New generation eco-friendly plant nutrients for sustainable agriculture
By: Rana P. Singh1*, Sanjeev Kumar1, Manish Sainger2, Anurag Kumar Singh1
For years together, the solo objective of fertilizer application to crop plants had been to provide nutrients to plants and in turn obtain enhanced or sustained optimal yield and hence the fertilizer producers and users had been and are being attempting to improve fertilizer use efficiency in terms of nutrient uptake and crop yield. It has been realized that the excessive use of inorganic fertilizers, which is the common agricultural practice of green revolution, is not a sustainable farming practice from either economic or ecological point of view.
Nitrogen is one of the major nutrients for the plant productivity. The inorganic forms of N i.e. nitrate, nitrite and ammonium are assimilated by the plants including bacteria into the primary amino acid, L- glutamic acid. Ammonium is the entry port for incorporation of inorganic N into the organic cycle. The primary sources of nutrient N are nitrate (in most of the arable soil), ammonium (mostly in anaerobic conditions) and biological fixation of di-nitrogen into ammonium (symbiotic and non-symbiotic). The nitrate gets reduce to nitrite in presence of the enzyme nitrate reductase (NR), which subsequently reduced to ammonium in presence of the enzyme nitrite reductase (NiR). Ammonium get assimilated into L- glutamine in presence of L- glutamine synthetase (GS) and subsequently produces L- glutamic acid by incorporation of amide nitrogen of the glutamine into 2-oxoglutarate which is catalyzed by L-glutamate synthase (Fd- GOGAT and NAD(P)H- GOGAT) . In this pathway one L- glutamic acid molecule is used to produced L- glutamine and subsequently two L- glutamate molecules are produced with a net benefit of one L- glutamate. This pathway was discovered in early 1970s in bacteria and subsequently in plants. Another enzyme, for which at least 14 isoforms have been reported, L- glutamate dehydrogenase (NADH-GDH and NAD-GDH), which occurs in almost all living organism has been reported to catalyze a direct amination and deamination of ammonium to and from L- glutamic acid. Being a reversible catalytic system GDH pathway of ammonium assimilation is considered as an alternative pathway which generally operates under the stresses when ammonium is available in excess amount. The assimilated N gets incorporated in proteins, nucleic acids and many other metabolites essential for the functioning of living organisms.
Nitrogen deficiency is one of the major yield limiting factors in plants especially in cereals, hence application of N- fertilizers are considered as an essential input to maintain high yield of the crops. Plants are responsive to the applied N which constitutes most of the vital macromolecules and metabolites related to its vegetative and reproductive growth and metabolism. The applied fertilizer N enhances crop productivity per unit area, as agricultural soil is deficient in N worldwide. The fertilizer application is thus considered essential to meet the requirements of the burgeoning population, particularly in the developing countries.
The losses of chemical fertilizers occur in many forms. Due to runoff and leaching it contaminate ground and surface water bodies which causes eutrophication and its environmental consequences. High levels of nitrate and phosphate etc. have been reported to be associated with the many kinds of toxicity to zooplanktons and aquatic animals and health hazards to cattle and ruminants and human being specially children. The nitrite may combine with organic pollutants and form nitrogenous xenobiotic compounds which affect nervous system, induces heart diseases and cause many types of cancers. The excessive use of N fertilizer are known to cause enhanced volatization of ammonia and emissions of NOx gases which are very potential threat to the global warming. Organic manures also emit methane in anaerobic conditions.
In most of the countries, applications of chemical fertilizers, pesticides, energy based tools and equipment and high water consumption for irrigation to gain high yield in plant agriculture have raised cost of production, on one hand, and have degraded soil, water and biosphere, on the other. Therefore, there is an urgent need to develop innovative procedures, tools, techniques, production, transportation, distribution and marketing systems, which are based on low input agriculture, sustained productivity and yield and sustainable resource management.
Organic manures e.g. cow-dung, compost, vermi-compost and farm yard manure (FYM) etc. have been recommended as an alternative to the chemical fertilizers in organic farming systems. However, these manures are slow acting and thus required to be applied in bulk to maintain high crop yield similar to that obtained by the application of chemical fertilizers. In recent years due to rapid urbanization and industrialization globally the populations of cattle and ruminants have decreased rapidly and thus the availability of organic manures in bulk is difficult. The practices to convert other organic wastes of agricultural and urban sources in manure have yet to be established at large scale in most of the developing countries.
Biofertilizers have been identified as an alternative to chemical fertilizers to increase soil fertility and crop production in sustainable farming. These are the products containing living cells of different types of microorganisms, which have been ability to convert nutritionally important elements from unavailable to available forms through biological processes. In recent years, biofertilizers have emerged as an important component of the integrated nutrient management programs (INMP) and hold a great promise to improve crop yield minimizing environmental degradation caused by the application of chemical N forms e.g. Urea in excess. Strains of Azotobacter, Rhizobium, Bradyrhizobium, Azospirillum, Pseudomonas, Bacillus and Acetobacter etc. have been developed as biofertilizers for cereals, pulses, vegetables, oil seeds, cotton, sugarcane, wheat etc. Though, Biofertilizers offers an economically attractive and ecologically sound alternative to the chemical fertilizers, for realizing the ultimate goal of increased productivity its efficacy is significantly low in relation to the crop yield when compared with the recommended doses of chemical fertilizers.
The microbial diversity is enormous and its evolution and adaptations to the changing environment is significantly better. A bio-prospecting of soil microbes from different agro- climatic conditions to isolate, select and improve plant growth rhizobacteria (PGPR) is a thrust area which will provide new and improved strains of biofertilizers who can enhance the nutrient availability by converting biologically non-available nutrients to its available forms. The genera, species, strains and ecotypes for symbiotic and non-symbiotic N2 fixers, phosphate solubilizing bacteria (PSB), potassium solubilizing bacteria and Fe solubilizing bacteria etc. have been discovered. Genetic improvement in the selected microbes can be done using conventional method and gene technologies. The selected microbes which are potential PGPRs are required to be optimized for its dose responses for different crops and different agro-climatic conditions. New carriers of such microbes are to be discovered and designed for providing more vibrant and productive micro-environment to these bio-fertilizers during storage, transport and application in the fields.
Slow- or controlled-release fertilizers containing plant nutrient(s) in a form which delays its availability for plants or which extends the nutrient availability to the plant for significantly longer duration (preferably for entire vegetative and reproductive cycles) than a reference ‘rapidly available nutrient fertilizer’ such as ammonium nitrate or urea, ammonium phosphate or potassium nitrate etc. Such retention of the nutrients for longer duration (slow release fertilizers; SRFs) or its release in rhizosphere as per the nutritional requirements of the crop (controlled release fertilizers; CRFs) can be obtained by the technical interventions which reduce the nutrient losses and provide nutrients to the plants for a comparatively longer duration. It plays an important role in improving fertilizer use efficiency by plants, thereby mitigating environmental pollution and can be seen as a major nutritional component of the sustainable agriculture.
Slow/ Controlled fertilizers (SRFs) have a potential to manage high crop productivity without the problems and risks associated with the soluble chemical fertilizers. Unlike the rapidly available soluble fertilizers, SRFs are slow acting due to the delayed release of nutrients often available in bound/immobilized form in or onto a non-toxic, biodegradable and inert matrix. A good SRF should release the nutrients in rhizosphere at the rates and amounts that match the need of the growing plants.
Several formulations of slow release fertilizers have been developed by condensation of small nutrient molecules, by coating to a matrix, developing super granules adsorbed or immobilized nutrients into a matrix, amending nitrification and urease inhibitors etc. The SRFs amended with nitrification inhibitors are also known as stabilized fertilizers. A new term i.e. customized fertilizers has also been introduced recently which indicate the modified forms of conventional chemical fertilizers to reduce nutrient losses and to increase its efficacy.
The commercial application of use of SRFs in main stream agriculture has been limited to a few countries e.g. Japan, USA, Israel, Australia, China and Europe and that too for the few cropping systems. The developing countries with extensive agricultural practices and pressure to produce more food to feed its burgeoning population, have not yet adopted it in its main stream plant agriculture. It is primarily due to high cost involved in production of SRFs/ CRFs which are not covered under the fertilizers subsidies in countries like India. Lack of appropriate endogenous technology, appropriate appreciation and awareness on SRFs by the existing agricultural network in these countries, ignorance to the associated environmental and health threats and overall the increased price of these fertilizers over the conventional soluble chemical fertilizers (subsidized), biofertilizers and organic fertilizers may be seen as the major limitations to realize the potentials of SRFs/ CRFs in the contemporary crop agriculture systems in many parts of the world.
We have developed certain formulation of SRFs which are based on use of organic matrix containing local non-toxic, biodegradable and comparatively low cost matrix to bind chemical nutrient forms which have performed very significantly for major cereals e.g. rice, wheat and Indian mustard etc. Some of our formulations are eco- friendly and cost-effective; however, it needs further validation in larger agricultural fields.
Use of low cost binders and other modifications to make it novel and yet cheaper are needed to meet the challenges in popularizing SRFs in the developing world. We are making significant efforts in this direction which has yielded encouraging results. We have developed organic matrix entrapped bio-fertilizers granules which have significantly higher efficacy as compared to the conventional bio-fertilizers available in market. Application of low cost slow/ controlled release fertilizers can be a viable alternative strategy to reduce excessive use of split doses of high N-responsive hybrid crops. These fertilizers can maintain high productivity with single basal application, reduce chemical fertilizer load and minimize nutrient (N) losses to the environment.
Further sustained N-availability to plant rhizosphere through chemical fertilizers as well as SRFs increase plant N-status and a significant portion of inorganic N-forms remain unassimilated in the aerial sinks especially in leaves which can cause a health hazard to the consumers on one hand and prove as an ultimate loss of the nutrient through the vegetative and reproductive parts of plant residue on the other. An improved NUE by manipulating N-assimilation, transport and sink-source relationship using recent biotechnological approaches can increase the yields of the chemical fertilizers as well as SRF applied plants further. Attempts should be made in the recent future to overcome these bottlenecks in N-relation to plants.
As alternative to the green revolution to agriculture, organic farming has been adopted in some regions, however, there has been over emphasis on the organic products with ignorance of practical problems and scientific know how. As a result the organic farming has become an elite farming system with expensive products and involvement of less quantum of human resources. The emphasis of the organic farming has lead on organic products with environmental concern only and economical concerns of the masses have been ignored. In place of organic farming an ecological farming is to be evolved to increase the efficacy of eco-friendly alternatives of the agricultural inputs by better selections and technological interventions. We have to develop the eco-friendly plant nutrients for ecological farming which can be cost effective, eco-friendly and can address the problems of productivity and yield of major crops. It has to be made available to a larger community including small and marginal landholders in the developing countries and must incorporate the interest of agricultural laborers and consumers at large scale. We understand that our work in relation to developing organic matrix based slow release fertilizers with amendments of reduced amount of chemical fertilizers, multiple nutrients, microbial biofertilizers etc. have a great potential in resolving the nutritional limitations in ecological farming system.
1Department of Environmental Science, Babasaheb Bhimrao Ambedkar University, Lucknow, India,
2Centre for Biotechnology, M. D. University, Rohtak, India
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