1. Biofertilizers: definition and general aspects

The increasing demand for safe and healthy food and the concerns on environmental pollution have led to the emergence and development of organic farming. It is globally an important priority area in the crop and livestock production, which promotes and enhances agro-ecosystem health, including biodiversity, biological cycles, and soil biological activity. Organic farming is based on the development and application of biofertilizers and plant strengtheners. The use of chemical fertilizers in large amounts has resulted in a manifold increase in the productivity of farm commodities but they also have an adverse effect on the soil. Continuous and excess use of chemical fertilizers and other agrochemicals to increase yield may lead to ground water contamination and depletion of soil nutrients, eventually resulting in reduction of crop yield. This problem could be overcome by the use of a different technology for the production of various biofertilizers. Biofertilizers from microorganisms can replace chemical fertilizers; they are less expensive and are more environmentally friendly than chemical fertilizers. The current global market for organically raised agricultural products is valued at around US\$ 30 billion with a growth rate of around 8 percent. Nearly 22 million hectares of land are now cultivated organically. Organic cultivation represents less than 1 percent of the world’s conventional agricultural production and about 9 percent of the total agricultural area. Biofertilizers, or more appropriately “microbial inoculants” in the strict sense, are not fertilizers, which directly give nutrition to crop plants. They represent natural and organic formulations that contain living or latent cells of beneficial soil microorganisms which, after being added to the seeds, plant surfaces or soil, colonize the rhizosphere or the interior of the plant and promote its growth by increasing the supply or availability of primary nutrients to the host plant. The inoculation with beneficial soil microorganisms is a promising method for raising soil fertility because, in this way, the accessibility of plants to a number of important elements, such as nitrogen, phosphorus and potassium, increases. As a result, the use of synthetic fertilizers can be significantly reduced. In the world literature, there is evidence of promotion of vegetable yields by inoculation with microorganisms. Microorganisms (bacteria, mycorrhizal fungi and algae) are the living components of the soil. Their activities related to soil fertility and plant nutrition are diverse. They affect the soil structure, the dynamics of nutrients in it, participate in plant nutrition and increase plant resistance to soil-borne pathogens.

These microorganisms are responsible for the process of nitrogen fixation, solubilization of insoluble soil phosphates, convertion of complex organic biomass into mineral compounds which are utilized by plants, and synthesis of growth-promoting substances such as amino acids, vitamins, etc. There are 17 essential non-mineral and mineral elements required for proper plant growth. The lack of any of these nutrients can result in severe damage to crop health. Three essential nutrients are carbon (C), hydrogen (H) and oxygen, which are taken up from atmospheric carbon dioxide and water. Of the mineral elements, the primary macronutrients (nitrogen, phosphorous, and potassium) are needed in largest quantities and are most likely to be in short supply in agricultural soils. Secondary macronutrients, such as Mg, S, Zn, Mn, Fe and Cu, are needed in smaller quantities and are typically found in sufficient quantities in agricultural soil, and therefore do not often limit crop growth. Micronutrients, or trace nutrients (B, Mo, Cl, and Ni) are needed in very small amounts and can be toxic to plants in excess. Silicon (Si) and sodium (Na) are sometimes considered essential plant nutrients, but due to their ubiquitous presence in soils, they are never in short supply. Microorganisms encourage plants to absorb a greater quantity of nutrients on their own which, even if naturally present in the soil, on occasion, cannot be assimilated by plants because of being in an insoluble form.

At present, biofertilizers are supplied to the farmers as carrier-based inoculants or as liquid formulations as an alternative technology, which has more advantages than the carrier-based inoculants.