A while back I read this statement on the internet forums;
"I have only been looking into root exudates a couple of years now, but not something that I dwell on as I have good root systems."
This made me realize that there is a large presence of misunderstanding about the function of root excretions as they relate to nutrient uptake and how they form the basis of natural (organic) growth.
I have written brief statements on the subject previously when discussing the microbial nutrient loop in the rhizosphere (root zone), plant control of homeostasis & nutrient provision and the microbial hierarchy of living soil.
I read through some of the more recent publications regarding root exudations with hopes new research might help me to give a simple explanation of the nutrient cycle related to organic acids secreted by roots and microbes. No such luck.
There are some advanced studies but they actually reveal more complexity and an overlapping role of the molecular compounds exuded by the roots into the soil. The (basic) exudates include organic acids, amino acids, carbohydrates (sugars) and hormones.
These influence many functions from nutrient assimilation/provision to pathogen & pest control to growth promotion or prevention of neighboring plants. There is new research which seems to validate some hypotheses I proposed around 10 years ago concerning plant roots discharging various molecular compounds (structures) to feed or attract specific microorganisms which in turn process (provide) specific nutrients or services.
In this small article I'll limit the discussion to exudates involved in the acquisition of nutrients into the soil solution where they can be up-taken by roots (plants). I'll be attempting to express this as simply as possible for the sake of the reader and the author. Please let me know if or where I have erred.
Bear in mind that this information is not given as a growing prescription but only to help growers comprehend what is going on and to be somewhat supportive of living soil horticultural systems.
Function In The Soil
To get an important definition out of the way, in this write-up, soil solution is that moisturized film adjacent to roots where nutrients become bio-available. This zone can be in constant flux as certain nutrients enter into it, mostly ionized and are immediately up-taken by roots and microorganisms.
Most growers have now been made aware of the meaning of CEC (cation exchange capacity), wherein positive charged cations are adhered to negatively charged organic matter or clay particles in the soil. The greater the CEC the greater the capacity to store these types of nutrients.
Furthermore, many growers know these nutrients can be released into the soil solution as (bio-available) ions by hydrogens (bonds) correlating to the positive charge (number of electrons lost) bonded to the nutrient (cation) molecule. This is the cation exchange where nutrient ions are made available for plant root uptake. This is the power of hydrogen. Indeed the power or potential of hydrogen in the soil solution is what pH is.
What growers may not be aware of is, where these hydrogens come from. Two major sources of them are soil microbes (bacteria, archaea & fungi) and roots. They are part of the molecular structures known as organic acids which are one of the root exudates. I'm only going to attempt discussing the nutrient acquisition role of organic acids, however they serve a number of functions, including soil pedogenosis (or development) and even as nutrients themselves.
Organic acids play a major role in nutrient acquisition for the plant, however as mentioned earlier there are some other compounds at play in the scenario. There is some cross over between function of organic acids, amino acids and carbohydrates wherein each sometimes is microbial food or functions to release nutrients. There are also still many unknowns. For the purposes of the situation I'm discussing, organic acids are more nutrient release agents, while amino acids and carbohydrates are more microbial food (attractant).
Please know that my interpretation is open to criticism as I endeavor to simplify the complex. I am encouraged that the unfolding pictures viewed in my mind some years back have been modestly validated.
In simple terms the plant itself excretes the organic acids which free up desired nutrients stored in soil and organic matter but it also excretes carbohydrates and amino acids that attract and feed bacteria, archaea and fungi which pump out these same (or differing) organic acids. In this way the nutrient economy multiplies for the plant, with less energy expenditure by the plant.
To try to understand what occurs when organic acids, exuded by roots and microbes, displace cations (nutrients) held by soil particles, let's first look at the net charges comprising these nutrient compounds.
Common Positively Charged Soil Cations
(can be nutrients, micronutrients and [neutral/harmful] )
calcium (Ca+2) - net positive charge; ionized by losing 2 electrons; 2 hydrogens required to release
magnesium (Mg+2) - net positive charge; ionized by losing 2 electrons; 2 hydrogens required to release
potassium (K+) - net positive charge; ionized by losing 1 electron; 1 hydrogen required to release
ammonium (NH4+) - net positive charge; ionized by losing 4 electrons; 4 hydrogens required to release
and so on.....
iron (Fe+2) - net positive charge; ionized by losing 2 electrons
manganese (Mn+2) - net positive charge; ionized by losing 2 electrons
zinc (Zn+2) - net positive charge; ionized by losing 2 electrons
copper (Cu+2) - net positive charge; ionized by losing 2 electrons
cobalt (Co+2) - net positive charge; ionized by losing 2 electrons
nickel (Ni+2) - net positive charge; ionized by losing 2 electrons
[aluminium (Al+3) - is toxic to most plant species at <5.5 pH soil solution]
[hydrogen (H+) - functions to affect pH]
[sodium (Na+) - rarely used as a nutrient; plays a role in pH and osmosis;]
Then look at the number of hydrogens bonded to the organic acids, considering that an equal number of hydrogens is required for the number of electrons to alter the compounds in order to release them as ions into the soil solution.
Some Common Organic Acids
(excreted by plants and microorganisms)
acetic acid, CH3COOH - total of 4 hydrogens
citric acid, H2C6H6O7 - total of 8 hydrogens
fumaric acid, C4H4O4 - total of 4 hydrogens
formic acid, HCOOH - total of 2 hydrogens
oxalic acid, H2C2O4 - total of 2 hydrogens
malic acid, H2C4H4O5 - total of 6 hydrogens
malonic acid, CH2(COOH)2 - total of 4 hydrogens
propionic acid, CH3CH2COOH - total of 6 hydrogens
succinic acid, C4H6O4 - total of 6 hydrogens
tartaric acid, H2C4H4O6 - total of 6 hydrogens
gluconic acid, C6H12O7 - total of 12 hydrogens
For example, by looking at the two lists above we can estimate that citric acid could potentially release 4 calcium ions, if citric acid is specific to calcium and all 8 hydrogens are exchangeable (8 divided by 2).
I've not researched information showing the specific combinations of organic acids exuded by roots and microbes to implement the corresponding release of specific nutrients into the soil solution (excepting citric acid mobilizing phosphorus & calcium). However one can see by looking at the numbers of hydrogens bonded to the various molecular structures of organic acids that there are corresponding positive charges [or numbers of electrons] on nutrient compounds which can be exchanged for (or knocked off) to ionize the molecule released into the soil solution.
"The process of gaining or losing electrons from a neutral atom or molecule is called ionization." ~ [boundless.com]
There are also anions which are negatively charged nutrient molecules. These are not stored in most soil types.
In most soils anions are mobile through the soil solution and are supplied ongoing by fertilizers or as they are degraded from organic matter and minerals and held within bodies of microbes until excreted or otherwise transported to the plant. There is involvement of organic acids in acquisition of anions in similar fashion to cations, particularly of insolubilized phosphate.
Common Soil Anions
chlorine (Cl-) - net negative charge; ionized by gaining 1 electron
nitrate (NO3-) - net negative charge; ionized by gaining 3 electrons
sulfide (S2-) - net negative charge; ionized by gaining 2 electrons
sulfate (SO42-) ....and so on
The Role of Predators
Beyond or on top of this method of nutrient assimilation is another step up of the nutrient economy initiated by the plant. Earlier I mentioned the plant attracts and feeds bacteria, archaea and fungi (with excretions of carbohydrates and amino acids) to in turn release the same organic acids. These organisms feed on some of the ions as well so one could think that the plant is stupid to encourage this competition, however as the bacteria and archaea multiply, protozoa (flagellates, ciliates & amoebae) are attracted to the rhizosphere (soil solution).
They begin feasting on the bacteria & archaea and dividing as quickly as every two hours [or even less?]. Nature's clever hedge fund has set up a system wherein the energy requirement for these soil protozoa is 10 to 40 percent of what they intake. What (energy) they expel is 60 to 90% of a multiplied ionic form nutrient, of course bio-available to the roots of the plant. Bacterial feeding nematodes attracted to the grazing area contribute similar nutrient value although with a lesser return on investment.
The fungi serve to degrade matter and materials to a form available to other organisms and some form mycorrhizal or endophytic relationships with the plant.
Does the predation cycle use a similar exchange system as we see in the cation exchange between plant roots and soil/clay particles? Perhaps in reverse so the microorganism's needs vary from those of the plant?
These cycles can take place for up to 24 hours (or more?) or may terminate within a couple of hours.
Because of all this hydrogen spilling into the soil solution, I am led to realize that the pH must fluctuate in different areas and at different times according to the needs of the plant, organisms & soil. If using natural growing techniques, hypothetically this is controlled by interplay between root excretions and microbial activity. I therefore wonder what effect, control of the overall pH in soil has beyond a gross scale target where soil is very acidic or alkaline.
Can one accurately check pH levels in the soil solution and is the time/nutrient phase it is tested in, a factor?
Boron, The Weird One
I've got to mention briefly that during researching for this little essay, I discovered a number of seemingly contradictory and incorrect (outdated) statements about boron and its assimilation by plants. Boron originates from cosmic rays along with two other elements found on earth lithium and beryllium. [This makes for some interesting reading for those interested; think black holes; or God's pixie dust]
Most information seemed to state that boron was just there, mobile in the soil and taken up easily if present and toxic if there is too much. My first clue was that boron (B2O3) carries a mix of positive and negative ions so requires more energy to ionize it to a form assimilated by roots. I could not resolve within my puny brain logic, how it is taken into the plant.
Some further looking revealed that it is actually the borate ion (BO3-) or boric acid (H3BO3) which is the form of boron taken up from the soil as an uncharged molecule. These are mostly stored in humus materials of organic matter. They are moved across (through) the cell wall membrane via protein transporters. These proteins were revealed through research within the last 16 years or so. [another fun research project for some]
So guess what? Uptake of boron is not a passive undertaking. It is regulated by plants. You might ask, then how do plants acquire boron toxicity from soils with high levels of the boron constituents? One needs to ponder again whether this could be the result of human interference in one form or another.
Like I said earlier, this is not meant to be any form of growing prescription. I've been accused many times of saying that growing is all about organic matter and microorganisms and even that one must have a microscope to grow adequately. Not so.
I've always stated that I'm just about trying to explain what is going on, to the best of my ability and when it comes to gardening, I say, be all inclusive so long as you are doing no harm. It's not about minerals OR microbes and compost, it's about minerals, organic matter AND microbes.
Many growers are in it to push the envelope, some for fun, like giant pumpkin growers, some for profit or bragging rights, like cannabis growers looking for those giant dense 'buds' [pot language for flowers]. The thing is; giant pumpkin growers don't eat their produce (I think).
Many have learned that natural growing produces higher quality vegetables, fruit and herbs (equivalent of nature farming, not the commercial meaning of natural). If you want your tomatoes or cannabis to increase in yield go with caution and read, watch and listen. Lest we forget the tobacco growers who thought phosphorus fertilizer was their key to the vault; the price was high levels of polonium 210 and lead 210 stored in tissues of glandular trichomes which some hypothesize is the true cause of lung cancer in smokers.
I hope I've managed to convey at least the basic function of root exudates for nutrient acquisition and that with natural growing the plant is not a sponge to just suck up the ratios of ingredients provided. One must just ensure that all components are provided in adequate amounts and in a stable form degradable by the organisms.
Examine all information, including mine, with skepticism.
(in no particular order)
Organic acid behavior in soils – misconceptions and knowledge gaps
D.L. Jones1,3, P.G. Dennis1, A.G. Owen1 & P.A.W. van Hees2
Plant and Soil 248: 31–41, 2003.
Root exudation of sugars, amino acids, and organic acids by maize as affected by nitrogen, phosphorus, potassium, and iron deficiency
Lilia C. Carvalhais, Paul G. Dennis, Dmitri Fedoseyenko, Mohammad-Reza Hajirezaei, Rainer Borriss, and Nicolaus von Wirén ~ J. Plant Nutr. Soil Sci. 2010, 000, 1–9
Aliphatic, Cyclic, and Aromatic Organic Acids, Vitamins, and Carbohydrates in Soil: A Review
Valerie Vranova, Klement Rejsek, and Pavel Formanek
The ScientificWorld Journal Volume 2013, Article ID 524239
Organic acid induced release of nutrients from metal-stabilized soil organic matter – The unbutton model
Marianne Clarholm, Ulf Skyllberg, Anna Rosling
Soil Biology and Biochemistry; vol. 84, May 2015
Gluconic acid production by bacteria to liberate phosphorus from
insoluble phosphate complexes
M. Stella and M.S. Halimi ~ J. Trop. Agric. and Fd. Sc. 43(1)(2015): 41 – 53
Sodium as nutrient and toxicant
Herbert J. Kronzucker, Devrim Coskun, Lasse M. Schulze, Jessie R. Wong
& Dev T. Britto ~ Plant Soil (2013) 369:1–23
Interaction of micronutrients with major nutrients with special reference to potassium UJWALA RANADE-MALVI
Institute for Micronutrient Technology, Pune - 411 048, India
Karnataka J. Agric. Sci.,24 (1) :(106-109) 2011
Aluminium Toxicity Targets in Plants
S´onia Silva ~ Journal of Botany; Volume 2012, Article ID 219462
Role of proteinaceous amino acids released in root exudates in nutrient acquisition from the rhizosphere
DL Jones, AC Edwards, K Donachie, PR Darrah ~ Plant & Soil, Jan. 1994
Amino acids in the rhizosphere: From plants to microbes
LUKE A. MOE ~ American Journal of Botany 100(9): 1692–1705. 2013
BC. Open Textbooks - Introductory Chemistry
Michigan State University Extension
University of Hawaii - Soil Management Manoa
Arkansas State University - Department of Chemistry & Physics
Elcamino College - www.elcamino.edu
GPB Media - gpb.org
The Only Three Heavy Elements In The Universe That Aren't Made In Stars by Ethan Siegel - Forbes - July 1, 2015
Separation and Analysis of Boron Isotope in High Plant by Thermal Ionization Mass Spectrometry
Qingcai Xu, Yuliang Dong, Huayu Zhu, and Aide Sun
International Journal of Analytical Chemistry Volume 2015, Article ID 364242
Unravelling the interactions of Boron with natural
organic matter (NOM) on a molecular level
András Gáspár ~ Thesis presentation 2008
Lithium-Beryllium-Boron: Origin and Evolution
Elisabeth Vangioni-Flam, Michel Casse and Jean Audouze
astro-ph/9907171 June 1999
Effect of Composted Organic Matter on Boron Uptake by Plants
U. Yermiyahu, R. Keren, and Y. Chen ~ SOIL SCI. SOC. AM. J., VOL. 65, SEPTEMBER–OCTOBER 2001
Boron transport in plants: co-ordinated regulation of transporters
Kyoko Miwa and Toru Fujiwara ~ Annals of Botany 105: 1103–1108, 2010