Hello everyone!
There are several things to consider.
Dr. Bugbee's fertilizer recommendation is for use with tap water, and here's why:
Firstly tap water contains calcium and magnesium (more often than not approx ~80 ppm Ca and ~25 ppm Mg), which together with some from the dolomitic lime is sufficient. The 20-10-20 peat lite does not contain calcium and almost no magnesium, because it is designed for tap water and takes this into account. Micro nutrients (Fe, B, Cu, Mn, Mo, Zn) are included in this fertilizer in sufficient quantities, so no need to worry about that.
Secondly, but not less important, tap water has alkalinity (not to confuse with ph) which consists of mostly carbonates and bicarbonates ("hardness", for example CaCO3). This alkalinity will drive the ph of the substrate more and more up towards 7 over time, and this is too high. To counteract this, the 20-10-20 contains approx 40 percent of its nitrogen as ammoniacal nitrogen (NH4), which in turn has an acidifying effect in the substrate. The result is that the ph remains somewhat constant over time. Both, alkalinity and the acidifying effect of ammonium nitrogen on their own have much more impact on the substrate ph than the ph of the irrigation water.
If you use reverse osmosis water (which has no alkalinity), the high ammonium nitrogen content of the fertilizer will drive the substrate ph rapidly down over time since there is no alkalinity to buffer that, even if the ph of the irrigation water would not be acidic.
So you have to consider several things if using reverse osmosis water:
1) The use of a fertilizer which contains most of its nitrogen in the nitrate form, so that substrate ph remains somewhat stable.
2) Further you need a source for calcium and magnesium.
The best way to achieve this is to use a three part system, consisting of
Part A: a fertilizer with no or just a little nitrogen, but a ~1:2 ratio of P2O5:K2O, like Peters combi sol 6-18-36 or Peters hydroponic special 5-11-26. They are designed to work in conjunction with different nitrogen sources, depending on the source water and type of substrate (ammonium nitrate, calcium nitrate, ammonium sulfate etc.).
Part B: calcium nitrate for nitrogen and calcium
Part C: magnesium sulfate for magnesium and sulfur.
Here is an example (also perfect for coco and other soilless substrates etc.)
Peters combi sol (6-18-36): 0,34 g/l
Calcium nitrate (15,5 N 19 Ca): 0,65 g/l
Magnesium sulfate (10 Mg 13 S): 0,3 g/l
gives N-P2O5-K2O-Ca-Mg-S:
121-61-122-120-35-48
The ratio Dr. Bugbee recommends gives superb results in terms of both yield AND quality. Just be sure to NOT use any bloom boosters as it is really not needed and only throws you NPK ratio off. Just this ratio from veg till the end, no extra flowering formula or changing ratios, no reduced nitrogen. All the "cannabis specific" bla bla is just marketing bullshit and broscience.
So to summarize it:
It's super cheap, gives perfect results, better than all the overpriced, down watered high PK ripoff "nutrients lines", it is super easy, no constantly running to the hydro store.
You can get these in 50 pound bags and have a lifetime supply (!) for a couple of bucks.
As a side note: I would trust Dr. Bugbee more than most if not all others because for sure he knows what he is talking about and first researches what he claims. He's a well-known, respected professor doing pioneering work for almost 40 years, photobiology, nutrition, soilless substrates, hydroponics to only name a few topics, and has released many peer-reviewed papers. They are professionals who do REAL scientific research.
Watch his USU Cannabis lab tour video. They are doing state of the art cannabis research in controlled environment growth chambers and testing also parameters like temperature, humidity etc.
The world of cannabis is so full of myths and false claims and marketing bollocks, it is ridiculous, and i'm glad that finally there is scientific research going on and people like Dr. Bugbee uncover the truth!
