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Work in Progress - unedited. We're in the process of migrating information from Integral Hydroponics to our new site @www.manicbotanix.com
The full Beneficial Microbes in Hydroponic Settings article can be found here
Overview of Beneficial Microbes in Hydroponic Settings
Microbial inoculants are used in agriculture as soil amendments that use beneficial bacteria and fungi to promote plant health and nutrition. They are applied as seed dressings for direct contact with roots as they germinate. Foliar sprays are used for leaf coverage and they are applied through irrigation to inoculate the soil. While they are applied to improve plant nutrition and health their exudates can also promote/stimulate hormone production in plants, therefore promoting plant growth. Many of the beneficial bacteria and fungi form symbiotic relationships within the plant that are mutualistic. Roots themselves release exudates into the soil that are beneficial to the microorganisms which suggests a degree of co-evolution between microorganisms and plants that form the ecosystem of the rhizosphere. The use of inoculants in agriculture has been shown to extend beyond their benefits as biological fertilizers (for instance mycorrizhae fungi fix phosphorous while other microbials’ fix nitrogen). Research into the disease resistance of microbialinnoculants in crop species shows they can initiate systemic acquired resistance (SAR) to several common crop diseases. In plants SAR is a resistance response that occurs following a previous localized exposure to a plant pathogen. Once stimulated SAR can provide resistance for several days to a wide variety of pathogens. When a plant recognizes a pathogen it induces a rapid defence response called the hypersensitive response (HR). HR results in localized cell or tissue death at the site of infection, which limits further spread of infection. (Dixon et al 1994) This localized response provides non specific resistance throughout the plant; a phenomenon known as systemic acquired resistance – SAR (Ryals et al 1996). Plants produce salicylic acid as a result of the HR and this increase in concentration of salicylic acid is an activator of SAR. Research has shown that aspirin (acetyl salicylic acid) can work as a trigger for SAR. Beneficial microbes such as plant growth promoting bacteria (PGPB) and fungi can improve plant resistance to pathogens and even some insects by inducing systemic defence responses. Beneficial bacteria and fungi exudates are recognized by the plant, which results in a mild activation of plant immune responses. Plant Growth Promoting bacteria (PGPB) are considered to promote plant growth directly or indirectly. PGPB can exhibit a variety of characteristics responsible for influencing plant growth. The common traits include production of plant growth regulators (auxins etc), siderophores (iron chelating compounds), HCN (amino acid precursor) and antibiotics. Indole acetic acid (IAA) is one of the most physiologically active auxins. IAA is a common product of L-tryptophan metabolism by several microorganisms including PGPB. Microorganisms inhabiting rhizospheres of various plants are likely to synthesize and release auxin as secondary metabolites because of the rich supplies of substrates exuded from the roots compared with non-microbe inhabited soils. There is evidence that the growth hormones produced by microbes can in some instances increase growth rates and improve yields of the host plants. It is also possible that microbes capable of phosphate solubilization may improve plant productivity both by hormonal The majority of soil living beneficial bacteria require oxygen for cellular respiration (also termed “oxidative metabolism”). Bacteria that require oxygen are classed as aerobes. Aerobes also require organic material or molecules (such as glucose) to produce energy. For this reason this class of bacteria are also called aerobic heterotrophs (i.e. aerobic heterotrophs are organisms that cannot live without free oxygen and do not produce their own food). The main elements required for beneficial bacterial nutrition are C, H, O, N, S, P, K, Mg, Fe, Ca, Mn and traces of Zn, Cu and Mo. ‘Aerobic heterotrophs’ require a source of organic carbon, gaseous oxygen (air) (see note 1) and water along with the aforementioned mineral elements. Their source of energy is produced by the aerobic oxidation of organic material by metabolism to water and carbon dioxide. The energy released is stored in the phosphoanhydride bonds of ATP. When the energy is required it is released from ATP by hydrolysis. Certain environmental conditions are also required for the growth and division of bacteria like O2 concentration, pH and temperature. Note 1. In ‘Integral Hydroponics, Indoor Growing Principles for Beginners and Intermediates’ I make note of the fact that liquid bacteria and fungi products should be avoided due to stability/shelf life issues. While this statement is arguably oversimplified (i.e. there are some extremely good liquid products containing sporulating micros sold through the agricultural sector), the reason for this recommendation is due to aerobic bacteria requiring oxygen/air to survive. For the most part, liquid friendly (live) bacteria products sold through the hydro industry are unreliable; that is, an airtight bottle does not provide a viable environment in which bacteria or fungi can survive long term. Compound this with often less than ideal storage conditions and shelf life issues and you are typically purchasing a product of unknown quantity/quality (i.e. beneficial microbes may or may not be present and at what rates???). In some cases (e.g. Bacillus and Trichoderma strains) a microbe species will sporulate when faced with nutritional and/or oxygen stress. This means the living bacteria or fungi hibernate (put simply) until food or oxygen is available. For instance, Bacillus subtilis sporulation is an adaptive response to nutritional stress and involves the differential development of two cells. However, other species (e.g. Mycorrhizae) do not sporulate and their populations typically die out or are significantly reduced when faced with nutritional or oxygen stress. When purchasing bacteria and/or fungi products always purchase microbes as dry suspended spores. Microbes that sporulate • Trichoderma harzianum About Mycorrhizae (AMF) The term "mycorrhiza" literally means fungus-root. It is estimated that 80 to 90 percent of all plant species form mycorrhiza. The relationship between plant and micorrhizae is a symbiosis, the main function of which, while complex, is the transfer of carbon produced by plants to fungi (sugars created in leaves of the plant move downward and into the fungal hyphae via the roots) and the transfer of nutrients acquired by fungi to plants (the plant receives phosphorus, nitrogen, potassium, and micronutrients such as copper, sulfur and zinc). Elements that are critical in the plant/mycorrhizae symbiosis are CO2 concentration, nitrogen levels, phosphorous levels, soil matrix, pH and carbon.
AM fungi colonize the root cortex of the host plant in which the fungi are able to acquire organic carbon as food to build 'the infrastructure' for P uptake and transport.
AM fungi also acquire substantial quantities of N from organic sources and play an important role in the nitrogen cycle, intercepting inorganic N released from decomposing organic matter before roots can acquire it and passing some of this on to plants as arginine (CH2CH2CH2NH-C(NH)NH2). Additionally, a plant ammonium (NH4 N) transporter that is mycorrhiza-specific and preferentially activated in arbusculated cells has recently been discovered, suggesting that N transfer to the plant may operate in a similar manner to P transfer. 2
H.J. Hawkins et al (2004) note that a nutrient medium containing a P concentration of 0.9 mM (27.876384ppm P) failed to produce viable mycorrhizal colonisation.8 Similar findings by G.Nagahashi (1996) demonstrates that mycorrhizae grown in the presence of P at 1.0mM (30.973ppm) showed significantly less hypal branching than in lower P environments.9 Evaluation of P in Hydroponic Working Solutions
Author’s note: When considering that many hydro growers use further P via P and K additives during flower this too needs to be factored into the P equation. For instance, with a product that contained PK 13- 14 %w/w listed as P2O5 and K2O with a specific gravity of 1.25, used at 1.5mL/L this would equate to an additional 104.8ppm of P in working solution. More simply, additive + nutrient equals over 5 times the P that has been found to be detrimental to AM fungi colonization.
While the symbiosis between plants and AM fungi is complex and while more research is needed, based on current knowledge it seems probable that any potential benefits of AM fungi in hydroponics is negated by the presence of high bioavailable P in hydroponic solutions. Additionally, high bioavailable N in hydroponic solutions likely reduces the efficiency of AM fungi further. It is also possible the presence of AM fungi in hydroponic settings may be detrimental to growth rates and yields as a result of carbon drain.
References
ATP stands for Adenosine Tri-Phosphate. ATP consists of an adenosine molecule and three inorganic phosphates. ATP is the most important energy-transfer molecule in all living cells. ATP transports chemical energy within cells for metabolism. ATP is produced during photosynthesis and cellular respiration and used by enzymes and structural proteins in cellular processes, including biosynthetic reactions and cell division. Phosphorous/phosphate plays a vital role in the ATP chain. Inorganic phosphorus in the form of the phosphate PO43- plays a major role in biological molecules DNA and RNA where it forms part of the molecular structure. Living cells use phosphate to transport cellular energy in the form of ATP. Nearly every cellular process that uses energy obtains it in the form of ATP. ATP -------> ADP (Adenosine Diphosphate) + Pi (orthophosphate) + energy. For beneficial bacteria to survive in a hydroponic environment they will need ideal environmental conditions. Most hydroponic nutrients lack organic carbon sources for beneficial bacteria to survive. They can metabolise humic and fulvic extracts but one of the best sources of food for beneficial bacteria is molasses. Molasses typically contains Total Digestable Nutrients (TDN) in excess of 60%, as well as containing a number of the major elements and trace elements required by bacteria, molasses is very high (50%+) in sugars. The sugars contained in molasses are an ideal source of carbon for heterotrophs. Cobalt and molybdenum, which are not usually listed in the typical analysis of molasses, will still be found in small traces. Another property of molasses, due to the high percentage of sugars, is its’ sticking ability when used in foliar sprays. Molasses, along with a wetting agent, increases the coverage and surface holding, optimising foliar nutrition. While discussing foliar sprays and biological inputs, saponins can be used as an organic wetting agent that not only reduces the surface tension of water (i.e. surfactant – surface active agent) it also has bio stimulating properties. Saponins are chemical compounds (phytochemicals) found in abundance in various plant species. To be specific they are amphipathic glycosides. The foaming ability of saponins is because of their surfactant like structure with hydrophillic (water soluble) and hydrophobic (fat soluble) chains. Their name is derived from the plant soapwort (genus Saponaria). Most commercial saponins are extracted from Yucca schidigera (Spanish Dagger) and Quillaja saponaria (the soap bark tree). Two other prominent organic additives that act as microbial nutrients/stimulators and plant fertilisers are kelp and fish products. These not only supply nutrients but they…. More soon. ... The full Beneficial Microbes in Hydroponic Settings article can be found here
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