Soils and Climate
Onions can be grown successfully on any fertile, well-drained, non-crusting soil. The optimum pH range, regardless of soil type, is 6.0 to 6.8, although alkaline soils are also suitable. Onions do not thrive in soils below pH 6.0 because of trace element deficiencies, or occasionally, aluminum or manganese toxicity. Onion is cool-season biennial, and is tolerant of frost. Optimum temperatures for plant development are between 13 and 24°C, although the range for seedling growth is narrow, 2O-25°C.
High temperatures favor bulbing and curing. Prior to planting, soils should be plowed and disked sufficiently to eliminate debris and soil clods. In most commercial areas, beds 0.9 to 1.0 m wide are formed, and two to six rows are seeded or planted on the bed. If two rows, they may be two-line (twin) rows with plants staggered to achieve proper spacing and high population density.
Three systems of planting may be employed:
- Direct seedling is preferred and gives excellent results where the season is sufficiently long to provide early prebulbing growth.
- Transplants normally have three to five well-formed leaves at transplant time. Transplant leaves are pruned during growth prior to field setting, facilitating handling and increasing plant hardiness.
- Sets are used in some areas to ensure large bulb size and uniform maturity. Sets are small dry bulbs, approximately 12 mm in diameter, produced the previous season by seeding thickly or growing under conditions favoring rapid bulbing. Any of the above systems may be used for early green onion production.
The ultimate yield of onion is determined by the number of leaves that are formed prior to bulbing. Since bulbing in each cultivar is triggered by a specific daylength, early planting is the most effective method of improving bulb size and is a primary factor contributing toward yield. If, however, early planting coincides with cool air temperature or cool wet soils, the stand and ultimately the maturity of the crop will be erratic.
Some cultivars of the Bermuda type also may bolt if substantial growth precedes exposure to cool temperature. Seeds are sown 6 to 18 mm deep in heavy mineral soils, deeper in light mineral soils and mucks. Excessively thick seedlings of bulb onions may delay maturity, however necks tend to be thinner than in sparse seedlings, and bulbs are somewhat more globular in shape. Using coated seed (shown) and precision seeding, the seeding rate can be adjusted easily for projected bulb size. For normal storage onions, seeds are spaced 7.5 cm apart. When small boiling, pickling or pearl onions are desired, spacing would be reduced to 2.5 cm in the row. Large bulb size is promoted by spacings of 10 cm or more.
Onion responds very well to organic manure. Organic manure at 25 to 40 ton/ha is recommended to obtain high bulb yield. Fertilizer is applied either as a broadcast, or more commonly, as a band 5 to 10 cm directly below the seed set or transplant. Onion plants utilize substantial amounts of nutrients. Based on a yield of 18 ton/ha of bulbs, the plants remove an average of 66, 11, and 70 kg of N (Nitrogen), P (Phosphorous), and K (Potassium) respectively. Soils differ widely in fertilizer needs, depending on production history, soil type, and analysis.Mineral soils on average contain 90 to 112 kg/ha of N and 56 to 168 kg/ha of P2O5 (Phosphorous Pentoxide) and K. N, P, and K application of about 160, 90, and 40 kg/ ha, respectively, is recommended for mineral soils.
One or two side dressings of nitrogen are applied during a season. These side dressings may be applied through the irrigation system. Fall seeded onions require only P2O5 before seeding and require N when active growth starts in the spring and twice thereafter. Insufficient N will induce early maturity and reduce bulb size; high N may increase bulb size and cause large nicks and soft bulbs with poor storage quality.If heavy fertilization rates are indicated by soil tests, the material should be incorporated throughout the plow layer, or if banded, placed 15 cm to the side of the row.
Minor element deficiencies, particularly zinc and copper, may be encountered. Suggested corrective rates are 11 kg/ha of zinc or 17 to 28 kg/ha of copper, applied every two to three years. Relatively high levels of sulfur are utilized by onions, but corrective applications vary widely, according to soils, leaching losses, and presence of sulfur contaminants in the atmosphere. If applied, sulfur will acidify the soil, and therefore, liming rates should be adjusted accordingly.Similar fertilizer ratios are recommended for green onions; however, due to short growing period, the application rates would be reduced.
Onions require uniform moisture throughout the growing season. Fields that suffer growth retardation may produce excessive numbers of doubles or splits, reducing the number of Grade i bulbs. Furrow irrigation is generally used. Light sandy soils are irrigated with overhead systems or by subsurface seep irrigation where the soil profile allows. Onions at the bulbing stage utilizing substantial amounts of water, although excessive moisture must be avoided during the growing season.
Onions are not good competitors with weeds. Cultivation, if used, must be shallow to avoid root damage, and growers usually favor chemical control. Pre-emergent broadcast applications of DCPA or one of several organic compounds have been used with some success.
Both field and storage diseases reduce profitability. Field diseases include purple blotch, Stemphylium blight, anthracnose, downy mildew, Botrytis leaf blight, pink root, smut, smudge, and several basal rots. Storage diseases include some of the common field rots, black mold, botrytis neck rot, and bacterial soft rot.