Fertilizers provide substances to the soil to ensure proper plant growth and increased agricultural productivity. They are essential for the production of food and forages, especially when using high-yield varieties that provide a positive response to nitrogen fertilizers rich in phosphorus and potassium. However, this solution has also become a problem given that the yields of these crops have decreased as a result of poor planning in soil care, an unbalanced application of fertilizers and a continued reduction of organic matter, in addition to poor cultural and/or agronomic practices derived from modern agriculture, also known as Green Revolution.
It is noted that nitrogen is the main limitation to increase productivity in the soil, this coupled with a higher economic value and environmental cost; therefore, currently the agricultural sector is looking for other options that guarantee greater efficiency and better use of nutrients, as a result the so-called biofertilizers (BF) have emerged, which have been generated from biotechnological methods and are introduced to the market as an excellent option.
Today, biofertilizers are recognized under different names, such as bacterial fertilizers, phytostimulants, biopesticides, bioinoculants, among others. But the fact is that they do not have the function of a fertilizer as such, since they do not work directly on the nutrition of the plants, instead they are microorganisms such as bacteria, fungi, and green and blue algae that are packaged in an inert material (vehicle).
BF are microbial inoculants that can be found in solid or liquid formulations, containing live or latent cell strains that are efficient for the fixation of nitrogen, phosphate solubilizers, or celolytic microorganisms used in the application of seeds or roots in order to increase these microorganisms and accelerate microbial processes that increase root growth, and also to provide the availability of nutrients.
In this regard, the availability of nutrients occurs through two main actions: the production of hormones and the suppression of pathogens. When applied to seeds, on the surface of plants, or directly to the soil, BF colonize the rhizosphere or the interior part the plant (endophytes), thus stimulating growth by increasing the supply or availability of nutrients.
Microbial biofertilizers are classified as follows:
- Biological nitrogen fixers such as Rhizobium sp, Bradyrhizobium sp, Azobacter sp, and Azospirillum
- Phosphorous solubilizers such as Bacillus sp, Pseudomonas sp, Penicillium
sp, Trichoderma sp, and Aspergillius sp.
- Phosphate mobilizers such as endomycorrhizal or arbuscular mycorrhizal fungi (AMF), emphasizing Rhizophagus sp, Endogone sp, Gigaspora sp, Acualospora sp, and Scutellispora
- Plant Growth Promoting Bacteria (PGPV) or Plant Growth Promoting Rhizobacteria (PGPR) such as Pseudomonas sp, Agrobacterium sp, Bradyrhixobium sp, Azobacter sp, Azospirillum, Streptomyces sp, and Xhanthomonas
The techniques related to biofertilizers are not new; however, they are little known and widespread and, particularly, little standardized; an example of this can be seen in the use of beneficial microorganisms.
It is worth mentioning that microorganisms used in the application of BF play a crucial role since they minimize the impacts of conventional fertilization and ensure the permanence of sustainable agriculture, especially in cases where priority is given to implement conservation measures and reduce environmental impact.
By using microbial BF (mycorrhizae), not only sustainability and agricultural productivity with low environmental impact is ensured, but also an increase in production is achieved, soil fertility is improved, and populations of phytopathogenic microorganisms are reduced. With these contributions, savings are obtained in fertilizers and agrochemical products, thus minimizing costs while implementing components that contribute to the sustainability of agriculture. Undoubtedly, this type of product is a feasible alternative for the partial or total substitution of mineral fertilizers with high environmental costs.
Fungi have highly efficient mechanisms that manage to establish a symbiotic mutualistic relationship between some soil fungi and plant roots. Mycorrhizae are neither the roots nor the fungi, but the associated structures formed from these partners, where both benefit. “The autotrophic host (the plant) provides carbon compounds (sugars), from photosynthesis to the heterotrophic symbiont (fungi), as well as a protected micro habitat” (Lira-Saldivar et al., 2013); and the fungi provide the minerals required by the plant, mainly «phosphorus, growth hormones, protection against certain pathogens and better water absorption… and with it, greater tolerance to drought, increased photosynthetic rates, lower concentrations of toxic elements such as cadmium and arsenic in plant tissues, in addition to improving the physical properties of the soil» (Muchovej, 2001).
The classification of mycorrhizae has been generated from various morphological, anatomical, and systematic criteria, both in plants and in fungi, recognizing five groups: 1) erical mycorrhizae; 2) ectomycorrhizae; 3) Orchidaceae mycorrhizae; 4) ectoendomichorhizae; and 5) endomycorrhizae or arbuscular mycorrhizae.
Recent research on arbuscular mycorrhizal fungi (AMF) shows that they are associated with greater growth of various plant species due to the effective acquisition of water and nutrients attributed to the increased growth of hyphae, beyond the depletion surrounding the root; which provides a better tolerance to extreme environments such as water stress, soil salinity, and some pathogens, in addition to the production of growth-promoting substances, reduced impact on transplantation, and synergistic interaction with other beneficial soil microorganisms, such as N (nitrogen) fixers and P (phosphorus) solubilizers, thus allowing superior growth even under extreme conditions for the plant.
Problems of synthetic fertilizers
Alcántar and Trejo (2007) emphasize that “losses due to sublimation or volatilization and leaching reach up to 60% due to large emissions of NO and N2O into the atmosphere.” As we know, NO is a greenhouse gas that causes global warming, but not all is lost because a timely solution for agriculture lies precisely in biological fertilization, as a “cutting-edge technique for sustainable agricultural development based on the use of natural inputs to improve the fixation of nutrients in the rhizosphere, produce growth stimulants for plants, improve soil stability, facilitate biological control, biodegrade substances, recycle toxic, xeno-biotic, recalcitrant substances” (Carvajal and Mera, 2010).
Over time, it has been found that synthetic fertilizers manage to solve certain production difficulties, but also that they represent a serious problem for soils since they destroy their fertility, affect groundwater, both for agriculture and fresh water wells for human consumption, and cause high pollution and eutrophication of aquatic systems, implying their high cost. As mentioned by Covarrubias et al. (2009), the excessive use of nitrogen fertilizers can turn into nitrosomine bacteria, cancer precursors. On the other hand, the use of biofertilizers (BF) is recommended as this guarantees agricultural sustainability, reduction of costs, and a low environmental impact.
Alcántar, G.G. and Trejo, L. (2007). Nutrición de cultivos. Colegio de Postgraduados.
Primera edicion. México: Multi Prensa.
Carvajal Muñoz, J.S. and Mera Benavides, A.C. (2010). Fertilización biológica: técnicas de vanguardia para el desarrollo agrícola sostenible. Producción +Limpia 5(2):77-96.
Available at http://www.scielo.org.co/pdf/pml/v5n2/v5n2a07.pdf.
Covarrubias-Ramírez, J.M., Sandino Salazar, R., and Cortes Bracho, J.J. (2009). Impacto de los biofertilizantes en los cultivos y en la producción. Memorias del XXXIV Congreso Nacional de la Ciencia del Suelo. Torreón, Coahuila, México.
Lira-Saldivar, R.H. (2017). Uso de Biofertilizantes en la Agricultura Ecológica. Serie Agricultura Orgánica 14: 9. Artículos Técnicos de INTAGRI. México. Disponible en https://www.intagri.com/articulos/agricultura-organica/uso-de-biofertilizantes-en-la-agricultura-ecologica.
Lira-Saldivar, R.H., Tucuch-Pérez, M.A., López-López, D., Borjas-Banda, C.L. (2013). Respuesta del chile habanero (Capsicum chinense Jacq.) a la biofertilización y acolchado plástico en producción órganica. Agricultura Sotenible 9:538-552.
Muchovej, R.M. (2001). Importance of Mycorrhizae for Agricultural Crops. Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.