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Soybean is the most important vegetable source of oil and protein for food use worldwide. Before the Second World War, China, Manchuria, Korea, and Japan were the countries most affected by its cultivation. In the United States, during the aforementioned conflict, there was a significant increase in the cultivated area following a government program aimed at obtaining a greater production of vegetable oils whose interchange had become particularly difficult. The USA is today the most important soybean producer, followed by Brazil and Argentina. The interest in this crop is mainly due to the oil content of the seed (18-21%) and the protein content (38-41%). Soybean oil is used both as an inedible oil and as an edible oil; while the soybean extracted meal is used for human nutrition in strong competition with other high protein foods and is appreciated as a zootechnical concentrate. Soybean is a kind of ancient cultivation. It is mentioned in Chinese literature before 2838 BC, but it is believed that it was extensively cultivated in China as early as 5000 years BC. Much later it was known in the US, in the early 1800s, and then in Europe: soybean was imported to England in the early 1900s as a starch-free food for diabetics.
Soybean belongs to the legume family and all cultivated varieties to the species: Glycine max. It is a bushy, erect, rather leafy plant. The stems of almost all varieties are covered with thick silvery or brown hair. Most cultivars are over one and a half meters tall, branching mainly from low knots. The type of growth habit can be indeterminate, semi-determinate or determinate. In general, the determinate forms present in many Japanese, Korean and southern United States varieties adapt well to environments characterized by a long vegetative season, high temperatures, high soil fertility, while the indeterminate ones are more suitable for opposite environmental conditions. In indeterminate types, flowering occurs from the bottom up; while in the determined types the first flowers to open are the flowers located in the apical part. Determined plants generally have 5-8 nodes in the early cultivars and 12-14 in the later ones and are characterized by lower risks of lodging. The leaves are of different types, in addition to the cotyledonary ones there are two simple oval-shaped primary leaves and trifoliate leaves, almost always thickly covered with hair. As ripening approaches, the leaf color begins to turn yellow and generally the leaves fall off before the pods ripen. The flowers are typical of legumes, white or purple in color. The pods are small, straight, or slightly curved, covered with fuzz and tend to open when ripe. They have a color that varies from yellow, to numerous shades of gray, brown to almost black; commercial varieties contain 2-3 seeds. The shape of the seed changes with the variety, it can be spherical, flattened and elongated, and in most cases, it is round or elliptical. The color can be straw yellow, yellow-gray, green, brown, black or a combination of these colors.
The root system is composed of a main tap root from which the secondary roots branch off. The crop cycle develops through different phenological stages: stages of vegetative and reproductive development.
Stages of vegetative development: ranging from emergence, they include the development of single and trifoliate leaves, up to the development of n nodes of the stem.
Stages of Reproductive Development: these include flowering, pod development and ripening.
The climatic needs are about the same as for corn. The minimum growing temperature is around 4-6 ° C: a temperature of around 24-25 ° C seems to be the optimum for all varieties. Lower temperatures delay flowering. At the seedling stage and in the ripening stage, soybean is less sensitive than corn to temperature changes. It adapts to a wide range of soils, from clayey to highly organic ones. As a legume crop, if its cultivation is managed correctly, it can represent an important economic and agronomic resource. In fact, it is considered a crop that improves soil fertility. It increases the fertility of the soil leaving it in better fertility conditions from a physical (structure), microbiological (biodiversity) and chemical (symbiotic soybean bacteria fix nitrogen in the soil) standpoint.
Nitrogen nutrition represents one of the main processes of all the metabolic activity of the soybean plant and is normally carried out through two ways:
1) The absorption of nitrate through the root system
2) The fixation of atmospheric nitrogen by radical nodules
Nitrogen fixing is due to specific rhizobia bacteria (Bradyrhizobium japonicum) that penetrate the root hairs of the soybean giving rise to numerous nodules or tubercles which are the seat of nitrogen fixation. To ensure optimal nitrogen fixing, it is important that the soil has a good structure, that the environmental conditions are favorable for the development of the crop and that selected bacterial strains are used. The inoculation to the seed is performed on the seeds with microbial consortia also containing Bradyrhizobium japonicum, with the aim of having an initial advantage in the formation of root nodules as well as improving the emergence and development of roots. Moderate nitrogen inputs are recommended to stimulate growth in the early stages. The critical moment for nitrogen requirements occurs just before the start of flowering.
In order to obtain high productions, contributions of phosphorus and potassium are also required. Phosphorus favors the increase in the percentage of protein and the potassium content, while potassium increases the amount of oil. The need for phosphorus is relatively constant throughout the cycle, but the crop shows a peak of absorption during the early stages of seed formation. In the last phase of ripening, the phosphorus is translocated from the vegetative parts to the seed. The soybean plant absorbs more potassium than phosphorus. Potassium is absorbed throughout the growing season and about half of the total is localized in the seeds.
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