Fertility, Soil Crusting, & Cultivation
Onions require timely applications of nutrients to achieve maximum plant development and yield. Onion roots are mostly confined to the top 18 inches of soil, which can make supplying nutrients to the crop difficult. A soil test in the fall, while forming the seed bed, is the most accurate way to address fertilizer requirements. Soil test results, field experience, and knowledge of specific crop requirements can help to determine the nutrients needed and the rate of application. Select fertilizer type and rate to insure that all important nutrient levels are adequate for high productivity. Optimum fertilization is essential for top quality onions and yields.
Nitrogen is one of the most important nutrients for onion plant growth and development. A typical onion crop will use about 150-200 pounds of actual nitrogen per acre during the growing season, with a majority of the nitrogen taken up after the plant has started to bulb. Side-dress with nitrogen by applying low amounts to avoid burning the plants. It is critical to avoid late (after mid-July) and heavy applications of nitrogen after bulb initiation as it will encourage late maturity and large necks that are difficult to cure. Excess nitrogen in the bulb at harvest will result in soft onion bulbs and poor storage quality.
Most of the phosphorus and potassium should be applied and worked into the seedbed prior to planting. Phosphorus is essential for vigorous early growth of seedlings. If phosphorus is banded at planting time, it should be placed two inches to the side and two inches below the seed. Onions require medium levels of potassium and most soils in Utah contain sufficient levels for onion growth and development.
Springtime weather can bring heavy rain storms that can lead to crusting in seed beds with heavy-textured soil. To break the soil crust prior to onion emergence, run a harrow, spiked rollers, or finger-type cultivators lightly over the soil surface. Take extra care not to disturb the seed row during this process. If seeds/seedlings are disturbed prior to emergence, onion stands can be severely reduced.
Cultivation can begin as soon as onion seedlings emerge from the soil. Many types of equipment are used to cultivate; however, the standard set-up uses disks, knives, duck feet, and furrow openers. The disks are placed on either side of the onion rows to cut the crust. A knife is mounted behind each disk to undercut weeds on either side of the onion row and fill in the furrows made by the disks. A single duck foot might be centered in the furrow to undercut weeds, followed by the furrow opener which remakes the ditch for the next irrigation. Most onion fields need to be handhoed at least once to eliminate weeds that escaped the herbicide treatments and mechanical cultivation.
Onion field being cultivated.