It's all about understanding ratios and nutrient ppm levels in plants. Go outside those ratios and expect the plant to react negatively. Even then we can only guess since ratios are based on tissue samples. It would be assumed that we are controlling those ppm levels through the nutes, but does 200 ppm of K in nutrient form equate to 200 ppm of K in the tissue sample?
Potassium, Ca and Mg appear to be needed in larger quantities in comparison to other floriculture species. Increasing the levels of K, contrary to the Mulder's Chart, decreases the availability of Ca and/or Mg. Provide K, Ca and Mg in a 4
1 ratio avoids antagonisms. The image below shows how increasing K effects the uptake of Ca and Mg.
Poinsettias need around 200 ppm K to 100 ppm Ca to 50 ppm Mg and since cannabis is similar in that it's a short-day plant it's a good rule to follow.
So now you know that part. The next is understanding how much P a plant needs. A graduate student, Josh Henry, worked on optimizing P fertilization rates for his master’s degree thesis at NCSU. Plants require a baseline level of P to grow adequately. For a continuous fertilization program for plants grown in a soilless substrate, the target concentration is between 8 ppm and 15 ppm of P. Providing levels below that will result in less plant growth, while concentrations above that level provide little benefit.
Since there really is no data on P needs for cannabis increasing those levels from 15 ppm to 25 ppm would be an acceptable range. Higher than that and you risk antagonisms or stimulations. North Carolina State University has begun an experiment looking into optimal P rates supplied at a constant level throughout the cannabis crop cycle and most likely will refine those recommendations in the near future.
The image below shows the response of Alternanthera to increasing concentrations of P from 0, 5, 10, 20 and 40 ppm (left to right).
You can see the growth slowed after 10 ppm and basically stopped after 20 ppm. Phosphorus is also the primary contributor to plant stretch. Too much P will lead to excessive internode elongation and tall plants. That’s why it’s important to limit excessive P in grow applications. We often hear about growers wanting to add bloom enhancers and yet the plants don't want it and ultimately it leads to deficiencies because of overuse. The next image shows the nutrients that are from a tissue sample of a plant. Again notice the percentages and acceptable ranges.
You can use the following ratios as a guide to growing cannabis:
10 parts Nitrogen
1 to 2 parts Phosphorus
10 parts Potassium
5 parts Calcium
2 to 3 parts Magnesium
1 to 3 parts Sulfur
The micronutrients are important and most can be found in tap water(get a water report) but if you don't use it then of course you need to add those which most ferts carry enough of anyhow.
The nitrogen should be mainly in the nitrate form as it provides more compact and controlled growth. More extensive leaf and stem growth occurs with ammoniacal and urea nitrogen. In general, to avoid excessive stretching and oversized leaves, more than 60 percent of the nitrogen provided to plants should be in the nitrate form. Ideally 70 to 80 percent as this will provide a moderate growth response and avoid overly large cannabis plants.
I'm sure I bored many here but there's more that is readily available online to look up. Is it a science to figure it all out? Sure it is and that's why people spend years understanding it all. But if you try and follow the basics you'll do fine. Hopefully that's enough to get you on your way.
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