Question on pH and KH.

[Paph_Person]

We've all known the importance of adjusting pH when watering our orchids. However, the topic of water kH (carbonate hardness) often gets overlooked. Recently, I had a conversation with Ray, who pointed out something interesting: the pH of the water doesn’t directly correspond to the pH in the potting media. In fact, the media pH can vary significantly regardless of the water's pH.

This raises an important question: what should the ideal kH of our watering water be? Especially ones whoo use RO water. Since kH acts as a buffer, it helps stabilize pH and prevents sudden fluctuations. Getting this right seems crucial for maintaining a stable environment for our orchids.

I’d love to hear your insights on the ideal kH levels.
Thanks.

 

[CharlesD]

I agree that potting media has biggest effect on root zone pH. Fertilizer solution pH is very important but if the potting mix is way out of range then it is hard to make an adjustment with just the water you pour through. I have 130ppm well water with 25ppm caco3 alkalinity. The pH is 7 so with fertilizer added closer to 6.4, 6.3. With the bark potting mix I use I think this gets pH in the right range. I flush with RO every 4th watering. I think a lot of people grow very nice paphs mixing fertilizer with city water or other alkaline hard water because their bark mix is acidic and fertilizers with some ammonium or urea tend to lower pH in fertilizer solution and in the root zone. Hence the end result is pH at the roots around 5.5 - 6.5

 

[Ray]

Alkalinity or KH targets will vary based upon the fertilizer used.

As a plant takes up ammonium ions, it secretes compensatory H+ ions, which acidify the substrate. Urea is also a source of ammonium ions. A plant taking up nitrates secrete OH- or HCO- ions, which raise the pH of the substrate.

Since KH is really just a measure of buffering, you need to figure out what CaCO3 content would provide that for your choice of fertilizer.

Bill Argo, the PhD for formulated the so-called “MSU” fertilizers and both K-Lite versions for me, wrote a series of articles on plant nutrition several years ago. They are worth a read (Primer on Plant Nutrition), but the 3rd installment addresses this directly.

 

[Stone]

A bit off topic, but it's worth remembering too that leaching is a common (if not the most common) reason pH drifts down even with 50/50 nitrate ammonium fertilizer. Nitrate added as fertilizer will not reduce pH but fertilizer added as ammonium will do so as, apart from the reason Ray mentioned, it is converted slowly to nitrate in the pot which then binds with cations (mainly Ca but others too) and takes the Ca with it as it is leached from the pot. (as Calcium nitrate) This is believed to be the main reason why pH drifts down in nurseries which mainly use ammonium or urea as their N.
Anions (nitrate, phosphate, sulphate, molybdate, (and boron behaves similarly) are negatively charged so they mainly remain in the water in the medium because most media surfaces have a negative charge and they are repelled. This is the reason we need to periodically add Calcium carbonate to reduce the acidity caused by ammonium ions. For example, for every 1 kg of Urea, the amount of Ca carbonate needed to restore pH is 0.75 kg. 1 kg of Ammonium sulphate requires 1.1kg of CaCO3! Just how much CaCO3 to use on the pot is for you to work out. My fertilizer is mainly Osmocote which is high in urea and Nutricote which is 50/50 ammonium/nitrate so I sprinkle a mix of ground limestone and dolomite (giving me about 4 or 6 to 1 Ca to Mg) about twice/year or more. My mains water is very pure. If you use a ratio of 2-3 nitrate to ammonium you may eventually need to acidify your solutions. But you may run into other problems.....

 

[CharlesD]

A bit off topic, but it's worth remembering too that leaching is a common (if not the most common) reason pH drifts down even with 50/50 nitrate ammonium fertilizer. Nitrate added as fertilizer will not reduce pH but fertilizer added as ammonium will do so as, apart from the reason Ray mentioned, it is converted slowly to nitrate in the pot which then binds with cations (mainly Ca but others too) and takes the Ca with it as it is leached from the pot. (as Calcium nitrate) This is believed to be the main reason why pH drifts down in nurseries which mainly use ammonium or urea as their N.
Anions (nitrate, phosphate, sulphate, molybdate, (and boron behaves similarly) are negatively charged so they mainly remain in the water in the medium because most media surfaces have a negative charge and they are repelled. This is the reason we need to periodically add Calcium carbonate to reduce the acidity caused by ammonium ions. For example, for every 1 kg of Urea, the amount of Ca carbonate needed to restore pH is 0.75 kg. 1 kg of Ammonium sulphate requires 1.1kg of CaCO3! Just how much CaCO3 to use on the pot is for you to work out. My fertilizer is mainly Osmocote which is high in urea and Nutricote which is 50/50 ammonium/nitrate so I sprinkle a mix of ground limestone and dolomite (giving me about 4 or 6 to 1 Ca to Mg) about twice/year or more. My mains water is very pure. If you use a ratio of 2-3 nitrate to ammonium you may eventually need to acidify your solutions. But you may run into other problems.....

@Stone interested to know what bark you use. I use orchiata so do not add lime until it is closer 6-12 months old. I think a lot of people prefer kiwi bark because it starts out in the ideal pH range for a lot of people’s water type. I don’t have experience with how stable its pH is under ammonium/urea fertilization. I do lime other orchids in sphagnum moss right away and every 6 months. Though still test pour through periodically to make sure I’m finding the right balance