Fertilizing belongs to one of the most important gardening duties. Yet many cannabis cultivators do not know when the right time is to fertilize, how much to use, or how often. This is in part also caused by the lack of information supplied by the companies selling the fertilizer. To further complicate matters the recommended dosages are often quite confusing and tempt customers to use large amounts of fertilizer! If cultivators lack experience or, despite a good amount of gardening knowledge, they cannot find the cause of certain plant symptoms, it is very likely that they are constantly overfertilizing the plants.
To put an end to this negative trend, and because our strains grow perfectly well with only small amounts of fertilizer, we would like to share important tips from our many years of cultivation experience. By following these guidelines fertilizing your plants can become a secure routine and not a matter of life and death for your plants.
Lets start with the small print on the bottles. What many do not know: the recommended EC value is based on osmosis water with an EC of 0,0!
Example: Ionic recommends an EC of 2,0 for flowering in hydroponics. From this value the cultivators must deduct the EC of their tap water in order to calculate the actual recommended dosage. Let’s say your water has an EC of 0,6. Then we calculate 2,0 – 0,6 = 1,4 EC. The recommended maximum dosage is therefore 1,4 EC.
Canna and other Dutch companies generally recommend even higher values. So the necessity of being aware of this formula is quite apparent. Although one really must ask how the plants should survive long term on values of 2,0 – 2,2 EC, most customers actually use such strong dosages in their grow because they are unaware of this essential information. The result is not only a waste of fertilizer in hydroponic set-ups. Plant care also becomes much more complicated and problematic. Regular flushings with water are necessary at such high EC levels. On the one hand the plants grow very fast (they stretch, get leafy, etc.), and on the other hand they develop complex symptoms of over- and under fertilization from the high salt content in the substrate. In the end massive overdoses of nutrients lead to a high risk of mold in the flowering phase, reduced levels of resistance against pests, lower yields, and possibly pose a health risk from radioactive traces in the cannabis (from phosphate).
For the cultivation on soil there is a common rule among cultivators to use 50% of the recommended dosages. Growers follow this rule without actually knowing what salt concentration they are feeding their plants with. For soil cultivation an EC meter is the most important tool to find out what the proper dosage is. This is why everyone who values their plants should not save on this rather moderate investment of 50,- Euros!
To demonstrate how important the EC value is we have tested 5 popular types of fertilizer. Only the fertilizer for flowering was selected because during this phase growers fertilize the most (after the motto: more fertilizer = more yield) and during this part of the life cycle plants become increasingly sensitive to excess salts.
Before we look at the results it is important to understand one thing: generally one should never fertilizer more than 0,8 EC on soil. That’s why the first important step is to find out what the EC of your tap water is. In this example we are using a 50-50 mix of osmosis water and normal tap water to get 0,43 EC. Obviously, if you have a value over 0,8 in your tap water you have to lower the EC value through filtration. For small gardens it´s sufficient to use a Britt Filter if you do not have the money to buy a household osmosis filter (approx. 150,- Euros). The Britt filter can reduce the EC value by 0,15-0,20. Another alternative would be to buy 5L bottles of mineral water when you fertilize. Good mineral water has approx. 0,25 EC.
Here are the test results on our water with 0,43 EC (measured on Hanna Dist 3 meter).
One Part Bloom
Recommended min. dosage (Hydro): 8ml/1L or 1,5 EC
Recommended dosage on soil: “less” (?!)
Frequency on soil: every second watering
* 2ml/1L = 1,30 EC
* 4ml/1L = 2,00 EC
* 0,5ml/1L= 0,70 EC
The amazing thing about these instructions is the inaccuracy of the amount to fertilize on soil. What does “less” mean? If we were to use the common rule and mix 50% of the recommended dosage we still get an EC of 2,0! That would lead to immediate symptoms of overfertilization. Even at 25% of the dosage we are still in the danger zone. With 0,5ml/L or 6% of the recommended dosage we finally have a safe solution! This example shows how important it is to have an EC meter at hand.
Recommended dosage: 5ml/1L
Frequency: 1-3 x weekly
* 5ml/1L = 1,83 EC
* 2ml/1L = 1,05 EC
* 1ml/1L = 0,76 EC
Out of interest we tested the dosage of 5ml/L. If you were to give your plants this dosage they would not be very happy about it. Surprisingly, the manufacturer recommends this high salt level for soil cultivation – a level that is not even used in most hydro grows! The ratio of nitrogen and potassium is quite dangerous in this fertilizer. Both minerals are present in high amounts but flowering cannabis plants cannot store excess N and K as easily as phosphorous and magnesium. At 20% of the recommended dosage we can water with a safe nutrient solution.
Recommended min. dosage (Hydro): 7ml/1L
Recommended dosage for soil: 1,0 ml
Frequency: no info
* 3,5ml/1L = 1,62 EC
* 1 ml/1L = 0,81 EC
At least the manufacturer offers an EC value for soil cultivation. But, again, if you don’t have an EC meter and mix 50% of the dosage the plants are over fertilized. At 15% we found the correct dosage. The downside is that the N-P-K values are not disclosed on the label which is below standard .
Recommended dosage: 4ml/1L
Frequency: every second watering
* 2ml/1L = 1,38 EC
* 1ml/1L = 1,00 EC
* 0,5ml/1L = 0,75 EC
This concentrated and excellent fertilizer is a good choice for cannabis cultivation – but it is also dangerously potent. Even half the dosage is still too strong and it has to be diluted to 12,5% before we get the ideal value of 0,75 EC. If you have a higher EC level in your tap water you can water with only 10% and get good results.
Feeding frequency on soil
Apart from the actual salt concentration of the nutrient solution the frequency and amount you water is an important aspect. Generally, cannabis plants prefer small but regular feedings. “Small” means in our case a moderate level of 0,6-0,8 EC. You can compare fertilizing to real mealtimes. People also get a stomach ache when they overeat. It’s much healthier to eat small meals regularly. In the same way cannabis appreciates getting small portions of nutrients that can be “digested”. So watering high dosages of fertilizer is like force feeding. But plants aren’t pigs that have to be fattened.
With a nutrient solution of 0,6-0,8 EC you can’t do much wrong unless you make a mistake with the feeding frequency. That’s why it´s important to wait 7-10 days after fertilizing and observe the plants. How are they taking up the nutrients? Do they need a bit more or are they looking healthy enough? A slight deficiency can be easily leveled out (and does not impair flowering) but over fertilization causes irreparable damage. As you learn to observe the plants you will develop an intuitive understanding and can read small signs of nutrient deficiency that signal the best time to apply a mild nutrient solution.
How we fertilize during flowering
To provide the plants with many useful nutrients we first lower the EC level of the water to about 0,45 EC. With this level we either mix a nutrient solution of 0,55 EC and water with this solution every 5 days, or we mix a solution of 0,75 EC and water that every 8-10 days.
If we use this feeding plan during a typical 70 day flowering cycle a plant receives approx. 5 fertilizations with the maximum dosage of 0,75 EC. Why only 5 times? Because in the last 2 weeks of flowering you should stop fertilizing. Let the plants use up the stored nutrients in the plant tissue and soil naturally. In the last weeks before harvest the plants are practically “dying” and absorb only small amounts of nutrients.
This product from General Hydroponics is a real “calorie bomb” with NPK: 0-6-5 and 2,5% magnesium. It’s almost a complete fertilizer. In cannabis cultivation Ripen is a good choice for fertilizing extreme Sativas that quickly respond to nitrogen with unwanted stretching, leafy buds, and retarded flowering. Ripen assists in halting vegetative growth in tall plants quickly during pre-flowering. It can be applied in soil grows with quality potting soil during flowering to accelerate bud development and up till the final 15 days before harvest. Since it does not contain any nitrogen, use it during early flowering only if you have the plants growing in fresh soil and sufficient pot size, so that they have a source for their minimum requirements of nitrogen. The recommended dosage is very high (4-5ml/L) so please check your EC levels for the correct dosage.
What you should watch out for
The quality of the soil and size of pots decides a great deal about the nutrient needs of your plants. If you use strongly pre-fertilized soil it´s possible that your plants don’t require any additional feeding till harvest! A good example are organic grows that use composted material with a high mineral content (like manure, etc.). Some manufacturers sell very potent soil mixes that are almost toxic in their pure form. An example is All-Mix from Plagron which has 2,4 EC! If your pots are too small compared to plant size or maturity you will find that your plants quickly suffer from nitrogen deficiency. Another factor is light intensity. A fluorescent grow cannot be compared to a sodium vapor grow. More light means bigger plants and higher nutrient requirements.
Not only the EC but also the amount of nutrient solution determines salt levels in the soil. The amount you water should not exceed the normal requirements of the plant. If a grown cannabis plant in your garden uses 500ml water daily then you should water that same amount with the nutrient solution. Never fertilize on dry soil!
Outdoors the quality of the soil, plant size, and climate play an important role in nutrient uptake. It’s best to fertilize when warm/sunny days are predicted so that the plants can transform the nutrients directly into growth. Due to plant size the most common deficiency is nitrogen. This deficiency is easy to detect: first the leaves lose their leaf shine and become dull, then the lower sun leaves start to yellow, these symptoms gradually move upward and affect more leaves, in advanced stages the lower leaves dry up and die and the plant has yellowed up to the shoot tips. A flowering fertilizer with a good amount of nitrogen (like AlgoFlash) should be used when nitrogen deficiency appears during or after sexing. When a deficiency is already apparent you can use a higher EC of about 0,9-1,0 to compensate the low level of nutrients quickly.
The second most common deficiency is phosphorus which is required in large amounts for root and bud growth. Phosphorous is stored generously in plant tissue so that if you have a regular feeding plan the plants will get enough phosphorous to last till harvest even when feeding stops about 2 weeks beforehand.
Things to consider
Always purchase high quality fertilizer from reputable companies such as General Hydroponics, Hesi, Bio Bizz, Canna, AlgoFlash, Compo, etc. With these fertilizer brands you will have the guarantee of a product that contains high grade minerals and nutrients that are easily absorbed by the plants. As you can see, fertilizer is not expensive when used at the proper dosage. All of the fertilizers we tested can be used without problems as long as the dosage is correct.
Some growers claim that their plants are thriving on very high EC levels and have no problems whatsoever. There are mainly two reasons for this assertion:
1. These are strains that are genetically predisposed to grow under high amounts of fertilizer (in another article we will elaborate on this)
2. The cultivator does not recognize the negative symptoms/damage from overfeeding (this occurs quite often)
We hope that these guidelines will assist cultivators in growing healthy plants and harvesting generous crops in a more plant and environment friendly way. Mandala growers will especially benefit from these tips because our strains are very efficient in nutrient uptake and grow extremely well with only minimum amounts of fertilization. In the second part, coming soon, you can read what makes Mandala strains so vigorous and easy to grow.
Questions? Our support team will be happy to answer any questions that you may have. Please understand that we can only respond to customers growing Mandala strains.
Fertilizing during the vegetative period
1. Use good size pots with quality soil instead of small pots & bad soil and fertilizing a lot to compensate for the lack of nutrients. If you give the plant enough natural nutrients in the soil then you don’t need to fertilize for the first 25-30 days of growth.
2. Never fertilize seedling or plants that are only 10-14 days old if you grow on soil!
3. Do not stretch commercial soil with a lot of perlite/vermiculite/hydroton/coco coir etc. This only reduces the amount of available nutrients. Don’t fill the bottom of the pots with stones/hydroton. The roots grow down and need a substrate that holds water at the bottom. Good potting soil like Compo Sana Universal already contains enough additives for a balanced air-to-soil ratio and roots get plenty of oxygen. If you are growing outdoors in very compact soil then you should add 20-25% perlite for a better air-to-soil ratio.
4. The same rules on EC levels during flowering apply here as well. Do not water with more than EC 0,8.