Biofertilizers

DEFINITION: Fertilizers consisting of either naturally occurring or genetically modified microorganisms

Biofertilizers provide a means by which biological systems can be utilized to supply plant nutrients such as nitrogen to agricultural crops. The use of biofertilizers could reduce the dependence on chemical fertilizers, which are often detrimental to the environment.

Plants require adequate supplies of the thirteen nutrients necessary for normal growth and reproduction. These nutrients, which must be supplied by the soil, include both macronutrients (those nutrients required in large quantities) and micronutrients (those nutrients required in smaller quantities). As plants grow and develop, they remove these essential mineral nutrients from the soil. Since normal crop production usually requires the removal of plants or plant parts, the nutrients are continuously removed from the soil. Therefore, the long-term agricultural utilization of any soil requires periodic fertilization to replace lost nutrients.

Nitrogen is the plant that is most often depleted in agricultural soils, and most crops respond to the addition of nitrogen fertilizer by increasing their growth and yield; therefore, more nitrogen fertilizer is applied to cropland than any other fertilizer. Up to 80 percent of all nitrogen on earth is present within the atmosphere, but it is not in a state that allows most plants to readily utilize it. Biofertilizers take this atmospheric nitrogen and convert it into a form that is accessible by plants. In the past, nitrogen fertilizers have been limited to either manures, which have low levels of nitrogen, or chemical fertilizers, which usually have high levels of nitrogen. The excess nitrogen in chemical fertilizers often runs off into nearby waterways, causing a variety of environmental problems.

Biofertilizers offer a potential alternative: They supply sufficient amounts of nitrogen for maximum yields yet have a positive impact on the environment. Biofertilizers generally consist of either naturally occurring or genetically modified microorganisms that improve the physical condition of soil, aid plant growth, or increase crop yield. Such fertilizers provide an environmentally friendly way to increase plant health and yields with reduced input costs, new products and additional revenues for the agricultural industry, and cheaper products for consumers.

While biofertilizers could potentially be used to supply a number of different nutrients, most of the interest thus far has focused on enhancing nitrogen fertilization. The relatively small amounts of nitrogen found in soil come from a variety of sources. Some nitrogen is present in all matter in soil; as this is degraded by microorganisms, it can be used by plants. A second source of nitrogen is nitrogen fixation, the chemical or biological process of taking nitrogen from the atmosphere and converting it to a form that can be utilized by plants. Bacteria such as Rhizobia can live symbiotically with certain plants, such as legumes, which house nitrogen-fixing in their roots. The Rhizobia and plant root tissue form root nodules that house the nitrogen-fixing bacteria; once inside the nodules, the bacteria use energy supplied by the plant to convert atmospheric nitrogen to ammonia, which nourishes the plant. Natural nitrogen can also be supplied by free-living microorganisms, which can fix nitrogen without forming a symbiotic relationship with plants. The primary objective of biofertilizers is to enhance any one or all of these processes.

One of the major goals in the of biofertilizers is to transfer the ability to form nodules and establish effective symbiosis to nonlegume plants. The formation of nodules in which the Rhizobia live requires plant cells to synthesize many new proteins, and many of the genes required for the expression of these proteins are not found in the root cells of plants outside the legume family. Many research programs have been devoted to efforts to transfer such genes to nonlegume plants so that they can interact symbiotically with nitrogen-fixing bacteria. If this is accomplished, Rhizobia could be used as a biofertilizer for a variety of plants.

There is also much interest in using the free-living, soil-borne organisms that fix atmospheric nitrogen as biofertilizers. These organisms live in the rhizosphere (the region of soil in immediate contact with plant roots) or thrive on the surface of the soil. Since the exudates from these microorganisms contain nitrogen that can be utilized by plants, increasing their abundance in the soil could reduce dependence on chemical fertilizers. Numerous research efforts have been designed to identify and enhance the abundance of nitrogen-fixing bacteria in the rhizosphere. Soil microorganisms primarily depend on soluble root exudates and decomposed organic matter to supply the energy necessary for fixing nitrogen; hence there is also an interest in enhancing the biodegradation of organic matter in the soil. This research has largely centered on inoculating the soil with cellulose-degrading fungi and nitrogen-fixing bacteria or applying organic matter to the soil, such as straw that has been treated with a combination of the fungi and bacteria. The introduction of this biodegraded material to nutrient-depleted soil has the potential to result in not only greater growth and yield for the plants growing within it, but better protection from disease, fungal attacks, and the harmful effects of pollutants.

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