By Stefan van Wyk (PhD), Head of Biological Research and Product Development, Victus Bio, Agri Technovation
Phosphorus (P) is a macronutrient required for the proper functioning of plants. Because phosphorus plays vital roles in every aspect of plant growth and development, deﬁciencies can reduce plant growth and development.
Phosphorus is the second most important macronutrient required by the plants, after nitrogen. Yet, availability of soluble forms of phosphorus for plants in the soils is limited because of its reactivity with iron, aluminium, and calcium in the soil forming insoluble phosphates. Most soils possess considerable amounts of phosphorus, but a large proportion is bound to soil components, most of it remains inactive and therefore unavailable to plants.
Soil with low total phosphorus can be supplemented with phosphorus fertiliser but are not able to hold the added phosphorus. About 75–90% of the added chemical phosphorus fertiliser is precipitated by metal-cation complexes and becomes rapidly ﬁxed in soils and has long-term impacts on the environment in terms of eutrophication, soil fertility depletion, and carbon footprint (Sharma et al., 2013).
Phosphorus gets immobilised by cations such as Ca2+ in calcareous or normal soils to form a complex calcium phosphate (Ca3(PO4)2) and with Al3+ and Fe3+ in acidic soils to form aluminium-phosphate complex (AlPO) and iron-phosphate complex (FePO) (Kahn et al., 2009; Kumar et al., 2018). These are insoluble forms and consequently unavailable. These accumulated phosphates in agricultural soils are adequate to maintain maximum crop yields worldwide for about 100 years (Walpola and Yoon, 2012) if it could be mobilised and converted into soluble phosphorus forms.
However, farmers who cannot aﬀord to use phosphorus fertilisers in order to reduce phosphorus deﬁcits, require alternative methods to provide phosphorus. Phosphate solubilising micro-organisms (PSMs) are a group of beneﬁcial micro-organisms capable of hydrolysing organic and inorganic insoluble phosphorus compounds to soluble phosphorus forms that can easily be assimilated by plants. PSM provides an eco-friendly and economically sound approach to overcome the phosphorus scarcity and its subsequent uptake by plants. Though PSMs have been a subject of research for decades, manipulation of PSMs is used for increasing fixed phosphorus in the soil and improving crop production at the field level. PSMs apply various approaches to make phosphorus accessible for plants to absorb.
These include lowering soil pH, chelation, and mineralisation.
- PSMs increase phosphorus availability by producing organic acids that lowers the soil pH (Satyaprakash et al.,2017). Production of organic acid, coupled with the decrease of the pH by the action of micro-organisms, resulted in phosphorus solubilisation (Selvi et al., 2017). Gluconic acid is reported as the principal organic acid produced by phosphate solubilising bacteria such as Pseudomonas spp. (Rodríguez and Fraga 1999).
- Organic and inorganic acids produced by PSM dissolve the insoluble soil phosphates by chelation of cations and competing with phosphate for adsorption sites in the soil (Pradhan and Sukla 2005; Kahn et al., 2009).
The hydroxyl and carboxyl groups of the acids chelate the cations bound to phosphate, thereby converting it into soluble forms. These acids may compete for ﬁxation sites of aluminium and iron insoluble oxides, one reacts with them, stabilise them, and are called “chelates”. 2-ketogluconic acid is a powerful chelator of calcium (Walpola and Yoon 2012).
- PSMs mineralise soil organic phosphorus by the production of phosphatases like phytase that hydrolyse organic forms of phosphate compounds, releasing inorganic phosphorus that will be immobilised by plants. Alkaline and acid phosphatases use organic phosphate as a substrate to convert it into inorganic form.
Mixed cultures of PSMs are most eﬀective in mineralising organic phosphate (Kahn et al., 2009). Some PSM produces siderophores, hydrolyse the organic phosphorus in the soil resulting in its availability (Kumar et al., 2018) (Dodor and Tabatabai, 2003).
Micro-organisms are integral in the natural phosphorus cycle. The use of phosphate solubilising micro-organisms (PSMs) as biofertilisers for agriculture enhancement has been a subject of study for years. PSM increases the availability of phosphorus without disturbing the biochemical composition of the soil. This is essentially applicable, where access to chemical fertilisers is limited.
PSM can be used for various crops and is not host speciﬁc. Several studies reported that the use of PSM enhanced growth, yield, and quality in many crops including walnut, apple, citrus, maize, rice, mustard, oil palm, aubergine and chili, soybean, wheat, sugarbeet, sugarcane, chickpea, peanut, legumes and potatoes.
PSMs have shown to enhance phosphorus uptake, the growth and the yield when applied to crop plants (Pandey et al.,2006; Vikram and Hamzehzarghani 2008). Adequate supply of phosphorus helps in seed formation and early maturation of crops like cereals and nodule formation in legumes (Dey et al., 2004).
Promoting plant growth with biofertilisers
SoluPHOSTM, which is a product offered by Agri Technovation, is an example of one of these biofertilisers that contain Pseudomonas fluorescence and Pseudomonas putida.
These multiple strains of Pseudomonas spp. are selected and combined for their synergistic plant growth-promoting benefits. Each strain produces different enzymes that degrade compounds in the soil contributing to the release of nutrients normally inaccessible to plants, inadvertently promoting plant growth. The competitive interaction of these Pseudomonas spp. further improves their plant growth-promoting effectiveness.
The application of PSMs by inoculating the
rhizosphere appears to be an eﬃcient way to convert the insoluble phosphorus compounds to plant available phosphorus forms, resulting in better plant growth, crop yield and quality. SoluPHOSTM is a combination of the most eﬃcient phosphorus solubilisers for increasing bio-availability of phosphorus in soil. PSM causes immediate plant growth by providing easily absorbable phosphorus form and production of plant growth hormones such as IAA and GA.
Furthermore, PSM supports plant growth through production of siderophore and increases eﬃciency of nitrogen ﬁxation. Therefore, PSMs represent potential substitutes for inorganic phosphate fertilisers to meet the phosphorus demands of plants, improving yield in sustainable agriculture.
Their application is an ecologically and economically sound approach. The combination of inorganic phosphate with PSM inoculum sounds preferable to minimise the risk of long-term total phosphorus soil deﬁcit.
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