media, transpiration and evaporation remove the water but leave behind the salts. This causes the EC of the nutrient solution suspended in the media to rise gradually between fertigations.
You may think that since the EC is rising in the media, you should lower the EC of the new water added (inflow). However, in most circumstances the best practice is to use fresh nutrient solution at the appropriate EC and apply it with at least 10-20% run-off. If “plain” water or dilute nutrient solution is used instead, you risk over-correcting and destabilizing the root zone EC.
Run-Off Removes Excess Salts
Run-off carries away the excess salts and allows you to maintain a consistent root zone EC. Because its function is to remove salts, you always need to remove run-off from the saucer or collection area quickly. If pots are allowed to sit in their run-off, it will be reabsorbed into the pot, and the salts are not removed. Be sure to see our DIY Tutorial for making your own “DIY Self-Draining Saucers”, which handle this task for you.
If run-off is insufficient, the salts will accumulate and the EC of the nutrient solution in the root zone will go up, potentially burning your plants. The EC of the run-off should be slightly higher than the EC of the nutrient solution inflow. When this is the case, the run-off is doing its job.
Principle #4: Fertigate Frequently: Do Not Let Your Coco Get Dry
Plants Grow Best Under High-Frequency Fertigation
High frequency fertigation is widely recognized as offering the best possible growth. High-frequency fertigation with run-off ensures that the plant always has access to all the water and nutrition it needs at the correct doses. In soil, high-frequency irrigation is not possible because soil retains too much water and the roots would drown from insufficient oxygen. However, it is important to remember that coco is not “soil”.
Don’t Treat Your Coco Like Soil!
Many guides suggest that coco can be managed just like soil with irrigations only once or twice per week. Although plants can survive for a while in dry coco, it is not good for them. Infrequent irrigations starve the plant of water in two ways and can potentially lead to “nute burn”. For one, plants have less water because there is less water being added. However, the more dangerous aspect of infrequent irrigation is that the EC within the media climbs steadily between irrigation events due to transpiration and evaporation. If the plant sits in the nutrient solution for many days the EC can reach damaging levels.
Coco/Perlite Enables Very High Frequency Fertigation
One of the miraculous properties of coco, especially when mixed with perlite, is that it is nearly impossible to over-water it. Unlike soil, coco maintains sufficient oxygen in the root zone even at field capacity (complete saturation). In soil, you need to allow it to dry because at field capacity there is insufficient oxygen in the root zone and roots can drown. This is not the case with coco.
Water with nutrients should be given every single day from the time the sprout emerges until the day before harvest. Once per day fertigation is best for small plants, but as they mature during vegetative growth and into flowering, it is best to increase the frequency of fertigations. Twice daily is often adequate throughout vegetative growth, but during flowering it is best to fertigate 3-5x per day depending on container size. Specific guidelines for determining the fertigation frequency in your grow are provided in our tutorial, “How to Water Cannabis in Coco”.
Automatic Watering Systems
Automatic watering systems are an excellent idea and are almost required to truly follow this guide. It is only reasonable to fertigate 3-5 times per day if you have an automatic watering system installed. We have a complete "Guide to Automatic Watering for Indoor Cannabis" and plans and instructions for you to build your own “DIY Automatic Watering System”.
Principle #5: Manage Electrical Conductivity (EC)
Managing EC is the key to unlocking the full potential of coco and achieving the best harvests. You need to monitor the EC of the nutrient solution you are adding (inflow) and the EC of the water that drains through and ends up in the saucer of each plant (run-off). The critical number is the difference between the two.
Be sure to read our article, “Understanding Osmosis and EC”.
Managing EC Allows You to Avoid “Nutrient Burn”
The wilting and burned tips that are often known as “nute burn” are not a result of an overdose of nutrients, but are actually caused by the plant being unable to get water through osmosis because the EC is too high. Tracking EC allows you to know and control the salinity of the water and the amount of nutrients that are available to your plant. It is important to set good targets for inflow EC. However, the EC of the inflow is only one factor that determines the EC of the water available to the plant. The number one cause of high EC is evaporation, which removes water, but not salt. Frequent fertigation and adequate run-off are both tools to control the EC of the nutrient solution available to the plant.
Measuring EC is Cheap and Easy
Reliable meters can be purchased for as little as $10-$20. I use a HoneForest meter, and I have verified it’s accuracy against other meters. The technology involved is simple and you do not need to invest in an expensive EC meter. Tracking both inflow and run-off EC is important.
If Salts Accumulate, The EC of Run-Off Will Rise
By monitoring the EC of the run-off you will have advance knowledge of any salt accumulation problems before they affect the plant. Salts don’t actually accumulate in the coco itself, because the cation exchange sites will be buffered with Ca and Mg. However, salt can accumulate in the nutrient solution that is suspended in the media. Measuring the EC of the run-off during a fertigation is a close proxy for measuring the EC of the nutrient solution available to the plant. If the run-off EC number becomes too high, then you can take the appropriate remedial actions. See our tutorial, “How to Manage Run-off EC”.
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