Medium ph vs nutrient ph??????

[grey1223]

For some reason I'm having a problem understanding this relationship. When you take promix, or coco and check the ph they come out 5.6 to 6.0. That's measuring by taking a cup of medium, filling with water (tap water at 8.0) and let set for an hour. Drain off and stick ph meter in.

Based on this, what is the use of measuring medium ph? If one hand waters with 6.5 nutrient solution the runoff will most certainly be lower than 6.5.

I guess I'm trying to understand if the ph of the medium and the ph of nutrient solution are independent of each other.

Promix is used all over the place commercially and growers really like it. So what am I missing here?

[stinkyattic]

First, if you want to get an accurate reading of the soil pH using that method, you must use deionized water. Tap water at 8.0 is that high because of dissolved minerals, which buffer the pH and will throw off your reading significantly. What you are seeing is not the pH of the medium, but the pH of the solution after the medium and your tap water have interacted chemically at ambient temperature and pressure for one hour.

If you water with 6.5 and the runoff comes out lower, your medium is causing that drop.

The medium pH and nutrient pH are only independent until they come in contact with each other. Once this occurs, whatever compounds are causing the pH of each to read as it does will interact and give you a pH that reflects EVERYTHING in the new solution- source water, dissolved atmospheric gases, medium, and fertilizer.

Coco and ProMix are both soilless and should be run about ~0.4 pH points lower than the soil range. Coco also scavenges calcium and may contain high levels of salts and potassium. Promix comes factory buffered with lime, which works well for 6-10 weeks depending on your water's pH and hardness, and if you are adding a fertilizer or supplement such as humic acid or silica blast that buffers pH.

[Greenthing]

I like to let my water stand for some time before i use it, gives it a chance for any chemicals to disperse and i allways run the tap for a while before i fill.:)

[stinkyattic]

Not all chemicals disperse when you let the water sit. The maximum amount of dissolved gases that can remain in solution is related to the temperature of that solution, so cold water can hold more than warm water- as the water warms to room temperature, some will come out. Many chemicals will remain in solution, especially minerals like lime and sulfur, which do not exist as gas at room temperature. Only compounds which exist as gas at RT will leave the solution.

[grey1223]

I thought when you hand watered soiless medium it is to be treated as if it is soil. Feed, water, feed etc... at 6.5. Are you suggesting a ph lower than 6.5 for hand watering?

[stinkyattic]

Good question, and I don't want to mislead you- I do not know if there is a difference that is dependent upon delivery method, but personally, I would actually hand-water soilless right AT 6.5, making sure my nutrient solution was correct. I'm not someone to ask on this one, since you can see in my grow log that I jsut royally fucked up a soilless (promix) crop by forgetting to check my EC and as a result never thought to add CalMag to it :(

[Greenthing]

Not all chemicals disperse when you let the water sit. The maximum amount of dissolved gases that can remain in solution is related to the temperature of that solution, so cold water can hold more than warm water- as the water warms to room temperature, some will come out. Many chemicals will remain in solution,
especially minerals like lime and sulfur, which do not exist as gas at room temperature. Only compounds which exist as gas at RT will leave the solution.
I am happy that i have good Quality water where i live Stinky not so many chemicals in it if any:thumbsup:.

[daihashi]

Good question, and I don't want to mislead you- I do not know if there is a difference that is dependent upon delivery method, but personally, I would actually hand-water soilless right AT 6.5, making sure my nutrient solution was correct. I'm not someone to ask on this one, since you can see in my grow log that I jsut royally fucked up a soilless (promix) crop by forgetting to check my EC and as a result never thought to add CalMag to it :(

I started off in promix and then transplanted into a 50/50 mix of promix and fox farms ocean forest.. followed by my latest transplant of going back into 100% promix.

I started off watering until my runoff read between 6.3-6.8 just like soil. My plants are doing fairly well considering and any problems had were due to my own doing. I can say that 6.3-6.8 has worked great for me, but at the same time I can't say for certain that I would've gained greater benefits fro a lower ph.

[grey1223]

If a chemical reaction causes the low ph reading why does the ph read the same when I use RO water??? Again, I just can't quite understand. Maybe it's not really important as these mediums have been around for awhile and are used commercially with success. My background and education are in business. Perhaps I need to go back to school and take some chemistry or botany???

[stinkyattic]

In ProMix, for example, the peat is the part that is most acidic. Now, if your tap water reads over 7.0 but has very low ionic strength and EC, that means there's not really a lot of stuff in it to react, and it will be influenced MORE by that peat, just like RO water is, than say a tap water that reads over 7.0 and has a lot of carbonate ion in it.