1. Introduction

Environmental issues, for example, freshwater contamination, energy saving, and soil erosion are compelling the farmers to present developmental strategies that have a lower polluting impact. The utilization of environmentally friendly practices is advanced by voluntary certification schemes (e.g., GlobalGAP or organic farming schemes) as well as by legally binding regulations (e.g., the EU Directive 2009/128 aiming at the implementation of sustainable pest management practices). In this context, the diminished utilization of chemical fertilizers with expanded use of organic fertilizers is viewed as compulsory route to improve the pressure on the environment derived from rural practices. In recent year’s history, the chemical pesticides and fertilizers have assumed an essential part in boosting the rural development; however they have a short history in modern agriculture. Their immediate action and low cost succeeded to bring them rapidly in to the centre of attention. On the other hand, their toxic effects on environment, plant, animal and human life diverted the focus on eco-friendly plant protection. Moreover, the development of resistance in insects against common pesticides has not been solved yet. Thus, practices such as Integrated Pest Management (IPM) have gained more importance.

Biofertilizers are vital segment of the IPM. They can be of extraordinary financial significance: they can in part replace different agrochemicals which are turning out to be increasingly costly and their improvement is in light of expanding requests for all the more ecologically agreeable farming practices. The term “biofertilizer” commonly refers to a product containing soil microorganisms applied to plants to promote their growth. However, it has often also been wrongly used as a synonym for a wide range of products such as green or animal manure, intercropping, or organic-supplemented chemical fertilizer. Vessey (2003) defined a biofertilizer as “a substance which contains living microorganisms which, when applied to seed, plant surfaces, or soil, colonizes the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant”. The microorganisms they contain are also called plant growth promoting rhizobacteria (PGPR) and result in benefits to the plant hosts after inoculation.

The enthusiasm for the utilization of these products is ascending due to the improvement in nutrient uptake efficiency and society demands for more green technologies and increased costs of agrochemicals. Moreover, biofertilizers and phytostimulators have optional helpful impacts that would increase their usefulness as bioinoculants. Indeed microorganisms such as Rhizobium and Glomus spp. have been shown to also play a role in reducing plant diseases. The practice of inoculating plants with PGPM can be followed back to 20^th century, when a product containing Rhizobium sp. was patented. Mycorrhizal fungi, even though utilized as biofertilizers since couple of decades, were reported to promote plant growth through P uptake since the late 1950s. Since then, research endeavours in these fields have consistently expanded, resulting in the selection of various strains demonstrating several beneficial characteristics.

The policies supporting sustainable rural development and broad research that has enhanced the adequacy and consistency of microbial inocula have resulted in the enrolment of several strains for both biocontrol and biofertilization, with mycorrhizal and PGPR preparations being marketed in several countries. Yet, a wider use of microbial inoculants, especially those acting as phytostimulators and biofertilizers, has been frequently hindered due to the variability and inconsistency of results between laboratory, greenhouse, and field studies. The explanation behind these discrepancies lies in the fragmented comprehension of the complex relationships established between the components of the system: the plant, the microorganisms, and the environmental conditions, particularly that of soil. In addition, the lack of correct formulations and the costly and tedious procedures of registration are also among the factors holding back the use of PGPM on a more extensive scale.

The real commercialization of PGPR began in 1995 in the USA and UK with the inoculation of legumes with rhizobia. However the enthusiasm for other PGPR has been increased over time and a range of new products have been developed more recently. Most of the nonrhizobial PGPR inoculants currently available contain bacteria from the genus Azospirillum (free living N₂-fixing bacteria) or Bacillus (phosphate-solubilizing bacteria (PSB) and biocontrol agents. Products containing arbuscular mycorrhizal fungi (AMF) are also becoming increasingly applicable worldwide. However, the diversity of PGPR and AMF populations potentially available in soil and the range of their modes of action are very broad and, for the vast part, incompletely understood and thus underexploited. It is also recognized that the various mechanisms involved in plant promotion may be host plant-specific and strain-specific and that the advantageous impacts may vary extraordinarily under various natural conditions. In addition, once introduced to the soil, microorganisms face competitive and often harsh conditions that may severely reduce their beneficial effects.

The four main types of formulation that have been used up to now are liquid, peat, granules, and freeze-dried powders (Fig.1). Their success relies on target crop, cost, market availability, environmental constraints, and usability. One of the real difficulties for the inoculant industry is to develop an improved formulation that combines all the above characteristics and that are suitable for use under field conditions. Moreover, while a microorganism may seem promising in laboratory, producing it commercially in order to obtain similar results under a wide range of field conditions is a difficult step. Some manufacturers included at least two types of microorganisms (e.g., rhizobia and AMF, rhizobia and PSB, various strains of AMF or PSB) in a single product, thus augmenting the subsequent benefits for the host plants. However, only a few reviews reported the positive effects of these co-inoculants. Their efficacy was not proven and their production and commercialization pose a number of technical difficulties. The most important aspect during inoculant development is assurance of the quality in a way that guarantees the reliability of the products with maximal chances for success. The absence of consistency in results obtained under field conditions because of conflicting quality has enormously influenced the commercialization of biofertilizers.

Fig. 1. Types of biofertilizers formulations: A – liquid; B - peat, C - granules, and D – encapsulated freeze-dried powders.