I've wondered if the Sohum was too hot. They do say it's too hot for direct seeds and seedlings. They say to make a cone of light mix for seeds and seedlings so they are more mature when the roots grow into the Sohum.
Thanks for asking this question!
This one? It has NPK 0.5 - 0.5 - 0.25
Compare that to Fox Farm Happy Frog at 0.3 - 0.3 - 0.05
So, the SOHUM has about 1.7 times the amount of N and P as Happy Frog, and 5 times the K as Happy Frog.
What is the best NPK value for potting soil for growing cannabis? Some quick googling, maybe not accurate...
0.030 - 0.006 - 0.022 (veg mix)
0.007 - 0.010 - 0.020 (flower mix)
0.025 - 0.008 - 0.030 (all-purpose mix)
So, these commercial potting soils – FF and SOHUM – would appear way too high in NPK.
Here's SOHUM compared to a hypothetical best "all-purpose mix":
SOHUM...... 0.500 - 0.500 - 0.250
ALL-PURP... 0.025 - 0.008 - 0.030
From this, the SOHUM looks to be too hot by a factor of: 20 - 63 - 8.4
For that matter, Happy Frog would also be too hot by a factor of: 12 - 38 - 1.7
Now let's compare ALL-PURPOSE to a premium cannabis potting soil...
BUILD-A-SOIL 3.0: 0.95 - 1.75 - 0.22
Hmm... not what I expected! BuildASoil 3.0 looks to be too hot by a factor of: 38 - 219 - 7.3
Let's look at another one... Roots Organics "Lush" Organic Potting Mix... roughly in the pallpark with BuildASoil 3.0...
Roots Organics Lush: 1.00 - 0.50 - 0.50
So by now you might be asking, what the heck is going on here?!
NOW let's add another level of complexity, and that is, the recognized best NPK for growing cannabis appears to be 3.00 - 1.00 - 1.00 (for veg). CAVEAT: This does not necessarily mean you create or acquire a grow mix that has NPK 3 - 1 - 1.
So, what we are seeing with these commercial soil mixes is that they contain a starting amount of nutrients that will last for a while, and then the grower will need to add nutrients. Or the grower will need to add nutrients basically immediately.
Now let's compare SOHUM to the ideal, 3 - 1 - 1 (for veg):
SOHUM: 0.50 - 0.50 - 0.25
IDEAL: 3.00 - 1.00 - 1.00
From this, we see SOHUM isn't hot at all... it's the opposite. SOHUM is deficient from the ideal by a factor of: 6 - 2 - 4. In other words, SOHUM has 1/6th the N, 1/2 the P, and 1/4th the K, than it should have to support optimum cannabis growth. (Can also be written, 16.6% N, 50% P, 25% K.) These deficiencies need to be remedied by adding liquid form NPK in the water.
From this, we see that basically all commercial potting soils are going to require added NPK (macro nutrients), and probably also micro nutrients (Ca, Mg, etc).
To conclude, here are the "deficiency factors" for the above mentioned soils*, as a percent of the ideal 3 - 1 - 1 (higher percent means more nutrients):
HAPPY FROG ......... 0.30 - 0.30 - 0.05 -> 10.0% - 30.0% - 05.0%
SOHUM .............. 0.50 - 0.50 - 0.25 -> 16.6% - 50.0% - 25.0%
BUILD-A-SOIL 3.0 ... 0.95 - 1.75 - 0.22 -> 32.7% - 175.% - 22.0%
ROOTS ORGANICS ..... 1.00 - 0.50 - 0.50 -> 33.3% - 50.0% - 50.0%
* I was going to include PRO-MIX soil as well, but it apparently has no added nutrients.
To bring this home to my own grow, I use my own custom-made organic living soil, with added organic fertilizers for macro and micro nutrients, at quantities that I control and experiment with. I also water with a high-N liquid fert with NPK 11-2-4, which also contains Na, Cl, Mg, and Ca. (Note, the Na and Cl are not in the form of sodium chloride, i.e. "salt," but are free-floating ions.)
My analysis of all this is... if you can build your own soil, then you can give it NPK values close to the ideal, and then add liquid fert only enough to maintain those levels. When going into flower, more P & K can be given, and less N (1-3-3 instead of 3-1-1). This is the ideal for a true soil grow, as opposed to a coco-type grow which is closer to hydroponic.
My next step is to determine the NPK of my soil mix, by having it tested at a lab.
Happy growing!