Adjusting PH in soiless mix?

[UrbanGardener]

Hi guys,

Got some satori from mandala about a week into veg, noticing some yellowing on the first set of leaves and slight brown, just not 100% healthy. Checked PH and its hitting 7. I read on the mandala website that they like 6.2 - 6.5 so Im guessing I need to adjust my ph, my question is what is the best way to do this? Im growing in coco coir with perlite. May have added too much dolomite lime when mixing up the mix. Any help would be appreciated.

UG

 

[Seamaiden]

I believe pH ranges are tied more to the medium than the strain, but I could be wrong about that. I also believe that a pH range of 5.8-6.2, while just as tight, is a better range for growing in coco coir.

The higher pH *could* be because of the dolomitic lime (I'm hoping your coco either came pre-rinsed/pre-charged or that you've already done so), but it could also be because of the source water you use for feeding. If your source water is alkaline (not base pH, alkaline in terms of buffering capacity, i.e. resistance to pH shift) then not only will it resist your adjustments, but the minerals that make it alkaline will build up in the coco, thus buffering *it* into a higher pH range that can be all but impossible to shift down.

If the source water is not so alkaline, or is RO, RO/DI, etc, then I would look to the dolomite, yes. If that is the case, then bringing the pH of your feeds down just a notch (say 5.7 instead of 5.8), then the dolomitic lime will dissolve more rapidly, which will free up the calcium and magnesium, and hopefully your girls will use that up.

For bringing pH down I like GH pH down, especially if I'm using unfiltered water.

 

[UrbanGardener]

Hi Seamaiden, thanks so much for the useful info. It is definitely caused by my mistake of adding too much lime when I made the mix. Havent fed them yet as they are so young.
I was told to add either acetone or sulphur to the soil to help alkalize it. Would this be the best way of correcting the issue? Just want to make sure I dont do anything drastic because of how young and fradgile they are.

 

[Seamaiden]

Sulfur acidifies, which means it will drive pH down, not up. Dolomitic lime and hydrated lime are two methods by which to drive pH up (make it more basic).

Since you're growing in coco (first time w/coco? Has anyone discussed with you how that stuff locks up calcium and magnesium?) I don't think you could have added such a drastic overage of the lime. In order to free up 'too much' you'd have to be watering with very low TDS, acidic water (per my own experiments) for it to be freed up too quickly.

Remember also that deficiencies look a lot like imbalances, and that coco chews up and locks up those two elements, Ca & Mg, at an amazing rate. Also, because you're in coco, they need feedings more than they'll need them in soil. In soil you can switch back and forth between waterings and feedings as needed. In coco you can upset the balance if you switch around like that, and so I've found that very mild feedings throughout the grow make it much easier to manage.

I don't think I mentioned that I add dolomitic lime to my own coco mix, but the plants still require regular additions of Cal-Mag or it's a bust.