Combined Effect of Enriched Compost and Microbial Inoculants on Soil Nutrients and Phosphorus Uptake by Rice

Abstract

Preparation of compost enriched with rock phosphate may be considered as a suitable option for utilizing the reserves of low-grade rock phosphate for enhancing the phosphorus use efficiency of crops. This research was carried out to study the effect of combined inoculation of mycorrhiza and selected rhizobacteria on soil nutrients and P uptake by rice. The experiment was conducted in Randomized Block Design with 10 treatments having three replications. The treatments included a control and a complete recommended fertilizer dose along with different combinations of rock phosphate enriched compost, mycorrhiza and rhizobacterial isolates at a lower P dose. Results from laboratory analysis of experimental soil and plant samples suggest that the treatment with rock-phosphate enriched compost @ 100% of the phosphorus dose along with phosphorus solubilizing bacteria and mycorrhiza proved to be the most effective treatment for maintaining a higher nutrient status in soils. On the other hand, rock-phosphate enriched compost @ 50% of the phosphorus dose amended with 50% of the recommended phosphorus from chemical fertilizers along with co-inoculation of phosphorus solubilizing bacteria and mycorrhiza resulted in a higher P uptake by rice. Co-inoculation of microbial inoculants in presence of organic matter can be considered as possible strategies towards improving nutrient status in soils as well as P nutrition to plants for a sustainable soil health Phosphorus (P) is the most limiting macronutrient after nitrogen in most of the Indian soils and Bihar is no different. Low phosphate availability in soils may be attributed to high phosphate fixation (60 to 70%) by hydroxides of iron and aluminium and calcium carbonate when water soluble P fertilizer is applied. This leads to poor yield of rice (Ghosal et al., 2012). Currently, a majority of the phosphatic fertilisers are imported and thus prove to be expensive for most of the resource-poor farmers in the country. Mineral wealth acts as the major resource for fertilizers which is finite and non-renewable. Thus, the optimal and economic use of this precious resource holds great importance. Although, local phosphate deposits exist in India, some of them are still unexploited and most of them are of low grade (Tarafdar, 2013). Rock phosphate has good P content (28-30%) but it cannot be directly used as a fertilizer owing to the poor release of plant available P (Reddy et al., 2002). The ability of the rock phosphate to release phosphates in plant available forms depends on their particle size, chemical and mineralogical characteristics as well as the properties of the soil in which they are applied. Also, solubility of phosphorus increases with the increase in organic matter content in soil. Moreover, farmyard manure (FYM) and traditional compost contain very small amount of nutrients, particularly P which are difficult to obtain in required quantity for substituting some part of chemical fertilizers. Rock phosphate enriched compost is a potent and suitable alternative for FYM or traditional compost to sustain soil physical, chemical and biological quality. Arbuscular mycorrhizal fungi (AMF) improve the phosphorus nutrition after colonizing the roots of host plants growing on soils with low or sparingly soluble forms of P (Shenoy and Kalagudi, 2005). On the other hand, beneficial rhizobacteria, also termed as plant growth-promoting rhizobacteria (PGPR) (Sikora, 1992), release low molecular weight organic acids (esp. Gluconic and keto gluconic acid) which dissolve phosphatic minerals (He et al., 2002). Certain identified phosphorus solubilizing bacteria like Pseudomonas striata can help in solubilizing the native soil P for increased availability to the plants. Not only providing phosphorus to the plants, the phosphate solubilizing microorganisms are also reported to facilitate the growth of plants by stimulating the efficiency of nitrogen fixation, accelerating the accessibility of other trace elements and by synthesizing important growth promoting substances (Mittal et al., 2008). There is great possibility to save our rock phosphate reserve as well as a huge amount of capital spent on raw materials to prepare P fertilisers by preparation and use of compost enriched with rock phosphate. In this paper, an attempt was made to study the combined effect of rock phosphate-enriched compost and microbial inoculants on soil fertility and enzymatic activity.