Zinc is a necessary micronutrient for the metabolic activities of coffee and associated multi-crops. More importantly, zinc is required by microbes to carry out many essential physiological activities. Zinc added as a micronutrient often gets fixed in the soil and is unavailable to the plant. Plants can uptake zinc as a divalent cation but only a very minor portion of total zinc is present in soil solution in soluble form. The rest of the zinc is in the form of insoluble complexes and minerals. Plant zinc uptake sometimes drops due to adverse environmental factors, resulting in reduced coffee yield and growth. Due to the unavailability of zinc in soil, zinc deficiency occurs which is one of the most widespread micronutrient deficiencies.
The main soil factors responsible for causing Zn deficiency are low total contents of Zn, high pH, high contents of calcite, high concentrations of bicarbonate ions and salts, and high levels of available phosphorus.
Zinc solubilizing microbes aid in the release of fixed zinc and enables the uptake in the available form. A majority of coffee Planters are unaware that the great majority of soil microbes, inside the coffee ecosystem are beneficial and absolutely necessary for various transformations, which yield energy-rich compounds needed for plant growth and development. Microorganisms are a vital link in the mineralization of organic constituents and provide nutrients in the available form for plant growth and development.
In addition to improving crop production and yield, researchers worldwide are Woking on biofortified crop development to prevent micronutrient-related deficiency. Biofortification is the process by which the nutritional value of food crops is enhanced by various methods including plant breeding, agronomic practices, and modern biotechnological techniques. Basically, biofortification is the process of growing crops to increase nutrition value from the seed on. This practice is not in vogue in coffee plantations. The deficiency in zinc and other micronutrients, such as iron and selenium, is known as “hidden hunger”.
It is a common fact that the coffee rhizosphere is greatly influenced by its surrounding environment, which in turn is influenced, depending on the distance to which exudations from the root system migrate to the surrounding area. Some of the microbes that inhabit this area are bacteria that are able to colonize very efficiently the roots or the rhizosphere soil of crop plants. These bacteria are referred to as plant growth-promoting rhizobacteria (PGPR). They achieve important functions for plant growth and health The rhizosphere effect indicates the overall influence of plant roots on soil microbes. Scientific data clearly reveals that a greater number of bacteria, fungi, and actinomycetes are present in the rhizosphere soil than in non rhizosphere soil. It has also been demonstrated that the rate of metabolic activity of the rhizosphere microorganisms is different from the non rhizosphere soil. Different strains of Plant Growth Promoting Rhizo Bacteria are capable of zinc solubilization. These bacteria improve plant growth and development by colonizing the rhizosphere and by solubilizing complex zinc compounds into simpler ones, thus making zinc available to the plants. Zinc-solubilizing bacteria act as natural bio-fortifiers that can solubilize the unavailable form of zinc by secreting organic acids, siderophores, and other chelating compounds. Zinc solubilizing microorganisms solubilize zinc through various mechanisms, one of which is acidification. These microbes produce organic acids in the soil which sequester the zinc cations and decrease the pH of the nearby soil.
Zinc Solubilizing Bacteria
Bacterial strains that have been reported to show zinc solubilization on lab scale include Gluconacetobacter dizotrophicus, Bacillus sp., Pseudomonas striata, Pseudomonas fluorescence, Burkholderia cenocepacia , Serratia liquefaciens, S. marcescens, and Bacillus thuringiensis, Thiobacillus thioxidans, Thiobacillus ferroxidans, Pseudomonas aeruginosa , Acinetobacter, Bacillus.
Isolation of Zinc Solubilizing Microorganisms.
One way of addressing this problem in the coffee ecosystem is by Isolation, Characterization of Zn Solubilizing Bacteria, specific to each Agro climatic region.
Periodic Addition of Soil Organic Matter
Another way to fix zinc deficiency in soil is to add organic matter like compost or manure. These substances help increase soil bacteria that can convert insoluble matter into soluble nutrients for plants. When these nutrients are available to plants, they can get better zinc uptake from the soil.
There has to be an urgent awakening among the coffee Planters to understand the roles of beneficial bacteria and utilize them to increase the productive capacity and sustainability of coffee agroecosystems.
The coffee Agroforestry model is truly symbiotic in nature because it complements the coffee ecosystem. Overall, the competition between and among crops is not a threat to the stability of the ecosystem. A few issues need to be addressed and one among them being the availability of micronutrients in coffee in the available form. Unfortunately, the application of zinc fertilizers has low use efficiency as 96–99% of the applied Zn is converted to an unavailable form due to the formation of carbonates, oxides, or phosphates depending on soil type.
We propose to circumvent zinc and other micronutrient deficiencies by isolating zinc-solubilizing bacteria from respective Agroclimatic zones and characterizing these isolates in the laboratory based on their efficiency. In doing so, it will also throw light on the microbial resource by analyzing the microbial community composition and isolating various beneficial microbes.
Anand T Pereira and Geeta N Pereira. 2009. Shade Grown Ecofriendly Indian Coffee. Volume-1.
Bopanna, P.T. 2011.The Romance of Indian Coffee. Prism Books ltd.