MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 9. THE ROLE OF MICROORGANISMS IN THE BIOGEOCHEMICAL CYCLES OF NATURE

BACTERIAL INOCULANTS FOR ENHANCING SOIL FERTILITY

Following the discovery of ROOT nodule Bacteria, the idea emerged to utilize them for improving crop yields by enhancing the fixation of molecular nitrogen. This concept was first put into practice in 1896 in Germany by F. Nobbe and L. Hiltner.

In recent years, root nodule bacterial inoculants have been widely used across various countries under different names: nitroculture, nitragin, N-germ, nitrazon, rhizotorphin, among others.

Nitragin is a preparation consisting of living Cells of a specific species of root nodule bacteria, with its primary function being the Fixation of Atmospheric nitrogen.

The application of root nodule bacterial inoculants is particularly necessary when introducing new legume crops to a region where the native soil microflora lacks the appropriate bacteria. Legume crops develop intensively only in the presence of specific strains of root nodule bacteria and in sufficient quantities, which is not always observed under natural conditions. Because the cultivation of certain legumes is restricted to specific areas, soils do not always harbor all strains of root nodule bacteria. In Ukraine, soils are sparsely populated with soybean root nodule bacteria. In transitional zones from the forest-steppe to the Polissya region, as well as in the Carpathian foothills and Transcarpathia, There is a scarcity of bacteria specialized for alfalfa, vetch, sainfoin, and peas. This is due to the fact that these crops have historically been sown sporadically or not cultivated at all in these areas. Furthermore, the root nodule bacteria of sainfoin and peas cannot tolerate the acidic environments characteristic of the soils in the northern and western regions of Ukraine.

How can we explain the positive effect of inoculating legumes with the Rhizobium culture in cases where soils have long been cultivated and already contain root nodule bacteria within their native microflora? First, cross-infection can occur under natural conditions, meaning that legumes become infected by bacteria from closely related plant groups. In such instances, nodules may form, but they function suboptimally, or ineffective nodules are produced. Artificial inoculation introduces an active strain of root nodule bacteria—applied directly to the seeds—into The Root System of the legume. Second, root nodule bacteria present in soil previously used for growing cereal crops often exist merely as Saprophytes. The conditions in such soils are frequently unfavorable for root nodule bacteria, causing their populations to decline significantly. Consequently, natural infection under these conditions fails to establish an effective Symbiosis.

A sufficient population density of indigenous (native) root nodule bacteria in the soil also does not always satisfy agricultural demands, as these natural strains may lack sufficient activity. Microbial breeders therefore recommend utilizing more active strains.

The industry produces two forms of nitragin: soil-based and dry.

Soil-based nitragin is a culture of root nodule bacteria grown in sterilized soil. One gram of such a preparation must contain at least 300 million cells. The application rate for seed Treatment is 500 g/ha. To produce this preparation, organic-rich soil (or peat) is mixed with up to 30% sand and chalk to adjust the pH to 6.6–7.0. The soil is sterilized, packaged into polyethylene bags, and inoculated with the appropriate strain of root nodule bacteria, followed by cultivation at 26–28 ^oC for 4–6 days (10–11 days for slow-growing strains).

Dry nitragin is a powder composed of living root nodule bacteria cells. One gram of the preparation must contain 8–10 billion cells. This facilitates transportation, although the preparation must be mixed with a filler prior to application.

Root nodule bacterial inoculants are species-specific and intended for treating seeds of a particular legume crop, as indicated on the packaging (e.g., "Soybean", "Vetch", "Sainfoin", etc.).

The economic return from applying the inoculant to forage grasses yields 10–15 units of profit per unit of cost, while for grain legumes, it reaches 20–30 units or more.

Azotobacterin is a preparation of living Azotobacter chroococcum cells. Its main function is the fixation of atmospheric nitrogen.

The production technology is identical to that of nitragin. One gram of the soil-based preparation must contain at least 50 million cells. For seed treatment, 3–6 kg of azotobacterin is applied per hectare. One gram of the dry preparation must contain at least 10 billion A. chroococcum cells.

Azotobacterin is also valuable because A. chroococcum synthesizes a range of BIOLOGICALLY ACTIVE SUBSTANCES: nicotinic and pantothenic acids, pyridoxine, biotin, heteroauxin, gibberellin, and Other Compounds that stimulate seed germination and accelerate plant growth.

The bulk of the fixed nitrogen is removed with the harvested crop, while a certain amount remains in the soil. Root nodule bacteria ensure the successful yield formation of most legumes without depleting the soil of nitrogen. Only after the cultivation of peas and soybeans might a slight decrease in soil nitrogen content be observed. Due to this property, legume crops serve as excellent predecessors in crop rotations, making it possible to reduce nitrogen fertilizer rates for subsequent crops or eliminate them entirely.

The cultures underlying both nitragin and azotobacterin are non-spore-forming bacteria, and their viable Cell counts decrease during storage; therefore, such preparations are manufactured immediately prior to use.

Phosphobacterin is a preparation containing spores of Bacillus megaterium var. phosphaticum, which transform complex organophosphorus compounds and sparingly soluble mineral phosphates into plant-available forms of phosphorus. Industry produces both soil-based and dry forms of phosphobacterin. One gram of the dry preparation contains approximately 8 billion spores. The per-hectare application rate for seed treatment is 5 г for cereal crops, and 15 г for corn, potatoes, and vegetable crops. The shelf life of phosphobacterin is virtually unlimited. The culture underlying this preparation synthesizes biologically active substances: thiamine, pyridoxine, biotin, pantothenic and nicotinic acids, and vitamin B₁₂.