Numerous soil bacteria which reside in the plant rhizosphere and which may grow in, on, or around plant tissues, stimulate plant growth. These bacteria are known as plant growth promoting rhizobacteria (PGPR).
Some of PGPR can promote growth by acting as both biofertilizer and biopesticides.
Figure 2: Integrated microbial actions in soils.
The screening for PGPR and investigation of their activities are expanding at a fast pace as endeavors are made to exploit them commercially as biofertilizers.
The most valuable activities of PGPR include fixing N2, increasing the availability of nutrients in the rhizosphere, positively influencing root growth and morphology, and promoting other beneficial plant-microbe symbiosis. The blend of these modes of actions in PGPR is also addressed, as well as the difficulties facing the more broad usage of PGPR as biofertilizers.
Two types of materials are used in agriculture, fertilizer or pesticide. It can be assumed that fertilizer is required for nourishment, and pesticide for medication of plants in conventional agriculture. On the other hand, biofertilizer and/or biopesticide represent respectively both materials in sustainable or environmentally friendly system (Figure 2).
The main sources for biofertilizer are nitrogen fixing becteria, phosphate solubilizer, and mycorrhizae. Similar to the functional foods, like restoratives and/or adjuvant, who are required for human health care; plant growth promoting rhizobacteria may be one of the compatible substances for better crops yield.
However, several limitations exist in the use of biofertilizer for agricultural system. Primarily, the efficacy for most biofertilizer is not reliable. This is due on the scarce data available about the mechanism of action of different biofertilizer in promoting plant growth. However, research into biofertilizer is increasing, trying to manage these issues.
Moreover, different parameters should be also assessed, such as: soil type, managements practices, and weather effect on biofertilizer efficacy. Furthermore, there is a block in biofertilizer development. It is difficult to test inoculant in field as routine experiments, as shown in Figure 3.
Figure 3: Experimental process for biofertilizer testing.
The microorganisms used for development of biofertilizers are bacteria of genera Bacillus, Pseudomonas, Lactobacillus, photosynthetic bacteria, nitrogen fixing bacteria, fungi of Trichoderma and yeast. Among the microbes, the most employed microorganism is Gram (+) endospore-forming bacteria from genus Bacillus. Usually, several species of microbes are used in microbial products with an available period of by- products of about 1~2 and/or 2~3 years.
Biofertilizers can be solid or liquid. Carriers used in solid type biofertilizers are generally clay mineral, diatomaceous soil, and white carbon as mineral. Other materials used are rice, wheat bran, and discarded feed as organic matter. However, the effects of carriers and/or supplements on microbial growth are of great importance and should be seriously consider in the control of microbial products. In fact, often farmers misunderstood this carrier effect as microbial action.
As displayed by producers, microbial products stimulate plant growth, decrease pest occurrence, stimulate composting and ameliorate the soil. However, the main effect generally is the plant growth stimulation. Nevertheless, in 40 % of the commercial biofertilizers manufacturers declare presence of multiple effects.
In this respect controlling the quality of biofertilizer is one of the most important factors. Thus their success or failure and acceptance or rejection by end-user, the farmers will be assured. Principally, quality represents the number of selected microorganism in the active form per gram or milliliter biofertilizer. Up to now quality standards are developed only for Rhizobium. Moreover, specifications of biofertilizer differ from country to country and maybe comprise parameters like: microbial density at the time of manufacture, microbial density at the time of expiry, the expiry period, the permissible contamination, the pH, the moisture, the microbial strain, and the carrier. Quality has to be monitored at different production stages (during pre-culture stage, carrier selection and preparing, broth formulation, mixing of broth and culture, packaging and storage). Main quality parameters to be respected during biofertilizer production are summarized in Table 1.
Table 1: Key quality parameters of biofertilizers
|Appearance of living target bacteria||Without strange smell||Brown or black||Brown|
|N fixation bacteria||>0.5x109/ml||>0.1x109/g||>0.1x109/g|
|Si bacteria P bacteria||>1.0x109/ml||>0.2x109/g||>0.1x109/g|
|Water content (%)||20-35||10|
|Non-target bacteria Contamination (%)||<5||<15||<20|
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