* All ppm numbers on this page use the U.S. 500 scale.

In our Case Study on Cannabis Root Zone pH Monitoring article we talked about using Osmocote and some of the extra things one must consider when deciding to grow cannabis with a time release fertilizer. Here we continue the conversation in order to point out some of the other issues one might find themselves experiencing with time release fertilizers.

Many people have the misconception that using a time release fertilizer for growing plants is a no brainer. You add Osmocote to the growing medium and then use plain water for the duration of the growing season, what could be simpler. Turns out that using a time release fertilizer indoors requires closer monitoring when compared to liquid fertilizers. For example, lets say you are monitoring the pH and ppm values of the runoff water every few days as you should be. Let’s also say that your plant(s) are in 2 gallon pots and each has 2 1/2 tablespoons of the time released fertilizer mixed into the top layer of an inert growing medium like peat or coco and perlite. Then you are watering your plants with a pH adjusted RO water of 10ppm tds and a pH around 8-8.5 to compensate for the acidity of the Osmocote. Then you test the runoff water on Monday, Wednesday, and Friday after watering the plants and you record the data. On each of the three days the pH values are about the same hitting your target area but you notice that your ppm tds values are kinda erratic and not what you might expect since you have added nothing to the plants other than pH adjusted RO water. So what could be happening here? One thought is that over time the Osmocote fertilizer sheds less or more nutrient causing the fluctuations, but your recorded numbers are 250ppm on Monday, 540ppm on Wednesday, and 300ppm on Friday. So the shedding theory doesn’t really work.

But then you realize that when you water your plants you are not measuring the exact amount of water being applied each time. So on Monday you maybe applied 1/2 gallon of water, on Wednesday it was more like 1/4 gallon, and on Friday maybe around 1/3 gallon. So each of the three waterings will have a different concentration of nutrients in the runoff water. This is a really important finding and has to be addressed or you will have problems. And maybe where you were pouring the water had the most Osmocote further distorting your runoff test data.

Larger plants might not be affected too much over time by these fluctuating ppm values but smaller plants could freak out. Consider if you were to apply 1 tablespoon of Osmocote to a small plant in a one gallon pot and then water it with 1 cup of water, the runoff would likely be around 260ppm tds since the soil is fresh and loose and the water runs straight through. However, if you have a larger plant that has been growing in a one gallon pot in a peat based medium (which tends to compact over time) for a few months, that same 1 tablespoon + 1 cup of water scenario could produce a runoff of 520ppm because it takes considerably longer for that water to get from the top of the soil and out the pots drain holes. Because the Osmocote has more time to sit in the water solution it will shed more nutrients and can increase the nutrient ppm concentration by 100% or more and that’s no good.

Another potential issue is that the popular Osmocote pictured above is a 15-9-12. A few weeks after you switch to a flowering cycle the plants Nitrogen needs will diminish. Not reducing the amount of Nitrogen being applied (since Osmocote is mixed into the soil) can cause a buildup in the soil with nowhere to go, so your plants might begin showing signs of N toxicity, dark green leaves with rust colored blotching and the extra Nitrogen can even delay flowering. The resulting increase of salinity in the soil can also lockout Potassium. A 15-9-12 fertilizer is fine for the vegetative cycle but not so much for flowering and this Osmocote lacks Calcium so you are going to need to address that as well. Click here to see the average nutrient levels of each element that are needed for growing cannabis.

So when using time release fertilizers you need to determine exactly how much water you would be applying against how much Osmocote is in the soil against how compact the growing medium is (how fast the drainage is). If you don’t consider these three variables at just about every watering then you can lose control of your plants soil/root zone environment. In addition to that, using a time release fertilizer like Osmocote requires pH upping your RO water to an unusual 8-8.5 to compensate for the acidic fertilizer in the soil even if you have mixed lime into the medium for pH buffering before planting. Multiply this by 10 or 20 plants and you might decide that growing with time release fertilizers indoors can be a bit of a hassle. Not to mention the cost of Osmocote has nearly doubled in the last year.