Hey everybody...
I've put together the following post based loosely around the use of Megacrop, although it can be applied generally to all fertilizers.
Lately I've come across a few grows where the grower has had an issue with their plants. Typically I find out first what type of media they are growing in. If it's not a hydro or passive hydro setup I don't worry about what the pH of their media is unless it's a product I'm not familiar with. I have to assume their water quality is fine and the correct buffering agents are in the media being used.
In a case where a plant is showing signs of a Ca def I would check how much the plant was being fed and compare that to the plants requrements for that stage of growth. Again, this is assuming we are using Megacrop. If the amount fed was below the recommended amount the simple solution would be to increase the dosage of MC. More MC gives more Ca eliminating the Ca def.
But what if you are at the high end of the calculator and are still having a Ca def? What is happening and what needs to be done? Most likely the pH in your media has been altered to the point that nutrient uptake becomes ineffective. Most of us understand that the pH of a nutrient solution does not effect the pH of the media. The akalinity of the water does and the higher the akalinity the faster the pH rises in the media. In most cases the pH of the media will increase over time.
So let's assume that most growers have a good water supply and their akalinity is considered normal. What other factor(s) can cause the pH to change in the media?
The most common factor would be the basicity or acidity of the fertilizer in use. The effect that a fertilizer has on media pH is dependent on the reactions that take place after the fertilizer has been applied to the crop. This reaction is determined by the nutrients (especially nitrogen) contained in the fertilizer, rather than the pH of the fertilizer solution that you can measure with a pH meter. The potential acidity or basicity should be interpreted as a general tendency of the fertilizer to raise or lower the media pH over time.
The two main types of nitrogen used in water-soluble fertilizers are ammoniacal nitrogen (NH4-N) and nitrate-nitrogen (NO3-N). Urea is another form of Nitrogen and is easily converted into ammonium in the substrate and therefore can be thought of as another source of acidic nitrogen, or NH4-N.
Ammoniacal nitrogen is acidic. When ammoniacal nitrogen is taken up by roots, the plant can secrete an acidic H+(charged hydrogen ion) into the soil solution. The more H+ contained in the root media, the lower the media pH.
In contrast, uptake of nitrate nitrogen increases substrate-pH because basic OH-(hydroxide ion) or HCO3-(bicarbonate ion) are secreted by plant roots into the root media. Since OH- and HCO3- are bases, nitrate uptake therefore can cause the media pH to increase.
Megacrop has a high N content at 9%. Using their calculator we know that 6g/gal is their max recommended dose. Using their elemental calculator it gives us the following types of N shown in PPM:
NO3 = 143
NH4 = 11
Knowing that, we can assume over time that the pH will rise in the media. If the pH rises too high we now cut off the nutrients being supplied to the plant, which causes the defs we see such as Ca late into a grow.
By not feeding extreme amounts of Megacrop you can limit the pH drift over time. If your water is at the high end of normal for akalinity then the drift is harder to slow down.
There are ways to reduce the pH and none that I am familair with. Here's an article on just that for those who would like to know.
To limit the upward climb of your growing medium's pH, acid may need to be injected into your water to reduce excess alkalinity.
www.pthorticulture.com
So what's the bottom line? if you are unsure of what the problem is then do a slurry test on your media. There's a link in this post that will show you how.
Danggnats! :lot-o-toke:
www.420magazine.com
If you constantly find the pH is rising in the media then it's most likely related to your water and switching to a nutrient that has a higher acidity(more NH4) could be the solution to all your troubles.
