Tricky PH question

[Rusty Trichome]

1. Different plants use nutrients at various ph levels

True. Cactus needs a different ph than cannabis. However, Cannabis prefers a slightly acidic environment to properly function, but you must still keep ph within cannabis' acceptable range. Shoot for the middle of the road, and all should be fine.

2. depending on whenever you use soil or hydro the ph should be different. ph for soil systems should be 6.0-6.5 and hydro should be 5.5-6.0

I stick with 6.3 to 6.8ish for soil grows. Not sure about hydro or coco, as I defer all those specs to those that use them daily.

I guess the reason the soil PH should be higher is due to the soil lowering it to the needs of the plant.

No, not really. Container soils are usually buffered with something (lime, gypsum, sulfates...) to raise ph to an approximatelly neutral 7.0 or so. As the buffering capabilities of the buffer fade with time, ph will take a dive. Each potting mix has it's own buffering capabilities, and some brands are buffered at different levels for different applications. Composts and mulches might or might not be buffered at all, and the lower (unbuffered) ph in those ammendments can kill a plant. (use the right tool for the job)

However, how do you factor in the soil ph levels? Does the formula of (ph = water PH level -/+ soil PH) work? Would it be safe to say that ph run off for soil should be around 5.5-6.0?

Doesn't really work that way. Each garden is dfferent, as are the soils, soilless mixes, water and nutrients. For instance...if ammending water, it would take me around twice as much PO2 (a weak phosphoric acid) than if I were to use PO4 (a stronger phosphoric acid) Same thing with soil buffering agents. Buffering capabilities of brand A are likely different than brand B's capabilities, and it is possible they are buffering with different needs in mind. Since there are different buffering agents used in the industry, (lime, gypsum, sulfates...) likely each brand will be slightly different at interacting with your specific nutrients and water. Runoff should be in the 6.3ish to 6.8ish range for container soil. Any lower and you risk nutrient lockout. (for container gardening with soil or soilless)
I monitor runoff and ammend my water if runoff isn't within a 6.2 to 6.5 range. Bad idea to ammend soil ph (mixing in additional buffers) when plant is actively growing/flowering. Best to ammend the water to control ph.

The best way to determine your runoff ph is to use a ph pen. On nutrient day, check runoff ph. Then, on a watering (non nutrient) day, check the plain waters' runoff ph. Color-coded test strips and aquarium test kits are no good for this, as the tinting of the runoff skews the color-coded results.

Any of this make sence...? Time for some coffee. :thumbsup: