Bleached leaves. Too much light or nutrition problem?

[Rick]

This interests me:drool: How can I in a relatively precice way determine pH of the "root-water"? With very diluted fertilizers (TDS 100ppm) and very open mixes, how do we measure it? I have tried pH meters and indicators, but frankly, this is not straight forward.
Another thing is an obesrevation I have made with fertilizer solutions containing urea. Let me explain;
Ok immediately after mixing, the pH is, say 5.1 at a conductivity of 160 microsiemens/cm (uncertain about the units, correct me if I am wrong) with quite a bit of urea this should be close to a TDS of 100ppm. If the same water is poured into a cup and kept for a day or two, the pH can be measured to 7.0 and conductivity to 190 microsiemens/cm. Here its imaginable that the urea splits to ammonia causing this change. So far so good?
Here comes the question: if the water changes during storage(hours!) how is the situation down at the root level? Would not ammonia (NH3) consume H+ and increase the pH of the mix? Ok, if it is absorbed by the plant, locally H+ is released and this can lower the pH locally. However, if you water every day or so (like I do), at every watering, the root system get flushed with fresh fertilizer and I would claim that there is no chance of a quantitative conversion of the urea to make the mix become acid. Chances are that the opposite happens, that the urea causes the substrate pH to increase instead.
Then my final question: for very open mixes with fairly low CEC; how do I measure the pH? Has anyone actually conducted this exercise?
:confus:

Most of us aren't watering every day. So I would agree if you water that freqently and have a low water retentive and low CEC mix, then pH in is probably close to pH out. But it sounds like many only water once or twice a week, and use water absorbant mixes. So in that case plug the pot and fill with RO water. Give it an hour or so and then test. Or pour through a pot several times until you get an equilibrium value.

 

[Rick]

Would not ammonia (NH3) consume H+ and increase the pH of the mix? :

Only if your mix is sterile.

But the nitrifying bacteria are very good at colonizing mixes and they "eat ammonia" and drop pH as they do so.

Basic waste water, and aquarium technology.

 

[DavidCampen]

You are assuming that an equal amount of N is being consumed by both the nitrifiers and the plants in this balancing act.

But in reality you can build a bacterial population that can handle way more ammonia than the plant is able to uptake NO3. So net balance you end up with alkalinity loss and acidification.

And you are assuming that most or all of the nitrogen is coming from ammonia.

Pot pH drop is a studied phenomena. Several years ago Orchids published a study that was designed to look at the percieved need of calcium for paphs. Using basic bark based potting mix different levels of lime amendment, and a standardized watering a feeding regime (some basic 20-20 -20 I believe) they were able to get pot pH to drop below 4 in unamended pots in less than a year (if I remember correctly). While ammended pots dropped more slowly.

What does this have to do with the use of ammonia as a nutrient? Was the change in pot pH correlated to the use of ammonia?

 

[DavidCampen]

Most of us aren't watering every day. So I would agree if you water that freqently and have a low water retentive and low CEC mix, then pH in is probably close to pH out. But it sounds like many only water once or twice a week, and use water absorbant mixes. So in that case plug the pot and fill with RO water. Give it an hour or so and then test. Or pour through a pot several times until you get an equilibrium value.

If so this is then a good argument for watering frequently, not for worrying about a portion of the nitrogen in your fertilizer coming from ammonium.

 

[Rick]

http://www.thewaterplanetcompany.com/docs/WPC_Nitrification & Denitrification .pdf

As it turns out there is a 7-1 alkalinity to ammonia consumption rate by nitrifciation.

So if you supply ammonia to a biologicially active substrate with nitrifiers, it will definitely acidify your mix at a very high rate.

 

[DavidCampen]

http://www.thewaterplanetcompany.com/docs/WPC_Nitrification & Denitrification .pdf

As it turns out there is a 7-1 alkalinity to ammonia consumption rate by nitrifciation.

What is said is that 1 pound of ammonia when converted to nitrate produces enough acid to react with 7 pounds of calcium carbonate. So what?

The nitrate produced from 1 pound of ammonia, when taken up by a plant releases alkalinity equivalent to 3.5 pounds of calcium carbonate and the uptake of the anion that accompanied the ammonia produces the other 3.5 pounds of calcium carbonate equivalent.

So if you supply ammonia to a biologicially active substrate with nitrifiers, it will definitely acidify your mix at a very high rate.

Yes, if you fertilize with pure ammonium sulfate and don't flush your pot.

 

[Rick]

What is said is that 1 pound of ammonia when converted to nitrate produces enough acid to react with 7 pounds of calcium carbonate. So what?

The nitrate produced from 1 pound of ammonia, when taken up by a plant releases alkalinity equivalent to 3.5 pounds of calcium carbonate and the uptake of the anion that accompanied the ammonia produces the other 3.5 pounds of calcium carbonate equivalent.

However the plant uptake rate is a fraction of the nitrification rate.

A full Cattleya leaf ways less than 1 gram dry and only contains 3% N (30 mg total). And it takes several months of (weekly?) fertilizer applications to get there. So the alkalinity addition by plants is insignificant compared to the alkalinity uptake rate of the potting mix bacteria.

And then regular heterotrophs are doing their magic and releasing CO2 from general respiration (going to carbolic acid and further reducing pH).

Biology happens, and a little bacteria can do a lot.

 

[DavidCampen]

However the plant uptake rate is a fraction of the nitrification rate.

A full Cattleya leaf ways less than 1 gram dry and only contains 3% N (30 mg total). And it takes several months of (weekly?) fertilizer applications to get there. So the alkalinity addition by plants is insignificant compared to the alkalinity uptake rate of the potting mix bacteria.

And then regular heterotrophs are doing their magic and releasing CO2 from general respiration (going to carbolic acid and further reducing pH).

Biology happens, and a little bacteria can do a lot.

Most all of the nitrate that plants take up in nature comes from conversion of the nitrogen in dead plants etc. to ammonia and thence to nitrate. It is amazing that the plants manage to survive in nature without our strenuous intervention.

 

[Rick]

Most all of the nitrate that plants take up in nature comes from conversion of the nitrogen in dead plants etc. to ammonia and thence to nitrate. It is amazing that the plants manage to survive in nature without our strenuous intervention.

http://elmu.umm.ac.id/file.php/1/ju...rimental Botany/Vol44.Issue3.Nov2000/1236.pdf

Yup orchid growers are generally in overkill mode.

The attached paper doesn't want to open fully for me, but a table near the end shows solution (looks like a hydroponic test) pH drop with use of NO3, NH3, glutamine, and various combinations thereof for nitrogen sources.

The biggest drops from the initial pH are for high percentage ammonia solutions.

 

[DavidCampen]

http://elmu.umm.ac.id/file.php/1/ju...rimental Botany/Vol44.Issue3.Nov2000/1236.pdf

Yup orchid growers are generally in overkill mode.

The attached paper doesn't want to open fully for me, but a table near the end shows solution (looks like a hydroponic test) pH drop with use of NO3, NH3, glutamine, and various combinations thereof for nitrogen sources.

The biggest drops from the initial pH are for high percentage ammonia solutions.

The paper also notes:
Our results suggest that organic N and NH4 ions are the main sources of nitrogen to C. fimbriatum. Some of the physiological characteristics detected in C. fimbriatum plants are typical of plant species adapted to tropical and subtropical rainforest climax communities (Stewart et al., 1992) and of plants species adapted to extreme soil conditions (Claussen and Lenz, 1999; Schmidt and Stewart 1999). Beyond representing an interesting model plant or physiological studies, the genus Catasetum comprises several species of economic value. Our results pointed out the advantageous use of glutamine for a more efficient development of Catasetum plants. It was also evident that NO3 alone is a poor nitrogen source for Catasetum growth.

 

[Rick]

Yes. But neither did it say that NO3 was totally useless either.


I also wouldn't be surprised given that they started at pH of 5.5 and let it go down from there.

Everything else (like K, Mg, PO4) concerning sugar/energy production (the primary criteria by which they assessed N choice) gets hard for the plants to access at those low pH values. So what would the results be like like with pH support in the typical 5.5-6.5 range considered optimal for orchids.

The best material was actually the glutamine (not ammonia), and from the what I picked up they felt organo-nitrogen sources were the best (for Catasetum fimbriatum). This also may not be that surprising since many Catestum are tended by ants that add all kinds of to the piles of shredded leaves they add around plants like this.

How much soluble carbon support do we give our plants? Of the dry matter in plants about 90% if it is in carbon based products. So plants need carbon much more than NPK......

The chelators you like to use may be a good source.

I use kelp.

 

[consettbay2003]

Rick, I recall that you use 1/4 tsp. of kelp/gallon of water once a week when you fertilize. Would there be any downside or upside in increasing this to 1/2 tsp/gallon once a week?

 

[Rick]

http://aob.oxfordjournals.org/content/early/2012/07/08/aob.mcs140.abstract

This is a very interesting abstract on ant plants, but doesn't include anything to the discusion of nitrate (maybe urea though).

 

[Rick]

Rick, I recall that you use 1/4 tsp. of kelp/gallon of water once a week when you fertilize. Would there be any downside or upside in increasing this to 1/2 tsp/gallon once a week?

Probably not, but the long term concern is hormonal overdose. Ray might have more to say on kelp dose, but I think even 1/2 tsp/gal is way below the suggested weekly dose suggested for food crops.

You might consider focusing increased dosing of anything on warm/bright times and big plants. Winter and small plants hold off.

 

[Ruth]

Very interesting discussion

I know your basic garden center should carry ammonium sulfate. You could probably add a pinch of that.

If you use it as a folar spray, how much would you use to a gallon of water

 

[Stone]

Here are a few points on what happens in pots regarding pH and fertilizers.

Repeated application of fertilizers which contain higher Ammonium and Urea than nitrate will acidify media over time so sooner or later lime must be applied if you want to avoid damaging your plants with a very acid mix.

This is a list of the Calcium carbonate needed to restore pH after using certain fertilizers: Kg per Kg........
Ammonium sulphate: 1.1
Urea: 0.75
Ammonium nitrate: 0.6
Ammonium phosphate 0.5
Dried blood: 0.2

If a fertilizer contains equal amounts of NH4 and NO3 should not alter pH wheather or not the NO4 is converted to nitrate before use by the plant because bicarbonate ions released by the roots as they take up NO3 will balance the acidity produced when ammonium is taken up or converted to nitrate.
BUT!!..... It is usually found in nurseries that pH drifts down anyway because of leaching of nitrate produced from ammonium. Not that lost from fertilizer NO3.
It is believed that acidification is due more to this than the direct effect of ammonium. So ferts with a 2 or 3 to 1 NO3 to NH4 ratio should prevent acidification.

Calcium nitrate will make a media more alkaline but the effect is small. 0.2kg of sulphur will neutralize 1 kg of Calnitrate.


So in theory you should be able to juggle your fertilizer components to sabilize your mix pH without resorting to lime but in practice I think its wize to keep tabs on mix pH with 6 monthly checks and use some lime if needed.

It seems that many orchids are happy with (or prefer) ammonium so I think the 2 to 1 nitrate to ammonium is probably a good ratio. Although most of my orchids are fed with 95% urea and ammonium N in a controlled release form and most are thriving! But I do sprinkle 50/50 dolomite/limestone on the pots once or twice a year.

 

[Stone]

How much soluble carbon support do we give our plants? Of the dry matter in plants about 90% if it is in carbon based products. So plants need carbon much more than NPK......

What do you mean by carbon support? Don't they get the carbon from air?

 

[Stone]

Then my final question: for very open mixes with fairly low CEC; how do I measure the pH? Has anyone actually conducted this exercise?
[/

Bjorn, I use an indicator liquid/barium powder all the time. Its easy and quite accurate enough for our purposes. Even if you check a few pieces of bark or whatever from a few pots, it will give you a good idea of your media pH. You can buy the kits at most hardware or garden centers.
Otherwize you have to use the saturated paste method with a meter which is more complicated and really not needed.

 

[Rick]

Here are a few points on what happens in pots regarding pH and fertilizers.

If a fertilizer contains equal amounts of NH4 and NO3 should not alter pH wheather or not the NO4 is converted to nitrate before use by the plant because bicarbonate ions released by the roots as they take up NO3 will balance the acidity produced when ammonium is taken up or converted to nitrate.

Noting that pH drop is observed even when balance of ammonia and nitrate is present is covered in that one pdf I posted. The bacterial acidification caused by nitrification of ammonia is disproportionate to the amount of alkalinity gained by the tiny amount the plant releases via nitrate uptake. You brought up what I think is a very important point in that bacteria are able to remove/convert considerable amounts of ammonia (mostly to nitrate anyway). And they are competing with the plant for that same ammonia.

But a plant in a pot is always going to loose the arms race to an active and growing bacteria population. As long as you keep shoveling in ammonia, and adding alkalinity the bacteria multiply at rates plants (or plant growers) can only dream about. Nitrifiers operate 24/7, and respond over temperature ranges better than the plants. Plant activity is focused on daylight activities.

The option of running higher nitrate/ammonia ratios and operating at lower alkalinity starves the bacteria population, and keeps it from predominating pot conditions. A periodic "shock" of ammonia has a greater chance of actually getting into a plant, when it doesn't have to compete with the nitrifiers.

 

[Rick]

What do you mean by carbon support? Don't they get the carbon from air?

The majority certainly does, and most plant carbon product is cellulose. But we also no that a lot of enzymes, vitamins, and amino acids (like glutamine C5H10N2O3) are "consumed' by plants. In the above example of Catesetum, it's "preferred nitrogen source" was the glutamine amino acid.

Maybe instead of playing with ammonia we should look at glutamine supplementation. I wonder if its in kelp.