Development of an Azolla-Vermiculite Composite fertilizer for enhancing nutrient use efficiency and soil productivity in Uganda

Abstract

Utilization of slow-release fertilizers is a sustainable agricultural strategy which steadily and consistently supplies nutrients throughout a plant’s life, thereby reducing fertilizer application rates. Such fertilizers also promote effective nutrient utilization by crops minimizing nutrient leaching and runoff to non-targeted areas, thereby maintaining a healthy environment. This study developed a slow-release fertilizer named Azolla-Vermiculite Composite fertilizer, which is an innovation from a blend of Azolla cristata powder as the mineral nutrient source, and exfoliated vermiculite as the carrier material. Azolla is an aquatic fern symbiotically associated with the nitrogen fixing cyanobacteria (Anabaena azollae). The cyanobacteria absorb atmospheric nitrogen and avail it to Azolla in form of ammonium ions. This makes Azolla a rich source of nitrogen, a characteristic that was manipulated in this study for developing the Azolla-Vermiculite Composite fertilizer. In this study, four species of Azolla were identified in Uganda through molecular characterization. These species had sequence identities of 100, 93.36, 99.22 and 99.39% to the reference database sequences namely; Azolla mexicana, Azolla microphylla, Azolla filiculoides and Azolla cristata, respectively. The four species were found distributed in four out of the ten agro-ecological zones of Uganda situated in close vicinity to large water masses of L.Victoria, L. Albert, and L. Kyoga. Principal Component Analysis results revealed that maximum rainfall and altitude significantly accounted for the variations in the distribution of Azolla in Uganda with factor loadings of 0.921 and 0.922, respectively. Following experiments of domestication and cultivation of the four Azolla species in the small ponds under uniform conditions, Azolla cristata was observed to have a significantly higher nutrient bio-accumulation of nitrogen, phosphorus and potassium as the primary mineral nutrients for plant growth (p<0.0001) in comparison with the other three species. It was on the basis of this superiority that it was selected out for massive cultivation in a larger pond in preparation for development of the slow-release Azolla-Vermiculite Composite fertilizer. These experiments were conducted at Bukobero village, Masuliita sub-county Wakiso district. Vermiculite is laminar hydrated clay like mineral with capacity to expand on heating and treatment with some chemicals like hydrogen peroxide. It is non-reactive, has an alkaline pH and xii a high water holding capacity. In this innovation, exfoliated vermiculite was used because exfoliation enhances the above properties. Laboratory analysis of both raw and exfoliated vermiculite indicated a significant difference in water holding capacity, bulk density, and electro conductivity with p-values of 0.0002, 0.0033, and 0.0002 respectively. Furthermore, analysis of vermiculite microstructure under the Scanning Electron Microscope exhibited a pure and asbestos free form of vermiculite that was used in the experiment. X-ray Fluorescence exhibited the richness of vermiculite in various minerals including; silica which contributed (>40%) and phosphorus (<1%) among other minerals. Therefore, it was on the basis of these unique properties exhibited by vermiculite after exfoliation, that it was selected for use as a carrier of Azolla powder in developing the slow-release composite fertilizer. When the Azolla-Vermiculite Composite (AVC) fertilizer was tested for growth performance, yield and nutritional composition of Zea maize (maize), the change in height and yield varied significantly between the composite fertilizer treated and control plants, with p values of <0.003 and ≤0.001 respectively. Proximate nutrient analysis of starch, reducing sugars, total carbohydrates and pro-vitamin A carotenoids all varied significantly between AVC fertilizer treated and control plants with p-values of <0.002. This significant variation was attributed to the availability of sufficient concentrations of mineral nutrients in the fertilizer that enhanced these nutritional components. In addition, all the fertilizer treated plants benefited from the water retention property of exfoliated vermiculite in AVC fertilizer. Since yield is a factor of various components with water as one of the major requirements, the water retention of AVC supported effective progression of various physiological processes including photosynthesis, cell turgor, nutrient uptake, among others. The result was enhanced accumulation of photoasssimilates in the sink tissues resulting into a boost in yield and nutritional composition. Therefore, AVC fertilizer is a promising Climate Smart Agricultural fertilizer technology for enhancing soil productivity translating into better food production and improved environmental health. As a sustainability measure, there is a need to improve this product so as to achieve its maximum potential in agricultural production. This will serve as a contribution in support of the FAO strategic framework 2021-2030 in the context of Agenda 2030 for sustainable development and Uganda’s vision 2040.