Microbial inoculants also known as soil inoculant

Soil inoculant

Soil inoculants are bacteria or fungi that are added to soils in order to improve plant growth by either:*Freeing up soil nutrients for plant use.*Entering into symbiotic relationships with plant root systems....

s are agricultural amendments that use beneficial microbes (bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 or fungi) to promote plant health. Many of the microbes involved form symbiotic relationships with the target crops where both parties benefit (mutualism). While microbial inoculants are applied to improve plant nutrition, they can also be used to promote plant growth by stimulating plant hormone production (Bashan & Holguin, 1997; Sullivan, 2001).

Research into the benefits of inoculants in agriculture extends beyond their capacity as biofertilisers. Microbial inoculants can initiate systemic acquired resistance (SAR) of crop species to several common crop diseases. So far SAR has been demonstrated for powdery mildew (Blumeria graminis f. sp. hordei, Heitefuss, 2001), take-all (Gaeumannomyces graminis var. tritici, Khaosaad et al., 2007), leaf spot (Pseudomonas syringae, Ramos Solano et al., 2008) and root rot (Fusarium

Fusarium

Fusarium is a large genus of filamentous fungi widely distributed in soil and in association with plants. Most species are harmless saprobes, and are relatively abundant members of the soil microbial community. Some species produce mycotoxins in cereal crops that can affect human and animal health...

 culmorum, Waller et al. 2005).

Rhizobacterial inoculants

The rhizobacteria

Rhizobacteria

Rhizobacteria are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties...

 commonly applied as inoculants include nitrogen-fixers and phosphate-solubilisers which enhance the availability of the macronutrients nitrogen and phosphorus to the host plant. Such bacteria are commonly referred to as plant growth promoting rhizobacteria (PGPR).

Nitrogen-fixing bacteria

The most commonly applied rhizobacteria are Rhizobium

Rhizobium

Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes and Parasponia....

and closely related genera. Rhizobium are nitrogen-fixing bacteria that form symbiotic associations within nodules on the roots of legumes. This increases host nitrogen nutrition and is important to the cultivation of soybeans, chickpeas and many other leguminous crops. For non-leguminous crops, Azospirillum has been demonstrated to be beneficial for nitrogen fixation and plant nutrition (Bashan & Holguin, 1997).

For cereal crops, diazotrophic rhizobacteria have increased plant growth (Galal et al., 2003), grain yield (Caballero-Mellado et al., 1992), nitrogen and phosphorus uptake (Galal et al., 2003), and nitrogen (Caballero-Mellado et al., 1992), phosphorus (Caballero-Mellado et al., 1992; Belimov et al., 1995) and potassium content (Caballero-Mellado et al., 1992).

Phosphate-solubilising bacteria

To improve phosphorus nutrition, the use of phosphate-solubilising bacteria (PSB) such as Agrobacterium radiobacter has also received attention (Belimov et al., 1995a; 1995b; Singh & Kapoor, 1999). As the name suggests, PSB are free-living bacteria that break down inorganic soil phosphates to simpler forms that enable uptake by plants.

Fungal inoculants

Several different fungal inoculants have been explored for their benefits to plant nutrition. The most commonly investigated fungi for this purpose are the arbuscular mycorrhizae (AM). Other endophytic fungi, such as Piriformis indica can also be beneficial (Waller et al., 2005).

Composite inoculants

The combination of strains of Plant Growth Promoting Rhizobacteria has been shown to benefit rice (Oryza, Nguyen et al. (2002)) and barley (Hordeum, Belimov et al. (1995a)). The main benefit from dual inoculants is increased plant nutrient uptake, from both soil and fertiliser (Bashan et al., 2004; Belimov et al. 1995a). Interestingly, multiple strain inoculants have also been demonstrated to increase total nitrogenase activity compared to single strain inoculants, even when only one strain is diazotrophic (Lippi et al., 1992; Khammas & Kaiser, 1992, Belimov et al. 1995a).

PGPR and arbuscular mycorrhizae in combination can be useful in increasing wheat growth in nutrient poor soil (Singh & Kapoor, 1999) and improving nitrogen-extraction from fertilised soils (Galal et al., 2003). In salinised soils, Rabie (2005) found that inoculating AM-infected Vicia faba plants with Azospirillum brasilense amplified the beneficial effects of AM inoculation.

External links

http://www.satavic.org/biofertilisers.htm

http://mycorrhiza.ag.utk.edu Mycorrhiza Literature Exchange, Plant Sciences, University of Tennessee

http://www.soilfoodweb.com.au Soil Foodweb Institute Australia

http://attra.ncat.org ATTRA – National Sustainable Agriculture Information Service