Effect of Compost, NPK and Plant Promoting Rhizobacteria (PGPR) on Growth and Yield of Three Vegetables cultivated on Arenosols

Soil fertility degradation, caused by erosion and depletion or imbalance of organicmatter/nutrients, is affecting world agricultural productivity [1]. Inorganic fertilizers have played a significant role in increasing crop production since the “green revolution” [2]; however, they are not a sustainable solution for maintenance of crop yields [3]. Long-term overuse of mineral fertilizers may accelerate soil acidication, affecting both the soil biota and biogeochemical processes, thus posing an environmental risk and decreasing crop production [4].

Composting is considered a sustainable and environmental friendly approach for the safe utilization of solid organic wastes such as farm manure and trash of crops [5]. Usually, composts are applied to get equivalent amounts of nutrients but that requires a large amount of application rates. Increase inorganic matter and vegetable production are seen in previous research at different levels (22.5, 56, and 112 t ha−1) of compost application [6]. Some reports stated that application of composts improved the soil physical, chemical, and biological as well as yield of crops compared to chemical fertilizers on sustainable basis [7]. Mineralization of compost in the soil results in nutrient release and soil quality enhancement. The useful effects of application of composted organic materials on soils are very extensively recognized allover the world [8]. Total organic matter contents, microbial activity, and nutrient release are also increased with the application of composts [9]. Compost improves the physicochemical properties of soils such as pH cation exchange capacity, bulk density, porosity, and water holding capacity. The major concern associated with the use of organic manures is their rapid rate of decomposition especially under high temperature. Organic matter may be mineralized within a single cropping season and its sustainability is a standing question. Practically, some limitations are associated with the application of composts at higher rates containing some toxic constituents such as heavy metals [10-11]. Excessive applications of composts may release some organic compounds which can contaminate surface waters by runoff and subsurface water when percolates in deep layers [8]. This concern can be addressed with the value addition of composts in terms of higher stability and fertility for sustainable agriculture.

Vegetables are rich source of vitamins, proteins, carbohydrates and minerals, which constitute an important component inhuman nutrition. Besides the nutritional value of vegetables, increased interest is being bestowed on the functional and therapeutic benefits of vegetables inhuman health. Agriculture is highly dependent on the use of chemical fertilizers, growth regulators, fungicides and pesticides for obtaining increased yield. This dependence is associated with problems such as environmental pollution, health hazards, interruption of natural ecology, nutrient recycling and destruction of biological communities that otherwise support crop production. The use of bioresources to replace these chemicals is gaining importance. In this context, plant growth promoting rhizobacteria (PGPR) are often considered as novel and potential tool to provide substantial benefits to agriculture. [12].

PGPR are a heterogeneous group of bacteria that can be found in the rhizosphere, which can improve the quality of the plant growth directly and or indirectly [13] as (i) their ability to produce plant growth regulators like indoleacetic acid, gibberellic acid and cytokinins [14], (ii) asymbiotic nitrogen fixation [15], (iii) antagonism against phytopathogenic microorganisms by production of siderophores [16], antibiotics [17] and cyanide [18], (iv) solubilization of mineral phosphates and other nutrients [19] and (v) active removal and bioaccumulation of heavy metals and their capacity to assist the root growth [20]. In addition, PGPR isolates must be rhizospheric competent, able to survive and colonize in the rhizospheric soil [21]. The variability in the performance of PGPR may be due to climate, weather conditions, soil characteristics or the composition or activity of the indigenous microbial flora of the soil that may affect their growth and exert their effect on the plant [22] Different bacteria that have been reported as PGPR belong to the following genera: Pseudomonas, Bacillus, Azospirillum, Agrobacterium, Azotobacter, Arthrobacter, Alcaligenes, Serratia, Rhizobium, Enterobacter, Burkholderia, Beijerinckia, Klebsiella, Clostridium, Vario vovax, Xanthomonas, and Phyllobacterium (23-24). Among these, Pseudomonas and Bacillus are the most widely reported PGPR.

The aim of this study was to evaluate the effects of composting and PGPR (including nitrogen-fixing bacteria and phosphate-solubilizing bacteria) on three vegetables (leaf-eating vegetable) as sweet cabbage (Brassica integrifolia O. B. Schultz), parchoi (Brassica chinensis L.),and mustard greens (Brassica juncea L.)