Preface Horticulture crops perform a vital role in the Indian economy by generating employment, providing raw material to various food processing industries, and higher farm profitability due to higher production and export earnings from foreign exchange. Fruits and vegetables account for nearly 90% of the total horticulture production in the country. India is now the second largest producer of fruits and vegetables in the world and is the leader in several horticultural crops, namely, mango, banana, papaya, cashew nut, areca nut, potato and okra (lady’s finger). Horticultural crops such as fruits, flowers, vegetables, spices and medicinal and aromatic plants occupy only 8.2% of the cultivated area and contribute 33% of GDP and 52% of export value of agriculture in India. According to the data provided by the Government of India for 2016–17, horticulture crops in India are being cultivated in 24 million hectares, which is about 7 per cent of India’s total cropped area. The annual horticultural produce is estimated around 295 million tonnes, which includes 175 million tonnes of vegetables and 92 million tonnes of fruitsduring 2016–17. India is the largest producer of okra (lady’s finger). Among vegetables, India ranks second in the production of potato, onion, cauliflower, brinjal and cabbage. In fruits, it is the largest producer of banana, mango, guava, lemon and papaya. Mango, walnut, grapes, banana and pomegranate are the major fruits exported, while onion, okra, bitter gourd, green chili, mushroom and potato have more exotic demand. Fruits and vegetables are mostly exported to the UAE, Bangladesh, Malaysia, the Netherlands, Sri Lanka, Nepal, the UK and Saudi Arabia. The horticultural industry offers a variety of jobs, both directly and indirectly. Chemical fertilizers are essential for increasing the plant growth and vigour for the food security of the world. In the process of increasing the yield with intensive application of chemical fertilizers for the ever growing world population, it has resulted in several environmental problems like polluted air, water, and soil, the degraded lands, depleted soils and also increased emissions of greenhouse gases. These chemically synthesiszed fertilizers are not only hazardous for our environment, but it also affects the humans, animals and microbial life forms. Hence, the major challenge in intensive production systems is to integrate intensive production with high nutrient use efficiency. Excessive use of the chemical fertilizer for a long time on the same soil may lead to soil degradation, loss of beneficial soil microorganisms, and many other losses. In order to ensure both sustainable agricultural production and safeguard the environment, an integrated use of nutrient supplements such as chemical fertilizer, organic manures, biofertilizers and other slow released or controlled released fertilizers should be the best option. In the current scenario, one of the cost-effective and eco-friendly inputs are the biofertilizers or microbial fertilizers which can act as a nutrient supplements for plant growth. Biofertilizer is defined as a substance which contains living microorganisms and is known to help the root system and improved seed germination. A healthy plant usually has a healthy rhizosphere which are dominated by beneficial microbes. Biofertilizers do not directly supply any nutrients to crops but through fixation or solubilization of nutrients. The biofertilizers are natural, beneficial and ecologically and user-friendly. Even though, the use of biofertilizer is not used in wide scale for all crops, there is a growing awareness among the farmers that production can be increased by the use of biofertilizers in case of cereals, pulses, oil seed and some cash crop like vegetable and sugarcane. Generally, fruit crops have received more attention than vegetables and ornamental crops. Therefore,the use of biofertilizers in cultivation of horticultural crops will result in protecting the soil health and also the quality of production. It also offers an economically sustainable means of reducing external inputs and improving the quality and quantity of natural land resources. Current resources should be overhauled in favour of the sustainable use of resources which will boost the horticultural crop production.

In modern agriculture, use of synthetic chemical fertilizers are essential in agriculture for sustainable yield but, these have resulted in environmental pollution. The use of synthetic fertilizer could result in serious human health issues and also affects the soil microflora, besides being costly due to high production cost. Further, excess use affects soil processes and result in nutrient imbalances in plants with increased susceptibility to pests and diseases and also affects the symbiotic associations. Under these circumstances, biofertilizers could be an alternative for integrated nutrient management system in horticulture. The entire crop production system depends on soil beneficial microflora and their activities, which makes them highly potential alternatives to synthetic chemical fertilizers. Biofertilizers or microbial fertilizers are essential for eco-friendly and sustainable cultural practices. The term “biofertiliser” has been defined in several ways due to its unique relationships between the plant and rhizosphere microflora. Biofertilizers mainly include nitrogen fixing bacteria, phosphate solublizing and potash solubiling bacteria.

2.1 Nitrogenous biofertilizers These biofertilizers augment nitrogen to the plants through fixation of atmospheric nitrogen, either symbiotically or asymbiotically. The major nitrogenous biofertilizers are: 2.1.1 Rhizobium The word Rhizobium is a Greek word where “rhiza” refers to root, and “bios” refers to life. Rhizobium was first named by Frank (1889) which means ‘root living’ in Latin (Fig. 3). Some species of Rhizobium were later moved to new genera based on phylogenetic analysis. It consists of 49 rhizobial species and 11 non-rhizobial species.

Biofertilizers improve the plant growth due to changes in the microbial community of rhizosphere by production of several substances. Normally, biofertilizers promote plant growth either by facilitating nutrient absorption or by secretion of plant hormones. Biofertilizers also cause inhibitory effects by acting as biocontrol agents on various pathogens so that the growth of plant is enhanced. The mode of action could be either direct or indirect methods

Benefit of biofertilizers have been extensively studied for various agricultural and horticultural crops. However, the effectiveness of inoculated biofertilizers are influenced by several factors. If the use of biofertilizers have to be popularized in horticultural crops, the major factors affecting their performance particularly in relation to soil, plant, quality of the product and application methods have to be addressed. Factors affecting the performance of biofertilizers are: 4.1 Quality of the product 4.1.1 Production process The high quality of biofertilizers are determined by the production process (Bashan et al., 2014). At the same time, the population density of mother culture and the quality of the final products are directly related (Stephens and Raskm, 2000). As the inoculum consists of a strain, the relationship among the microorganisms interaction in the rhizosphere have to be understood. In the case of more than one microorganisms, the efficiency depends on the various mode of action by various microorganisms which might overlap with antagonistic acitivities (Vassilev et al., 2001, 2006 b). The relationship of microorganisms with plants is not based on the taxonomic diversity, but their functional diversity (Maherali and Klironomos, 2007). The efficiency of biofertilizers could be assured by enhanced shelf life of the inoculant and biological traits which are a challenge for developing a formulation (Bashan, et al., 2014). Hence, either single or consortia of microorganisms formulation with or without additives determines the efficacy of the biofertilizers. The formulation determines shelf-life during storage and transport, which enhances the persistence of the strain in soil and get maximum benefits to the host plants after inoculation (Manikandan et al., 2010; Schoebitz et al., 2012). Different kinds of carriers are used in the formulation process which affects the overall quality and efficacy of the biofertilizers (Bashan et al., 2014; Herrmann and Lesueur, 2013). Granular inoculants showed good results under stressed soil conditions (Clayton et al., 2004; Lupwayi et al., 2006). Encapsulation of the microbial strain into polymers allow diverse compositions and structures (Vassilev et al., 2005) but, have limitations for commercial production (Bashan, et al., 2014; John et al., 2011). Use of additives might improve the shelf life of the product (Bashan et al., 2014; Malusà et al., 2012; Herrmann and Lesueur, 2013) with introduction of specific substances to enhance the efficacy of biofertilizers.

The inoculum dosage and method of application determines the efficacy of biofertilizers. The quantity of biofertilizers to be applied depends on the type of crop, seed rate and soil edaphic factors. There are 3 methods for inoculation of biofertilizers Seed treatment: One packet containing inoculant (200 g) is mixed with 200 ml of rice gruel or 40 % gum arabic solution to make a slurry (Fig. 23). Then the seeds required for an acre are mixed in the slurry in such a manner that they have a uniform coating of the inoculant over the seeds. Then, the inoculated seeds are shade dried for 30 minutes. The shade dried seeds should be sown within 24 hours. One packet of the inoculant (200 g) is normally sufficient to treat 10 kg of seeds.

Fruits, vegetables, ornamentals, plantation crops and spices, medicinal and aromatic plants, tubers, are the major horticultural crops contributing to the economic growth in India. Even though the horticultural sector is growing fast, with India being the second largest producer of fruits and vegetables in the world, the productivity and quality have to be improved in order to meet the demand of increasing population. Under these circumstances, ecofriendly techniques needs to be adopted which could ensure food security and sustainable production. Indiscriminate application of agrochemicals and chemical fertilizers in horticultural crops and their residues could harm the human health, soil and environment. Horticultural crops need an ecofriendly technique, as most of the horticultural products are directly consumed. Hence, there should not be any residues of agrochemicals in food products. In the current context, use of ecofriendly, cost effective and organic-based inputs like biofertilizers can maintain health of the soil and good quality in horticultural crops. The application of biofertilizers offers an attractive and sustainable means of improving the quality of food products. Biofertilizers like Azotobacter, Azospirillum and AMF and PGPRs not only supplies the required nutrients to plants, but improves the fruit quality and quantity (Pathak et al., 2017).

Biofertilizers can directly or indirectly influence the growth of plant. The direct influence can be through production of phytohormones or making the nutrients available to plants. The indirect effect can be through management of plant pathogens through biological control. Therefore, the purposeful introduction of biofertilizer inoculants to plant microbiome represents an environmentally sound option that holds a prominent position in order to reduce the dependency on agrochemicals (Adesemoye and Kloepper, 2009; Abhilash et al., 2016; Aloo, et al., 2019). Microbes in soil can rarely exists as single cells and survive near the plant root canopy or in bulk soils or endophytes or even on plant surfaces (Lorito, 2006). The soil microflora occurs as complex members of microbiota (Hacquard, 2015). Microbial consortia (MC) are unique mixture of organisms which respond to the environment stress and adapt to stresses due to internal beneficial interactions among the microbial community. Ecosystem’s physiological functions by microflora is carried out by more than one microbial species with the functional diversity and succession among different microbial components. The microbial balance among the different microflora of a MC will consist of a continuous shift between actively growing cells and non-dividing cells of the microbial consortia. It has been reported that different microbial populations “talk” to each other by “quorum sensing” mechanism in a given environment by exchanging specific chemical signals (Thompson, 2015). Natural microbial consortia holds many specific properties like stability, functional diversity and perform complex functions. The natural consortia have generated an interest in manpulating synthetic consortia for the benefit of application (Minty, et al., 2013).

Biofertilizer is already proven to be beneficial for agricultural and horticultural crops. The most important pre-requisite for the success of biofertilizer application is the presence of high population of viable cells which will show the desired effect on plants. Biotic interactions along with competition from native microflora is a major challenge to an applied strains of biofertilizers. The formulation is an important parameter for biofertilizer production containing efficient microbial strain with suitable carrier material and additives. The microbial strain needs suitable protective environment to increase their efficiency at the target site and also make it user-friendly. Performance of biofertilizers under field condition is the major requirement for inoculants. Inadequate formulations and poor inoculant quality are the major hurdles for the successful use of biofertilizers. Formulation of biofertilizers has to be consistant during production, distribution, storage and transport to the farmer/end users. The shelf-life of inoculated microbial fertilizer depends on the formulation with suitable carrier material. If local production and distribution of an efficient native biofertilizer formulations are developed, the present problems in the field performance of biofertilizers could be significantly solved. Specific formulations of native strains for use in saline soils, acid soils, drought, high temperature and eroded soils requires specific formulations. Therefore,use of native strains of biofertilizer formulations which are tolerant to the abiotic stress is an important approach. The formulations of biofertilizers are challenging and limited success has been observed in the commercialization of new biofertilizers. Hence, the search for new formulations has been a continuous approach. However, the choice of the formulation depends on the application method, available equipment, farmer’s convenience, plant inherent characteristics and developmental stage, cost, mode of action and colonization pathway of the inoculated strain. Conventionally, solid based formulations are used, which have problems of low viability during storage and application.

Quality Control is very important in biofertilizer production, which determines the efficiency of microbial strains. Generally, the quality control based on BIS are voluntary in nature. However, some of the biofertilizers are brought under the ambit of Fertilizer Control Order 1985 (FCO) during 2006 (Yadav and Chandra, 2014). At present, biofertilizers are under the FCO with specific quality standards. Biofertilizer production and its sales can be regulated and is a mandatory requirement for registration of manufacturing unit with the State Fertilizer Controller (who is generally the Commissioner or Director of Agriculture Department) under the statutory provision of FCO. Moreover, some officers of the Agriculture Department have been declared as Fertilizer Inspectors at the district level, who are authorized to inspect production and storage facilities and draw samples for quality analysis. National Centre of Organic Farming (NCOF), Ghaziabad under the Govt. of India and its six Regional Centre’s located at Bhubaneswar, Bangalore, Jabalpur, Nagpur, Hissar and Imphal have been declared as testing laboratories. Under the provisions of the FCO act, State Governments can also develop their own quality control laboratories and notify them under the FCO (1985). Peat, lignite, peat soil, humus, wood charcoal or similar material favouring the growth of microbial strain could also be used. The FCO specifications for different biofertilizers are presented below:

It has been well established that the biofertilizers are beneficial to plants and are cost-effective inputs for sustainable production system. However, there are some constraints with respect to its production, which are presented below: 10.1 Production constraints 10.1.1 Raw material Eventhough there are large number of biofertilizer formulations, only few are considered as good carrier materials for biofertilizers such as peat and lignite. Moreover, its availability in India is limited and the quality is lacking for these carrier materials. Hence, a cost effective and locally available material with increased shelf-life is the need of the hour. 10.1.2 Specificity and availability of strains As the microbial strains used as biofertilizers are specific to soil and agro-ecological conditions, availability of specific strains for the specific host and soil edaphic factors are inadequate.

Microbes present in the biofertilizer formulations help in microbial processes in the soil which augment the availability of nutrients to be taken up by plants. They are of different types based on their nature and function such as nitrogen fixing, phosphate solubilizing, phosphate mobilizing or biofertilizers for micronutrients. Plants use different strategies for growth promotion at different stages of plant growth and development. Biofertilizers are mass multiplied cultures of specific beneficial microbial strains which improves the fertility of soil and productivity. The nutrients are added through natural processes of nitrogen fixation, solubilization of phosphorus and synthesis of growth promoting substances to stimulate the plant growth. The main sources of biofertilizers are bacteria, fungi and cyanobacteria. 11.1. Bio-enrichment of chemical fertilizers- A new approach Yield response of different host plant depends on the mineral fertilizer and its fertilizer use efficiency. Generally, such fertilizers are low in efficiency due to environmental and soil factors. A large portion of synthetic fertilizers are lost due to fixation, leaching, run-off, erosion, volatilization, denitrification and precipitation without reaching plants. Hence, there is an economic loss and water and environmental pollution. Microbial inoculants applied to the soil enhances the uptake of nutrient by plants and increase the efficiency of synthetic or organic fertilizers. In this context, it is possible to impregnate mineral fertilizers with specific microbial strain. A new approach is to incorporate the microbes or microbial compositions into fertilizer compositions.

The soil fertility is depleting in the current scenario of gap between nutrient removal from soil and supplies. Moreover, increase in the fertilizer prices has become unaffordable to small and marginal farmers. There is also a growing concern about the environmental hazards and its threat to sustainable crop production. Indiscriminate use of synthetic fertilizers for intensive crop production has resulted in large quantity of nutrient (particularly P) accumulation in soils which has resulted in generating a dead soil. In this context, the long term use of biofertilizers are considered to be economical, eco-friendly, more efficient, productive and accessible to marginal and small farmers over chemical fertilizers (Subba Rao, 2001). Biofertilizers mainly consists of specific living cells which supplies nutrients to plants through their root system. Microbes in these fertilizers provide nutrients to the plants by nitrogen fixation, phosphate solubilization, phosphate mobilization, and promotion of beneficial rhizobacteria (Bhat et al., 2010). Biofertilizer application improves the supply of nutrients, organic carbon, accumulation of soil enzymes, productivity, soil fertility index, economy of farmers, and maintenance of sustainability in natural soil ecosystem and horticultural crops. The production of efficient and sustainable biofertilizers for crop plants needs to be undertaken to make the biofertilizers popular and efficient. The important and specific research in future may be focused on the following aspects.

There are many manufacturers of biofertilizers in both government and private sectors. Only some of them have been listed here for the information

The biofertilizer technology has paved way for the manufacturers and entrepreuners to establish a biofertilizer production unit due to an increasing demand and awareness among farmers. Biofertilizer production units have already been started with most of them in the southern states of India. The biofertilizers can be produced using different kinds of carrier materials (peat, coal, lignite, clays, inorganic soil), organic materials (composts, soybean meal, wheat bran, sawdust, etc.), or inert materials (e.g., vermiculite, perlite, kaolin, bentonite, silicates). Natural carriers like peat, plant waste materials, composts, sawdust, sludge lignite, clays, coal, vermiculite, perlite and bentonite and mixture of organic and inorganic materials may also be used. The Government of India is also encouraging a low cost technology by providing a subsidy to establish a biofertilizer production unit . The major equipment’s and other materials required for a biofertilizer production unit with the capacity of 150 metric tonnes/annum are given below

References Abdelaal, S., El-Sheikh, M.H., Hassan, H.A.S. and Kabeil, S.S. (2010). Microbial bio-fertilization approaches to improve yield and quality of Washington navel Orange and reducing the survival of nematode in the soil. J. American Sci., 6(12): 264–271. Abdel-Salam, M.A. and Shams, A.S. (2012). Feldspar-K fertilization of potato (Solanum tuberosum L.) augmented by biofertilizer. J. Agric. Environ. Sci., 12: 694-699. Abhilash, P.C., Dubey, R.K., Tripathi, V., Gupta, V.K. and Singh, H.B. (2016). Plant growth-promoting microorganisms for environmental sustainability. Trends Biotechnol., 34(11): 847-850. Adesemoye, A.O. and Kloepper, J.W. (2009). Plant–microbes interactions in enhanced fertilizer-use efficiency. Appl. Microbiol. Biotechnol., 85(1): 1-12. Ahkami, A.H., White III, R.A., Handakumbura, P.P. and Jansson, C. (2017). Rhizosphere engineering: Enhancingsustainable plant ecosystem productivity. Rhizosphere, 3: 233-243. Ahmed, M., Nadeem, S.M., Naveed, M. and Zahir, Z.A. (2016). Potassium-solubilizing bacteria and their application in agriculture In: Meena, V.S., Maurya, B.R., Verma, J.P. and Meena, R.S. (Eds.), Potassium solubilizing microorganisms for sustainable agriculture. Springer India, New Delhi, 293-313. Alam, M., Khaliq, A., Sattar, A., Shukla, R.S., Anwar, M. and Seema, D.S. (2011). Synergistic effect of arbuscular mycorrhizal fungi and Bacillus subtilis on the biomass and essential oil yield of rose-scented geranium (Pelargonium graveolens). Arch. Agron. Soil Sci., 57: 889–98. Albareda, M., Rodríguez-Navarro, D.N., Camacho, M. and Temprano, F.J. (2008). Alternatives to peat as a carrier for rhizobia inoculants: Solid and liquid formulations. Soil Biol. Biochem., 40: 2771–2779.