[coco] CEC & why measuring runoff pH is not accurate

[Ergot]

I'm not presenting this as fact nor my opinion, I don't know enough about this but hope to gauge your opinions on the subject. NebulaHaze it would be great if you could share your thoughts or any facts you have on this


It appears to be widely accepted on another cannabis forum that measuring the runoff from watered coco will not give you an accurate reading to the root zone pH.

Do not bother with run off pH in coir as it doesn't really give a good indication of whats going off due to the way the cation exchange works and due to way roots will change their immediate surroundings pH, run off E.C is a much better measurement to go by.

Assuming you're using good coir that is washed and buffered properly, and you can keep your trays/sponges or whatever clean of residual salts, then your run off E.C can give you an indication of whether you are over/underfeeding and/or whether you are irrigating enough.

[B]C[/B]ation [B]E[/B]xchange [B]C[/B]apacity (CEC) - The degree to which a growing media can adsorb and exchange cations
Cation - A positively charged ion, such as Na+, K+, Ca2+, Mg2+, NH4+ and H+(sodium, potassium, calcium, magnesium, ammonium and hydrogen)

Extracts from two articles on Canna's website:

Now what exactly do we mean by ‘CEC’? It all comes down to how easily the substrate releases nutrients that can then be taken up by the plant’s roots. In order to understand this, we should think of the substrate as a storehouse for plant nutrients. Many nutrients, such as calcium and magnesium, may be supplied to plants from reserves held in the substrate. Others, like potassium, are added regularly to the substrate as fertiliser so that they can be taken up by the plants as they are needed. The relative ability of the substrate to store a particular group of nutrients – called cations – is called the cation exchange capacity or CEC of that substrate

...

[[Above picture]] Schematic overview of the cation ion exchange in coco substrate. A coco substrate may consist of coir chips, coir fibre and coir pith. Chips are the biggest particles of coco used, and the dust is made up of the smallest. The coco substrate can be seen as a mix of microscopic particles which carry a negative charge. Each particle attracts positively charged ions. This is why natural, untreated coco particles have high amounts of sodium (Na+) and potassium (K+) ions. During cultivation, these ions are released making them available for the plants. Unfortunately, bivalent ions like calcium (Ca2+) and magnesium (Mg2+) bind to the coco substrate in their place. This means that these nutrients are less available for the plants, so there may be a shortage of calcium or magnesium. When we use a ‘washed and buffered’ coco substrate (right), the coco has been pre-treated with calcium fertiliser to enhance the potassium-calcium exchange.

Coco substrate that has been pre-treated in this way is unlikely to release excess potassium or bind the calcium and magnesium that the plants need.

Coco as a concept

The more cations that the soil or substratum can hold, the greater its ‘cation exchange capacity’ (or CEC). The proportion of cations in the soil directly influences the texture of the soil or substratum.

Ratio of plant nutrients explained

So I think I've copied and pasted a reasonable explanation of CEC, cations and their importance in a coco medium.


The extract below is basically is talking about CEC and because of how it works, Calcium & Magnesium deficiencies are more common in coco which we already know about.

What Does This All Mean for Growers?


As a grower, your goal is to create and use a well-balanced nutrient solution. If you are using an un-buffered coco product, a well-balanced nutrient solution goes into the coco and starts to buffer the coco as well as feed the plants, instead of all of the nutrients going directly to plants. So, the CEC in the coco is exchanging some of the K and Na for Ca and Mg.

This exchange is now unbalancing your nutrient solution, increasing the K and Na while decreasing the Ca and Mg. How much unbalancing, you ask?

Earlier, I mentioned the coco could have as much as 1.56 g of K and 0.35 g of Na per 100 g of coco. Your nutrient mix is not highly concentrated with Ca and Mg, but it is enough to get some of the K and Na released from the CEC.

About 15 years ago, I was growing roses in coco and we did a weekly chemical analysis of our feed and drain water. The first time we used coco, we noticed the Ca in our drain water was less than 40 ppm (we would have normally expected the Ca to read 100-150 ppm in the drain water), and we were feeding Ca at a rate of about 200 ppm.

For the next two weeks, we had the same result, so we doubled our Ca to 400 ppm. The analysis of our drain water went up to about 50 ppm of Ca. We watched that for about three weeks and then started feeding Ca at about 500 ppm and still had very little change in the Ca ppm in our drain water. It took about four months for our drain water Ca analysis to read about 100 ppm.

The loss of the Ca and Mg is one thing, but you also get an increase of K and Na. High levels of K will hinder the uptake of Mg by plants. Sodium can negatively impact plant health even at low levels and is toxic to some plants starting at 50 ppm.

The widespread use of calcium and magnesium supplements in the indoor gardening industry is an indication that many have experienced the CEC phenomenon in coco that I am talking about here.

The deficiencies are observed and can be corrected to a certain extent with calcium and magnesium supplements, but there are also coco products out there buffered to a higher level, which don’t need the calcium and magnesium supplements.

Adjusting the CEC in coco



So, going back to the title of this topic, I think I now have a basic understanding of how of CEC works but there's no explanation as to how this effects the pH and why it supposedly is a worthless measure with coco runoff. This is what I don't understand, all the articles I've linked to doesn't really explain how CEC will effect the pH, I get that roots can and will adapt and change the pH to an extent but with CEC I'm lost as to how all these interactions effect pH. Is it the chemical reaction between positive & negative ions that makes it a worthless measure?

Apologies for the mini essay and I'm sure I've presented it in a way that's all over the place, but I just wanted to share what little I understand of this and hopefully gain a greater understanding of this from yourselves

[Vapo69]

No apologies needed dude, this is a great piece of info.......unfortunately I can't answer your ph question but thanks for the info posted about the cation exchange....which I am currently trying to understand myself lol..Thanks dude

[Flockshot]

I like this stuff because it helps me understand the WHY for some of the things I do. I have had more than one calcium deficiency in my coco grow. I now hammer it with cal mag from the time I pour water on the dry bricks till the end of veg. This explains why.

As for checking PH in runoff, I am simply checking to see if the PH is dropping, and if so, how much. Also I am looking at the plants to see how they are growing.

I know that as the salts build up, it will lower my runoff PH. When I see more buildup than I want I can flush and continue. After a couple of grows I can predict where the flush will come and plan for it.

I may not know the PH of my root zone, but I don't need to know exactly, as long as my plants are doing well and I can repeat that from grow to grow.

The first runoff from you plant, may well be a different PH than the last runoff from the same watering (depending on how much runoff you have), so I don't consider it an exact science.

If my EC meter can become a better tool that is good to know. I will record my EC of runoff for a while and see what it tells me, and how EC levels coordinate with PH levels.

[GRAND]

Wow, interesting information! I haven't grown in coco ever, but find the information great if ever I choose to use coco. Thx for posting this Ergot !

[NebulaHaze]

What a post! I agree it isn't an exact science but the reason we say to test runoff and compare is because plants seem to grow well if you do it that way. You'll notice in a grower world that some of the things we do is just because it works in practice, even if we don't fully understand why. For example, no one is exactly sure why defoliation in the flowering stage can increase yields, but we know through trial and error that it definitely does work. At GWE, we focus more on "what works" than anything else.

When it comes to measuring the pH of the runoff, perhaps you're not getting the exact accurate reading of the root zone, but measuring the runoff helps you understand whether the root zone is causing the pH to go up or down. I think that's what's most important. If you know you put the water in at 6.5 pH and it's coming out at 7.5, you know that the root zone is increasing the pH. You can use that information to give your plant water at a much lower pH until it starts coming out at the right range. Using an EC or TDS meter can help you get an even better idea of what's going on at the root zone (for example you could determine if the water coming out has more or less "stuff" than when it went in), but simply measuring the runoff pH and reacting to that will work better for most people that simply using an EC/TDS meter.

I know that when I start seeing nutrient deficiencies in coco, I almost always find out that the runoff pH is coming out far higher or lower than what was put in, and correcting that causes the deficiencies to go away. It's important to remember that what's most important is the pH of the nutrient water itself. The reason we want a certain pH is because it makes certain minerals easier for the plant to absorb. This happens because the minerals literally take a different chemical form at different pH ranges. The plant can easily absorb some of these forms, but not others. So as long as you know the water with the nutrients is the right pH, you know your plant is getting access to the nutrients.