Potting practises, frequency of repotting

[naoki]

Most likely the main problem is the availability of oxygen dissolved in the water, and of course if the "mud" is caused by bacteria consuming available oxygen, then your roots will suffer. Of course, such an environment may be typical for most broken down mixes and if oxygen is not alloewd in the pot, well, then the roots perish. In such a case it might perhaps be a better option to increaser watering frequency in order to refill with fresh, oxygenated water.

Lance, both watering frequency AND how long it takes to dry out influence the water (and nutrient) availability to root (I'm sure that this is obvious and you tried to simplify this). If the media is moist for a longer time, they can get nutrient/water longer. The stable moisture level is considered to be one of the reasons why addition of 30% peat to bark-based media caused faster growths in Phal trials (e.g. Wang 1995. Medium and fertilization affect performance of potted Dendrobium and Phalaenopsis HortTechnology 5(3): 234-237). The bark only media dries quickly, so they did water twice as much as the pot with peat.

But higher frequency of watering might result in more oxygen as Bjorn said. Do you really think oxygenated water makes difference, though? The amount of O2 water can hold is tiny (around 10mg/l). In broken down media (or potting soil), top-watering is supposed to help exchanging the air in the pot. But it's a bit hard to imagine that it is a problem in pretty coarse orchid media. Soil respiration is an interesting point, too. I wonder how much this soil respiration contribute to the O2 level inside of the pot.

I briefly looked for an O2 probe to measure soil O2 content. I couldn't find a cheap one, but it would be fun to test how different watering is influencing the O2 level in the pot. Here is one from Apogee for $300. But it's a bit too pricey for me.

 

[gonewild]

Lance, both watering frequency AND how long it takes to dry out influence the water (and nutrient) availability to root (I'm sure that this is obvious and you tried to simplify this).

Yes that is correct I tried to simplify just to illustrate.

But higher frequency of watering might result in more oxygen as Bjorn said. Do you really think oxygenated water makes difference, though?

It might. Not so much for bringing O2 to the roots but rather keeping the media atmosphere fresh for the micro organisms. Since we really don't know what organisms exist in the media associated with orchid and nutrition giving aerated water more closely mimics nature.

The amount of O2 water can hold is tiny (around 10mg/l). In broken down media (or potting soil), top-watering is supposed to help exchanging the air in the pot. But it's a bit hard to imagine that it is a problem in pretty coarse orchid media. Soil respiration is an interesting point, too. I wonder how much this soil respiration contribute to the O2 level inside of the pot.

In coarse media the O2 should be good supplied from the atmosphere. It is in dense heavy medias that it may make a difference. It does make a difference where people have pots standing in wanter or roots submerged. Circulating the water is much better than still stagnant water. the circulation would mainly only be adding O2.

I briefly looked for an O2 probe to measure soil O2 content. I couldn't find a cheap one, but it would be fun to test how different watering is influencing the O2 level in the pot. Here is one from Apogee for $300. But it's a bit too pricey for me.

Cheaper to just aerate the water!

 

[Stone]

Do you really think oxygenated water makes difference, though?

Hydroponic people looked at this and found adding O2 to the water increased yeild in some crops but not in others.

 

[Bjorn]

Naoki, just for fun, I re-calculated the amount of oxygen in water into volume gas STP (STP= standard Temperature and Pressure, i.e., 25 (20?)C and 1BAR) and it is really quite low; around 7ml/l; Or expressed as air: 33ml/l. But its there and actually not insignificant amounts either.

I originally started that aerating to disinfect my water with ozone. Additionally the rain in the tropics have been found to contain quite a bit of ozone during thunderstorms, so I thought that it could not be bad? I have used it for the last 5 years or so and can frankly not say that it has done much in a positive or negative way.

BUT when that is said, because my source-water is reductive, by that I mean it sometimes smell sulphur (hydrogen-sulfide) it is improved by microbial oxidation in my tank and to perform well, that process needs some oxygen as well.

The reason for the reductive smell of the Source-water is that although I utilise rain-water, I have to top it with water from a nearby bog. You know the home of sphagnum etc? This water is relatively pure and contains little salts, and is quite neutral, but there is some sulphur in it causing it to smell slightly. Most likely, the bog-water contains most of the nutrients my orchids need, perhaps I should stop feeding? No, I do not believe it contains all the necessary elements. It does contain sulphur, which btw. should not be forgotten in the make-up of the fertiliser, right Mike?

 

[naoki]

It is cheaper, but don't we want to know the truth, Lance?

Mike, that's interesting. I guess if it is a continuous system (roots are under the water all the time), it probably makes a difference. I wonder if oxygenation is beneficial with ebb-and-flow type.

Bjorn, I guess whether it is significant amount or not depends on the type of media (I forgot the correct words; water holding capacity and porosity??). It can be significant with high water holding capacity with small amount of air space. But if there is lots of air space then air with 200ml/l may dominate as the source of O2.

How do you carry bog water? Do you have a pickup truck with a big water tank (lots of alaskans have to carry their drinking water like this)?

 

[Bjorn]

Bjorn, I guess whether it is significant amount or not depends on the type of media (I forgot the correct words; water holding capacity and porosity??). It can be significant with high water holding capacity with small amount of air space. But if there is lots of air space then air with 200ml/l may dominate as the source of O2.

How do you carry bog water? Do you have a pickup truck with a big water tank (lots of alaskans have to carry their drinking water like this)?

Its some 30meters from the green-house Pump and some PE-tubing takes care of the transport. I am the proud owner of some Norwegian bog and that why I have toads in my greenhouse. Each year the tadpoles transform to tiny toads and frogs and crawl all around in the garden. This year it was so plentyful that you had to watch your step in order not to kill too many. Lasts only a few days, they disperse into the surrounding woods eventually.

 

[Rick]

Oh, I see what you are getting. But the initial low pH with fertilization is due to the release of H+ (replaced by cations) which Bjorn explained in the first post. That's why pour-through measured after fertilization show much lower pH (pH 3.8 while fertilizer had pH 6) than the case where water was applied before pour-through. Pour-through was measured 30min after irrigation/fertigation. This initial drop in pH is indicative of high CEC. This is in the first paragraph of Discussion section.

Whether by H+ exchange or bacterial action. The scary part was that the incoming pH of the fert was adjusted up to 6.0 with NaOH and the media pH is shown to have pH of 3.8 almost instantly.

So the practice of raising pH of fert to compensate for substrate acidification seems to be pretty worthless.

Also if this is strictly a chemical CEC effect then you are doomed to never elevating pH with any base (Na, Ca, K, Mg OH) because you are saying that every cation going in is going to release H+ and run the pH down below that for neutraizing the incoming OH

 

[Bjorn]

Another reason why low CEC substrates are better! Joke aside, remember that the number of sites is finite and they will get consumed and even if new ones are produced by the break-down of the substrate, its not impossible to handle. If you add alkalinity for instance, the reaction between the acid and the hydrogencarbonate wil make the carbonate decompose and evaporate as carbon dioxide. That way, having hard water with some alkalinity may be beneficial for your growing.
I do not have alkalinity in my water, so my approach is to reduce the CEC by not using sphagnum nor charcoal.

 

[Rick]

Lack of air and space.
And perhaps it is overloaded with a nutrient imbalance.

and you could add increase in pathogenic species of bacteria and fungus.

If you have conditions suitable for bacterial decomposition then there's a good chance once they are done with the bark they make the concerted attack on the living.

 

[Erythrone]

Rick, are you sure bacteria and fungii able to decompose bark are harmfull to orchids ? I always thought wood decying organisms were unable to attack living roots of herbaceus plants.

 

[Brabantia]

Another reason why low CEC substrates are better! Joke aside, remember that the number of sites is finite and they will get consumed and even if new ones are produced by the break-down of the substrate, its not impossible to handle. If you add alkalinity for instance, the reaction between the acid and the hydrogencarbonate wil make the carbonate decompose and evaporate as carbon dioxide. That way, having hard water with some alkalinity may be beneficial for your growing.
I do not have alkalinity in my water, so my approach is to reduce the CEC by not using sphagnum nor charcoal.

A more mineral substrate then ... little by little we get there!

 

[Stone]

and you could add increase in pathogenic species of bacteria and fungus.

If you have conditions suitable for bacterial decomposition then there's a good chance once they are done with the bark they make the concerted attack on the living

No chance of that at all. Completely different species.

 

[Erythrone]

No chance of that at all. Completely different species.

I agree

 

[Rick]

I agree

You think the same species of microflora living in cesspool are the same as the species living in a mountain trout stream?

 

[Rick]

The microbes have a part in this picture as well, but they are just consuming nutrients and producing colloids and what else they produce.

Actually nitrifying bacteria don't "consume" ammonia, but convert it from ammonia to nitrate (in a 2 species 2 step process) liberating H+ in the process.

This is a very fast process which makes all municipal waste water treatment systems (and your home aquarium) work.

This process has been well documented in agricultural systems when higher percentage ammonia and urea feeds are utilized.

http://www.upc.edu/growingmediacomp...growingmediacomposting2011.ISHS/s4-8-restrepo

But check out the above presentation. A bacterial nitrification inhibitor was added to stop the conversion of ammonia to nitrate (seemingly to get more ammonia into the plants). However, plant N uptake was already maxed out so instead of blowing nitrate out of the pots they leached out straight ammonia.

 

[Bjorn]

From where did the nitrifyers come? What if you fertilise with K-liter only?
A nice presentation of the effect of nitrification inhibitor on the nitrification of urea in soil and coconut fibers.
1) these mixes probably have a retention time way above the potting mixes we use,
2) Isn't introducing nitrate vs urea a side-track? Unless the bacteria in question causes rot?
3) generally; if we tried to find results on monocots.. (but in this case I think its ok)

 

[myxodex]

I've had pot acidification problems as well. I live in a hard water area (HCO3 =264 ppm; Ca ~ 120 ppm). I use sphagnum moss in my medium. I used to pre-soak the sphag in the rain water I use to water with, but then I changed this to 3 changes of straight tap water for half an hour each, squeezing out the excess water after each change. The water from the first change shows a drop in pH, but not so much afterwards.
I also add 5% tap water to my fert mix and my pH says above pH 5.

 

[Rick]

From where did the nitrifyers come? What if you fertilise with K-liter only?
A nice presentation of the effect of nitrification inhibitor on the nitrification of urea in soil and coconut fibers.
1) these mixes probably have a retention time way above the potting mixes we use,
2) Isn't introducing nitrate vs urea a side-track? Unless the bacteria in question causes rot?
3) generally; if we tried to find results on monocots.. (but in this case I think its ok)

Nitrifiers are ubiquitous in soils and aquatic systems. They probably are on your hands when after you handle the bathroom door knoboke:

The residency time is short. Under aquarium applications with a trickle filter running on a tank with a 6X volume/hour turnover rate (or industrial/municipal waste treatment plant) the contact time with the biological media is on the order of minutes to get complete conversion from ammonia to nitrate. De-nitrification (from nitrate to N2 gas) is usually a little slower, and needs carbon input.

Note in the rose pot example that two levels of biological activity are occurring.
1) Urea is split into ammonia by bacteria
2) ammonia is converted to nitrate.

The conversion from ammonia to nitrate is actually a 2 step process using 2 species of bacteria living in spherical colonies. So in the pots stopping nitrification they should have measured nitrites in leachates to see which stay is effected more and do a more complete mass balance of N going through the pots.

Also note that plant parameters (including N in leaf tissue) were not significantly improved with more urea getting stuck as ammonia.

There is a full paper of this presentation on the internet, but I can't find it for free. From reading the free abstract there was also mention of pH changes.

Bottom line is that the plants took up almost nothing. What went into the pot (whether going in as urea or ammonia) most of it leaves the pot un-utilized by the plant as nitrate via bacterial activity.

 

[Rick]

A nice presentation of the effect of nitrification inhibitor on the nitrification of urea in soil and coconut fibers.

Not sure how much of this experiment was conducted in "soil".

The alternate media to coconut fiber was "burnt rice husks".

And one of the slide points was on growing in soil-less media.

Maybe in the full paper is there more detail of soil systems.?

 

[Rick]

Also the above look at aqueous leachates only looks at the aerobic side of pot microbiology.

Since the anaerobic side is taking nitrates to N2 gas (nitrous oxide if conditions are not efficient) you would need to sample the gas above the pots to complete the mass balance calculations for what happens to N going into pots.


http://naldc.nal.usda.gov/naldc/download.xhtml?id=54341&content=PDF

This paper does show significant nitrous oxide burping out of pots with fertilizer application.