1. Introduction
Onion (Allium cepa L.) is one of the most consumed vegetable planted widely across the world with a global annual production of 90 million tones. It is a natural part of the daily diet for most of the population and is a crop of great economic importance all over the world. It is considered as one of the most important vegetable crops produced on large scale in Ethiopia. Onion also occupies economically important place among vegetables in the country. The area under onion is increasing from time to time mainly due to its high profitability per unit area and ease of production, and the increases in small scale irrigation areas.
Raw and cooked onions are consumed as young green plants or as bulbs. They are valued for their distinctive pungency and flavor which improve the taste of other foods. Onion either green or bulbs are used almost daily in every home and are essential ingredient in Ethiopia diet.
Onion contains chemical compounds such as phenolics and flavonoids that basic research shows to have potential anti-inflammatory, anti-cholesterol, anticancer, antioxidant properties and mineral contents. It can protect against cancer, fight fungi and bacteria, promote cardiovascular health, reduce high blood pressure and insulin resistance, aid in weight loss, possess antioxidant activity, fight chronic bronchitis, infections, fever etc..
Minerals or chemical elements which are inorganic nutrients are usually required in small amounts from less than 1 to 2500 mg/ day depending on the mineral. The dietary focus on chemical elements derives from an interest in supporting the biochemical reactions of metabolism with the required elemental components. Appropriate intake levels of certain chemical elements have been demonstrated to be required to maintain optimal health. Some of the minerals of much biological importance are Ca2+, Mg2+, Mn2+, K+, P-, Na+, Fe2+, Zn+2, SO4-S. A rising from the use of onions in almost all meals in a typical Ethiopia family, the safety, nutritional and anti-nutritional composition of this plant which may be responsible for its reported medicinal abilities must be investigated to unravel new information, authenticate earlier claims or counter same. This study was designed to determine quantitatively the ant nutrients and mineral composition of onion bulbs. Onion has low mineral content but there is no clear research done weather or not and since its ingredient in various dishes, used in almost all food preparation and daily intake better to know essential minerals like Zn, Fe, Ca and Mg and if low enrich through biofortfication or food processing.
They also contain calcium, iron and have a high protein quality, ratio of mg amino acid/gram protein. Onions are low in sodium and contain no fat. Onions are also cholesterol free, and provide dietary fiber, vitamin C, vitamin B6, potassium, and other key nutrients.
Nutrient supply interacts with other agronomy management, pest, and climatic factors to affect quality and yield. Environmental factors that can delay maturation and increase storage loss include hail damage to plants, a cooler than normal growing season, or wet weather for field curing of bulbs. Understanding how the onion plant grows and develops is a key part of developing a strategy to supply nutrients for optimum bulb yield and quality.
So far five improved onion varieties namely Adama Red, Bonbay Red, Nasik Red, Nafis and Melkum were released through research from Ethiopian Institution of Agriculture Research, Melkassa agriculture research center and these varieties were demonstrate to consumers and they are used for house consumption, local and foreign markets. However, the information on their nutritional profile and quality parameters value in term of flavor, provide health-promoting phytochemicals, anti oxidant as well as mineral content is scanty.
The present study was conducted to determine the mineral profile, physicochemical properties of improved five onion varieties Adama red, Bonbay red, Melkum, Nasik red and Nafis.
2. Materials and Methodology
2.1. Field Experiment and Sample Collection
The field experiment was conducted at Melkassa agriculture research center (MARC) on station. Five released onion varieties and the onion seed samples were cultivated in three rapes, plot size 1.2mx5m with three triplicates for each variety and spacing 40x20x7cm on the same field to compare the nutritional quality of varieties which growing in the same condition (water requirement, soil fertility and field management). The center is found in the Ethiopian rift valley, 117 km away from Addis Ababa in the south east direction located at 8024'N and 39 012'E and an altitude of 1550 meter above sea level. The mean maximum and minimum temperature is 28.6°C and 13.8°C respectively. The center receives mean total annual rain fall of 825.9mm with erratic distribution.
2.2. Data Collected in the Field
Data on date of transplanting, depth of planting; plant height and number of leaves per plant, leaf diameters and length, dry biomass, weight of dry bulbs, diameters of dry bulbs were collected. Beside data on yield and yield attributes were collected from ten randomly harvested plants from central rows by leaving boarder plant from each plot. Accordingly data on bolter plants, days to physiology maturity, split bulbs, total and marketable bulbs weight were recorded. Maturity time was recorded based on time from transplanting to the time when 50-75% of the leaf falls down to determine the days to physiological maturity. One composite soil sample before planting and five composite soil samples from fifteen plots by treatment were collected.
2.3. Soil Chemical Properties Analysis Method
The soil pH was measured using potentiometer with a digital pH meter in the supernatant suspension of 1: 2.5 soils to water ratio while soil organic carbon content was determined by the dichromate oxidation was estimated from the organic carbon content by multiplying the latter by 1.724. Total N was determined using the micro-kjeldahl digestion, distillation and titration procedure as described by Bremner and Mulvaney. Soils available P was extracted by the Olsen method.
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