Published May 11, 2020 | By Mike Petersen
The group of P, K & Fe solubilizing bacteria and their activities
An overview of the mechanism used by bacteria and fungi to mobilize nutrients (P, K and Fe) in the soil. See graphic to the right. Courtesy: M.I. Rashid et. al, 2016, Microbiological Research, Elsevier Publ.
Legend of the graphic to the right of fungi and bacteria aid in the rhizosphere to make nutrients available.
In preparation for this tidbit of news I went deep into the weeds of scientific articles, journals and musings folks. I came out of it a changed soil scientist. What? I had read and been informed incorrectly in the past about the small number of microbes involved with potassium (K) releasing bacteria. It was known [to me] that only a handful (<6) bacteria and fungal microbes worked on the K from added K nutrients be they inorganic or organic. Now my head is opened and knowing that more microbes work on potassim latticed feldspars in many areas where K values in soil tests can be high(>250 ppm), it sure does not mean the K is readily available to a plant root system. In fact many crops we grow struggle to get K – why? Might it be our soils lack the right families and species of bacteria that make K available in an organic soluble form for the root to absorb I asked myself? Uhhh, duh!
The lifestream in the root rhizosphere again comes into the light so to speak. Similar to the flora in our gut and in the gut of lactating four legged critters, incredibly vital to digestion and absorption of key nutrients for all of us to sustain life, bacteria are the front-line soldiers. So how do they make K available?
Here is a quote from the authors of one of the research papers that made total sense to me as I consider the complexity of microbial action in soils: “Bacteria release various types of organic acids to solubilize K in the soil through various processes such as acidolysis, chelation, complexolysis and exchange reactions. Rashid et.al.” Those acids are citrate, malate, oxalic, tartaric, succinic, α-ketogluconic acid and oxalates. Now the complexity grows exponentially and I am not going to go into that because the weeds entangle your feet and mine way too quickly. As the microbes live, multiply and die they are making K as a nutrient available for the root hairs and epidermis to absorb, move into the cytoplasm and feed the plant for photosynthesis to keep going. I know I am throwing out a good deal of organic chemistry terms in this, but these acids break free chemical bonds holding tight the K, P, Fe ions on the clay particles and organo-complexes to become available to the root.
Deeper into the weeds: An associated group of bacteria first have to work on the mineral fraction, the rock or salt types of potassium. only few microbial strains have been isolated that have an ability to oxidize in the first step Fe2+ from primary phyllosilicates mineral in order that they release iron and K from these minerals. Bacteria species that split iron and potassium apart (Neutrophillic lithotrophs) utilize structural Fe2+ in biotite (the black felspar we see in granite) as an electron donor for their metabolism in order to produce energy and oxidize biotite. Who are those guys? The microbes which oxidized biotite (Fe2+-bearing mica) include Bradyrhizobium japonicum, Cupriavidus necator, Ralstonia solanacearum, Dechloromonas agitate, and Nocardioides sp. During the process of acidolysis, these rhizospheric microbes can chelate Aluminum and Silicon cations associated with K minerals and by doing so they also enhance the exchangeable K in soil solution. In a series of steps one group of bacteria work on the clay silicates, then another set who need to be there make the potassium available to the root for absorption. Some other scientific papers identified a number of species: Bacillus pseudomycoides, B.mucilaginosus, B. edaphicus, B. circulans, Acidothiobacillus feroxidans, Aspergillus tenens and Paenibacillus spp that were right in the soil matrix, thick as thieves on the organic components from old roots, manures, and dead/inert cellulosic materials. An important issue microbiologists have discovered that not all of the species are up and living in the soil complex together to make K available or P or N or S or Fe. I connected some dots in my head then, oh my no wonder we find in soils that crops need K — not all the players are on the roster. Wow!
You are reeling maybe some from all the chemistry, it is important to note ladies and gents that the microbiological world is so intricate and intertwined to make the soils medium an environment for the root/crop prosper, take up water and hopefully yield grain or fruit. So what in the dickens does this happen to do with Strip-Tillage on this website? When we place mineral fertilizers in the root zone with a tool like the 1tRIPr what happens besides us placing it in the root pathway and just expecting all the work out. Oh the itty-bitty critters that live in the soil (it is said that billions can be alive in 1 teaspoon) have such a gigantic role in making N,P,K,S,Fe,Zn and so on available. Every year more great research is discovering or uncovering secrets of the soil world. We are seeing that innoculating or invigorating soils with bacteria and fungi (which I did not talk much about in this blog article) has great promise to support the soil, supplement the line-up of players who belong in making the soil medium have a full roster (I am thinking of a baseball team analogy). Just placing the nutrients starts the game like throwing the first pitch. From there on it is the players swatting at the ball, hitting and catching or running it down in a certain amount of time before the harvest machine rolls in. For all of you, I played baseball, catcher in my high school years.
This kind of reading tells more of the story folks. I wrote in a previous article about potassium and its importance within the plants growth cycles, this offers you how the root obtains it. I am excited to offer this. The world of soil science is still after 50 years firing me up because of the incredible “digging deeper” gives me and hopefully you better understanding. Are you ready for more? How about Sulfur in the coming weeks
References I read to get this to YOU:
Bacteria and fungi can contribute to nutrients bioavailability and aggregate formation in degraded soils; 2016, M. I. Rashid, L. H. MujawaraT. S.Talal Almeelbi, I. M.I. Ismail, M. Oves; Microbial Research, Vol. 183, pp.26-41
Using Phyllosilicate-Fe(II)-Oxidizing Soil Bacteria to Improve Fe and K Plant Nutrition; Evgenya Shelobolina, Eric Roden, Middleton, Jason Benzine, Mai Yia Xiong, United States
(12) Patent Application Publication; 9/2014
Paving the Way From the Lab to the Field: Using Synthetic Microbial Consortia to Produce High-Quality Crops; Z Kong, M Hart, H Liu – Frontiers in plant science, 2018
Phosphate and potassium solubilizing bacteria effect on mineral uptake, soil availability and growth of eggplant, 2005; Han and Lee, 2005, H. Han, K. Lee, Res. J. Agric. Biol. Sci., 1 (2005), pp. 176-180
Beneficial plant-bacterial interactions; BR Glick – 2015 – Springer, Book
Physiology of Crop Production; 2006, N.K. Fageria, V.C. Baligar and R.B. Clark, Haworth Press, Binghamton, NY; Chapter 8
Research on potassium in agriculture: Needs and prospects; Römheld, V., Kirkby, E.A., 2010; Plant and Soil, Volume 335, Issue 1, pp. 155-180