Onion
Onions have a shallow, sparsely branched root system with most roots in the top 30 cm of soil. Rooting density decreases with soil depth. Thus, it is important to maintain nutrient and soil moisture within the shallow rooting area. Soils need to be well structured and fertile to maximize growth and produce high yields. Bulbs are harvested when the tops fall. For bulb production the plant should not flower, since flowering adversely affects yields. Flowering depends on day length, and requires low temperatures (lower than 14-16°C) and low humidity.
Seed bed condition
A fine, consolidated seed zone is required for maximum germination and good establishment. Clods and stones will hinder growth. Optimum soil temperature for germination is 15 to 25°C.
pH
Soil pH range of 6-7 is optimal for onion growth. When pH drops below 5.5, magnesium and molybdenum availability decline and when the pH exceeds 6.5 zinc, manganese and iron become deficient.
Nutrition:
Nitrogen (N)
Dry bulb production, from transplanting, requires 140-170 kilograms of nitrogen per hectare. Nitrogen application rates depend on soil type, rainfall, irrigation, plant population, and method and timing of application. Usually, it is advisable to incorporate 25-30% of the recommended nitrogen rate prior to planting.
Phosphorus (P)
Phosphorus is essential for root development and therefore is required at the establishment stage. Phosphorus uptake rate is more or less constant throughout the growth cycle. Since phosphorus is immobile in the soil and can be translocated from the old leaves to the bulb during the bulb development and maturing stages, it is possible to apply most of the phosphorus before planting and up to the bulb development stage, to simplify the fertilization program.
Onions are highly dependent on arbuscular mycorrhizal fungi for uptake of phosphorus from soils with low to medium soil test P concentrations. Mycorrhizal fungi also improve the uptake of zinc and other micronutrients in some high-pH calcareous soils. Mycorrhizal fungi are usually abundant in agricultural soils, except when non-host crops (e.g. sugarbeet, canola) are grown, soil is fumigated, or high P is present.
Potassium (K)
Potassium is an important factor in plant water relations, cell-wall formation, and energy reactions in the plant. It is recommended to split K applications by incorporating 30-50 percent of the recommended K before planting and to split the remainder during the bulb development stage. Appling potassium during the bulb development stage increases the bulb size and quality. Low K level makes plants more susceptible to cold injury.
Irrigation:
The onion crop requires 350 to 550 mm of water throughout the growth cycle. It is recommended to give frequent, light irrigations which are timed when about 25 percent of available water in the top 30 cm of soil is depleted. Irrigation intervals of 2-4 days are a common practice. Excessive irrigation sometimes gives rise to the occurrence of diseases such as mildew and white rot. Irrigation should be ceased 15 to 25 days before harvest. Late season irrigation might delay maturity and cause skin cracking.