Citric Acid

Another very useful additive is citric acid. The reason citric acid is a great additive is it bonds to P anions keeping them soluble in the rhizosphere and soil solution. Also citric acid helps the Krebs cycle. The reason P anions are not needed by plants at the same level of N, K or Ca is that P anions are not readily available to plants in nature, thus they have evolved to use lower levels of P than other major elements. I could explain all the why's but this post would get long and IME most people don't care about the why's as it gets technical in the topic of soil science.

In short, P anions are not very mobile in the soil solution (thin layer of water surrounding particles in media, soilless and soil) and are readily bound and made insoluble to media particles and other elements and organic substances; critic acid prevents this. The soil solution holds cations in equilibrium to the cations held in cation exchange sites of media particles. Roots exude large amounts of citric acid that keeps P anions soluble in rhizosphere and soil solution. Roots also 'feed themselves' by exuding critic acid (and other acidic chemicals/substances) that mineralizes P anions from organic matter like soft rock phosphate, etc.

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spurr said:

@ VG:

Here is some info about citric acid, and other ornic acids exudates by roots and microbes that solublize organic matter. For info on citric acid see the references below, esp. the first few, hope this is helpful. (thanks MM for the email)
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I hope I didn't come across as claiming their are saprotrophic plants; AFAIK there is no such thing, plants that where termed saprophytes** are really parasitic upon mycorrhizal fungi. Plants that were termed saprophytes are now termed "myco-heterotrophs" (see references below). I was referring to normal plants the can break down organic matter essentially feeding themselves, not myco-heterotrophs like non-photosynthetic orchards. Dicots tend to emit more organic acids than other monocots, esp. leugems, to my understanding.

** The term saprotrophic or saprophyte is now detritivores or saprophage. So saprophyte fungi are now called saprophage fungi.

I believe P is a major element that is freed (solubized via mineralization) from organic matter when roots emit or create weak acids in the rhizosphere and soil solution. Other elements like Ca, Fe, and I think N, etc, can be solublized too. The amount of ions the plant solublizes via acids isn't high, but can make a difference for P anion which has poor soil solution mobility and often gets tied up.

I'm sure you know most of this already, so please ignore any info that seems to assume you don't know something (like info about the soil solution).

Below I use "solublize" and "solublization" interchangeably with "mineralization"; to mean ions are freed from organic matter.


Here are some acids emitted or made by roots:

- Carboxylic acid: emitted by roots, lowers rhizosphere pH which effects ion availability (esp. P) and also solublizes some organic matter releasing ions the plant cant take up (ex. soft rock phosphate especially when the bulk soil pH is weakly acidic like 6 to < 7).

- Carbonic acid: similar to carboxylic acid but carboinic acid dissociates back into Co2 and water in the presence of water. Carbonic acid is formed when the roots emit Co2 and the Co2 mixes with water. Carbonic acids are pretty stable, however, in the presence of water it can dissociate into Co2 and water. Carbonic acid are similar to carboxylic acid in terms of rhizsphpere pH, and pH of soil solution and mineralization of some types of organic matter. Carbonic acid can be dissociated into Co2 and water via. exudates by bacteria (i.e. "carbonic anhydrase") that prefer neutral to basic pH levels.

- H+ protons: emitted by roots when roots take up ammonicial nitrogen such as ammonium (NH4+). This is why ammonium is said to be acidic, because of the reaction by the roots after taking in the cation. When H+ protons are emitted by roots it lowers rhizosphere pH and also can solublize some organic matter freeing ions the roots can use. When roots absorb nitrate N they emit bicarbontes, which are basic, and is why nitrate is said to be a basic (alkaline) form of nitrogen (roots also emit anions when nitrate is absorbed).

- Citrate (ex. citric acid): emitted by roots and lowers rhizosphere pH and can solubilize some organic matter. One of the acidic exudates with the highest level release by roots

- Oxalate: can provide nearly the same level of mineralization of soil P (from organic matter) as citrates. Oxalate is an acidic exudate of some mycorrhizal fungi (ex. AM fungi) used to solublize soil P.
​

The acids listed above is not a complete list, just an example. By the process of mineralization of organic matter the exudates (ex. citric acid) releases ions into rhizosphere and the soil solution (thin film of water surrounding media particles; where microbes and ions can be found). The roots can then use the ions in the soil solution or rhizosphere depending upon the pH of the liquid and solubility of the ions. The soil solution has an equilibrium of cations to those held by media cation exchange sites. Citrate has a large affect upon P anion in terms of increasing their bio-availability to roots, this is in regards to solubility of P anions already in the soil solution and/or rhizosphere.

Below are only a couple of quick references I have handy, I will put together other references for you. I think the weak acids I listed above are the same weak acids the Luebkes (e.g. Controlled Microbial Composting) refer to in terms of roots exudates that can solublize some organic matter.

Here is a good quote from the paper: Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review
References:

1. Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review
Philippe Hinsinger
Plant and Soil 237: 173–195, 2001.
(full text) http://www.plantstress.com/Articles/min_deficiency_m/P-bioavailability.pdf


2. Effects of root exudates on nutrient availability in the rhizosphere
A. Gransee
Plant Nutrition Developments in Plant and Soil Sciences, 2002, Volume 92, Symposium 9, 626-627,
(abstract only for now) http://www.springerlink.com/content/n216w5006k27754p/


3. The release of root exudates as affected by the plant physiological status
R. Pinton, Z.Varanini, Z. Nannipieri (eds.)
The Rhizosphere: Biochemistry and organic substances at the soil-plant interface. Marcel Dekker 2000.
(full text)
http://www-mykopat.slu.se/Newwebsite/kurser/SUMMER05/READING/Roemheld/NeumannRoemheld2.pdf


4. Root exudates as mediators of mineral acquisition in low-nutrient environments
Felix D. Dakora1,3 & Donald A. Phillips
Plant and Soil 245: 35–47, 2002
(full text) http://www.fsl.orst.edu/~bond/fs561/references/Dakora%20and%20Phillips%202002%20Root%20exudates.pdf


5. Carbonic Acid Decomposition
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
(full text) http://www.newton.dep.anl.gov/askasci/chem99/chem99661.htm


6. "Ecosystem Physiology: The Plant-Microbe Dance"
(I do not agree with everything written in this reference, but the info about carbonic acid is sound)
by Leslie H. Kirkegarrd
(full text) http://grow-orchid-grow.com/science_corner/Ecosystem_Physiology_2_The_Plant-Microbe_Dance.html


7. "A Touch of Chemistry"
http://www.treedictionary.com/DICT2003/shigo/CHEM.html


8. "Troubles in the Rhizosphere"
http://www.treedictionary.com/DICT2003/shigo/RHIZO.html


9. Plants parasitic on fungi: unearthing the fungi in myco-heterotrophs and debunking the ‘saprophytic’ plant myth
JONATHAN R. LEAKE
Mycologist, Volume 19, Part 3 August 2005.
(full text) http://www-mykopat.slu.se/Groningen/Leake.pdf


10. Myco-Heterotrophs: Hacking the Mycorrhizal Network
Peter Werner
Mycena News, March 2006
(fell text) http://www.mykoweb.com/articles/Myco-Heterotrophs.html

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spurr said:

Sounds good, it would get lost here you're right. FWIW, I use citric acid I buy from a wine and beer brewing shop to drop water pH from 6.00 to 6.10. Dropping pH to that range allows me to add a decent amount of critic acid which is my goal.

I don't use citric acid to drop pH but it's a side effect of using it to increase the Krebs cycle, organic matter mineralization and P anion solubility in rhizosphere and soil solution. As has been discussed elsewhere, for organics a water pH from 5.5 to 7.5 is fine, even up to 8 is OK, and water pH has very, very little effect upon pH of media (rhizosphere and soil solution). The alkalinity of water is what has much greater effect upon pH of the media than the pH of water; and roots/microbes have a great effect upon pH of rhizosphere and soil solution.

Water with high alkalinity has pH over 7 but water with pH over 7 doesn't necessarily mean high alkalinity.

In hydro with chems water pH is much more of a concern, as well as alkalinity and level of ammonicial N (ex. ammonium, which makes roots release acidic H+ protons lowering pH) and nitrate (which makes roots release basic bicarbonates increasing pH).

On the topic of ammonicial N, that is one reason why fungal rich media has lower pH. Because fungal rich media often has greater pools of ammonicial N than does bacterial rich media; thus when roots take in the ammonicial N (mostly as ammonium, NH4+) they release acidic H+ protons. Bacterial rich media often has greater pools of nitrates (NO3-); thus when roots take in the nitrate N they release basic bicarbonates increasing pH.

P.S. I have other papers on the topic of critic acid I can post in your thread you will start.

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spurr said:

@ VG,

Below is a good article from Greenhouse Grower about P and plant growth. It covers what I wrote about in more detail, such as Fe making soluble P anions insoluble (not bio-available to plant roots) and low movement of insoluble P anions within soil solution.

I think you read my other thread I wrote elsewhere ("Tissue nutrient analysis of Cannabis = rethinking Lucas and N-P-K-Mg-Ca-S") providing lots of cannabis tissue analysis and references about why lower levels of P is good for cannabis. And that the very high levels of P given to cannabis is silly and not helpful. This article covers these topics well. For my ongoing tests using chem ferts with compost teas and microbial rich inputs (using a microscope to see the effects upon microbes) I keep P level below 50 ppm all the time, but most cannabis ferts, ex. Lucas formula, provides P over 100 ppm!

High P means greater plant stretch (internodal length) and less root growth (root to shoot ratio), along with other potential problems. This is why boosting P during pre-flowering isn't wise IMO.

In the quote below the author discusses what happens to P anions in rhizosphere and soil solution, however, we as cannabis growers tend to provide much more Fe than greenhouse growers not growing cannabis. Thus Fe has greater impact upon making soluble P anions insoluble. As well as the effects of media particles making P anions insoluble (binding to them, but not in terms of anion exchange capacity). This is why I think adding citric acid is wise because it prevents/reduces Fe and media particles from making P anions insoluble.


"An Eye On Phosoporus Nutrition: Understanding how phosphorus acts in soil and in the plant will help you regulate vegetable transplant growth."
By Shiv Reddy; September 2010

http://www.greenhousegrower.com/magazine/?storyid=3753

Isn’t Phosphorus A Starter Fertilizer?

Phosphorus as a starter-fertilizer concept seems to have come from professional cousins to vegetable transplant growers: field farmers. In the field, high-phosphorus fertilizers are applied at the start of planting and are indeed useful there. Why? Most mineral soils have aluminum and iron. Aluminum and iron react with phosphorus and precipitate and render most of the applied phosphorus insoluble and not available to the plants immediately. A great portion of the applied phosphorus ends up fixed by these soils. High levels of phosphorus are thus needed to satisfy the soil and then have some available for the plants.

Also, early plantings in cool spring weather experience phosphorus deficiency: You may have seen purpling of leaves. In cold temperatures, phosphorus solubility decreases and phosphorus moves less in the soil. Plant root growth is also slow. To cope with these conditions, farmers apply high rates of phosphorus as a starter fertilizer, so at least some of it is accessible to the small root system of young plants.

But vegetable transplants are not grown in mineral soils or in cold temperatures. Vegetable transplants are grown in soilless, peat-lite mixes and in temperature-controlled greenhouses. Peat or other components of mixes contain little aluminum and iron. Whatever phosphorus you apply in these conditions is immediately available to the plants. High phosphorus levels, as in field farming, are not needed for growing in peat mixes, either at the start or later.

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For anyone who wants to read the thread I wrote "Tissue nutrient analysis of Cannabis = rethinking Lucas and N-P-K-Mg-Ca-S" I uploaded here: (link) and the password to decrypt the file is "references" (without quotes). The file host is in Russia and doesn't require Javascript, etc., thus it's more anonymous IP wise than a U.S.file host.

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MrFista said:

Spurr, I'm trying to make a citric/dolomite buffer and my software is spouting nonsense (Curtipot) when I add ions to it. I can't find pKa for dolomite anywhere's and calculating it it as 4 ions is what is returning gibberish.

Any decent acid/base freeware you know of. The math is ridiculous for me to attempt about 4 papers over my pay grade.

All I've got is from breaking down the molecule itself - it's a triprotic acid, 3 -OH attached to C=O groups.

If you know a reference explaining davis general equation a bit better, or better freeware, or just that damned elusive dolomite pKa, would help a lot.

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Clackamas Coot said:

...I believe that part of the reason that it's added is to keep the fruit and vegetables from oxidizing (turning brown) - that's what I think I've heard.

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