Microbial Tools – A Key to Sustainable Productivity

Dr.  Dhananjaya P. Singh

One of the dreams of human endeavour is the sustainable increase in the pace of agricultural productivity to fulfill the demands of rapidly growing human population. During the last decades, interaction of the following factors of modern technology resulted in increased agricultural production-a) development of high yielding varieties, b) multiple cropping practices c) improved irrigation technologies and d) use of chemicals. However, the need and greed for crop yield has resulted in over-exploitation of the soil ecosystem. The effects are manifested in the form of degraded soils depleted of soil nutrition and organic matter.

In the Indian context, the seriousness of the issue can be understood from the following facts. About one fourth of the nitrogen, more than half of phosphate and total potash are being imported either in the form of raw material or finished product.  However, only a considerable portion of these nutrients put into the soil remains available to the plant roots and a major part is lost due to the physio-chemical as well as biological activities.  Long term experiences have shown that neither the organic manure nor the chemical fertilizers alone can achieve sustained high yields. In modern intensive farming, the nutrient turn over in the plant-soil system has been quite large. Integrated use of organic manures, bio-fertilizers and chemical fertilizers, therefore, remain the only promising option in improving crop productivity.

Microbes balance soil ecology

Microbes are integral parts of every soil ecosystem.  Various kinds of biological activities are continuously going on in the soil. Metabolic activities of the microbes such as bacteria (plant growth promoting rhizobacteria, PGPR), fungal organisms (mycorrhiza, cyanobacteria) and soil fauna (nematodes, worms, protozoans etc.) promote soil health and crop productivity.  Studies indicate the beneficial role of microorganisms in increasing productivity by soil enrichment with organic matter and nutrients.

Almost all the beneficial microbes essentially need carbon source for their survival.  This explains as to why the soils poor in the organic matter content are usually poor in microbial activities.  In the past decades, huge chemical inputs have made soils poor, or even worse in terms of affecting microbial activities. This led to decrease, or in many areas, extinction of a number of soil biota such as beneficial growth promoting bacteria and fungi, earthworms, actinomycetes etc.  This situation prevails in almost all types of soils across the country.  For example, in Chandauli district of Uttar Pradesh state, both qualitative and quantitative change in the rice-wheat cropping system has been observed in the past 10 years. The yield as well as the quality of the harvest declined severely.  Emergence of high alkaline patches in the field and crop failure due to increased fertilizer doses are reported by the farmers.  Such a situation is common in many parts of the Eastern U. P.

The Foundation for Agro-Technology Development and Resource Management working in that area helped the farmers in the region to assess their situation and adopt remedial measures.  The farmers adopted biofertiliser application in rice ecosystem in a step-wise manner.  Soil was first mixed with FYM and Sewage Sludge Waste (SSW) in equal quantities (1: 1 ratio) at the time of field preparation for rice cultivation following flooding.  This was followed by the application of mix culture of cyanobacterial strains (originally isolated form the field and then cultured under similar conditions) @ 15 kg per hectare, 10 days prior to transplantation.   This resulted in the improvement of both quality and quantity of rice crop and also improved soil conditions for the next wheat crop.

Biological Nitrogen-Fixation (BNF): A Nature’s Gift

Nitrogen is one the most abundant gases in the atmosphere but unusable by the plants in its gaseous form. Many microbes, however, are capable of converting the gaseous nitrogen to nitrate. The nitrates can be easily utilized by the plants.  This process is called ‘Biological Nitrogen Fixation'(BNF).

BNF has an assured place in agriculture mainly as a source of nitrogen for legumes and other important crops. On a global scale, BNF provides the largest input of nitrogen to agricultural soils.  Inoculation of Rhizobium as biofertilizer in the crops such as groundnut, pigeonpea, soybean etc. provided 19-22 kg of nitrogen per hectare which increased production by 17-33%.  Similarly, use of non-symbiotic bacteria Azotobactor in wheat, sorghum, tomato, cotton, sugarcane and Azospirillum in wheat, maize, rice, sorghum crops augmented nitrogen supply to crops to an extent of 20-30 kg per hectare and resulting in increased crop yield by 10% to 30%

Blue-green algae (BGA) (or cyanobacteria) are phototrophic organisms that grow wherever water, sunlight and carbon di-oxide are available.  The rice ecosystem is an ideal environment for the growth and development of these self-supporting organisms.   A large variety of nitrogen-fixing BGA such as Aiiabaena, Nostoc, Aulosira, Calothrix, Tolyphothrix etc. colonize the rice field soils.  Upon application in the soil, blue-green algal organisms compete well with the native strains, grow profusely near the root zone in soil. Nitrogen fixed by them is released either through exudation or through microbial decomposition after the alga dies.  In paddy fields, the death of algal biomass most frequently results in gradual build-up of soil fertility. The residual effects influence the succeeding crops also.  Apart from fixing N and adding organic matter to soil, BGA are also known to produce and excrete plant promoting substances like indole acetic acid etc.   Also, continous use of the BGA biofertilizers for 2-3 years adequately builds up the population of these organisms in the soil.

The relative contribution of BGA as a percentage of total nitrogen-fixed in paddy fields varies widely and is estimated to be 15-35 kg nitrogen per hectare in India.  In areas where chemical nitrogen is not used for various reasons, algal inoculation enhances a minimum of 4% to a maximum of 32.8% crop yield with an overall average of 16.1%.  Even at the levels of chemical N fertilizers being used in different states, the application of BGA biofertilizer resulted in an increased crop yield of 8.85%. Overall the application of BGA biofertilizer with the existing technology led to a net saving of 25-30 kg nitrogen per hectare every season.

PGPRS: Low-Cost Input from Nature

Besides nitrogen fixers, many bacteria colonize plant roots. Some of them promote plant growth significantly.  They help in mobilization of the soil nutrients and production of phytoharmones or growth regulating substances.

These phytohormone producing microbes have been classified as PGPR.  These PGPRs readily colonize the rhizopheric zone of roots where they produce a number of natural growth promoting substances.  Of the many such bacteria identified, the role of fluorescent Pesudomanas and Bacillus species have attracted much attention. The substances produced by them have natural biocontrol and plant growth promoting capabilities.  Increased amount of nutrient uptake by plants inoculated by Pseudomonas putida has been attributed to the production of growth regulators by the bacterium at root surface which stimulates root development.

Pseudomonad (group of Pseudomonas species) inoculants produced indole acetic acid-like substances (plant harmone) in the rhizosphere of wheat grown in field conditions.  Many PGPRS, for example, Pseudomonas fluorescence, Pseudomonas aerug7nosa and Bacillus subtilis) also produce substances such as siderophores and saponins, which are responsible for the removal of heavy metal toxicity.  These organisms are also responsible for enhancement of rhizospheric competitive ability by antagonistic effects on other harmful bacteria, control of plant diseases that affect root density, production of chemicals that interfere with the organisms infecting plant roots, enhancing the availability of nutrients that improve the efficacy of plants.  PGPRs are therefore being widely evaluated for their role in sustainable resource management as biocontrol agent and biofertilizer.

An indegeneous method has been developed by the Foundation for the production of biofertilizer rich in bacterial population at the farmers level. The idea is to use beneficial microbial population, already acclimatized with the pH and other physicochemical conditions of local soil. Roots of existing crops, grasses and weeds were cut from the rhizospheric region along with the adhered soil and chopped into fine pieces.  The whole material was dipped (suspended) in a bucket of water containing 5% molasses solution overnight.  Further, this solution along with the chopped material is transferred to field at the time of transplanting.  By this low cost, indigenous process mixed population of root-rhizospheric bacteria can be increased manifold and maintained by the local farmers as per their requirement.  The process reduces synthetic fertilizer inputs. (Farmers of Varanasi and Ghazipur region, U.P. (Villages- Bhopapur and Ramna, Hansrajpur and Yusufpur, respectively) are being trained by the workers of the Foundation for such processes, who may utilize naturally existing microbial population in their field).

 Mycorrhizal Fungi (VAM)

It is now well established that mycorrhizae play a dominant role in making unavailable soil nutrients available to plants roots and increasing the potential gain of available resources.  These organisms ensure easy availability of organic carbon and complex organic nitrogen and phosphorus sources, increase phosphorus solubilization and availability in clay soils.  These fungi work upon large volumes of soil. Their hyphae extend outwardly from the roots ranging from a few centimeters to several meters in the soil. This results in increasing the effective absorbing surface of the host root by as much as 10 times, resulting in enhanced absorption of immobile nutrients such as phosphorus, zinc, copper etc. in the soil by 60 times.  Mycorrhizal fungi also transport many other nutrients including calcium, magnesium, sodium, sulphur, iron, chlorine etc., all essential for plant growth and development. It has been observed that plants with mycorhizal association are more tolerant to heavy metal toxicity.  These plants survive well in drought and arid conditions as improved water movement is facilitated by mycorrhiza.

Theoretically, the most efficient level of nutrients is the concentration of mineral elements in the plant tissue just above the ‘critical level’ necessary for optimum growth.  Further addition of chemical fertilizers may be taken up by plants, as ‘luxury concentration’. This adds very little to plant growth.  Now, these microorganisms help in constituting the ‘optimum level’ of minerals in the plant tissue even at low level of fertiliser inputs. They fix nitrogen, solubilize phosphorus and facilitate uptake of minerals by roots.  Thus, these microorganisms in the form of biofertilizers may provide essential tool for maintaining good soil fertility, better soil conditions and sustainable agricultural productivity.

Dr. Dhananjaya .P. Singh, Ph.D (Biotechnology), Director, Foundation for Agro-Technology Development and Resource Management, 57, Lane No. 1, Om Nagar, Akatha, Pahariya Varanasi (U.P.), Ph. 0542-2373775, e-mail- samar_ps@yahoo.com

Correspondence : F-165, NR, Lahartara Colony, VARANASI – 221002


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