Onions, Leeks and Shallots - Fertility

Nitrogen

If manure is applied or legume sod is plowed down, reduce the nitrogen (N) application. See Table 1-8, Adjustment of Nitrogen Requirement Where Sod Containing Legumes Is Plowed Down, and Table 1-7, Average Amounts of Available Nutrients for Different Types of Manure, on page 9.

Broadcast and incorporate the recommended preplant nitrogen with the required potash and phosphate. On muck soils, apply the recommended side-dress nitrogen when plants are 8-10 cm (3-4 in.) tall (see Table 9-50, Onion Nitrogen Recommendations).

On shallow muck soil or soils with a high proportion of mineral soil, band 200 kg/ha (180 lb/ac) monoammonium phosphate (MAP), applied 5-7.5 cm (2-3 in.) below the seed. Reduce the broadcast fertilizer by 22 kg/ha (20 lb/ac) nitrogen and 96 kg/ha (86 lb/ac) phosphate.

Leeks require higher amounts of nitrogen than onions, normally around 200 kg N/ha (180 lb/ac), applied in three instalments: one preplant and two side-dressings.

Table 9-50. Onion Nitrogen Recommendations

Phosphorus

Test the soil to determine phosphorus requirements. Where the soil test values are 61 mg P per L soil (ppm) or higher, onions grown on both muck and mineral soils are unlikely to respond to additional phosphate applications. For soils testing lower than 61 mg P per L soil (ppm), see Table 1-10, Phosphorus Requirements: Vegetables on Mineral Soils, on page 12 , or Table 1-11, Phosphorus and Potassium Requirements: Vegetables on Muck Soils, on page 13 .

Potassium

Test the soil to determine potassium requirements. Where the soil test values are 181 mg K per L soil (ppm) or higher, onions grown on both muck and mineral soils are unlikely to respond to additional potash applications. For soils testing lower than 181 mg K per L soil (ppm), see Table 1-12, Potassium Requirements: Vegetables on Mineral Soils, on page 14 , or Table 1-11, Phosphorus and Potassium Requirements: Vegetables on Muck Soils, on page 13 .

Micronutrients

Crops vary greatly in their response to micronutrient fertilizers. For complete information, see the section Micronutrients, on page 16.

Copper

Copper deficiency occurs on organic (peat and muck) soils. Bulbs show poor-coloured skins. Copper may be mixed with the fertilizer and applied at 14-29 kg Cu/ha (12.5-26 lb/ac) on newly cultivated muck. Subsequent applications may be made every 2 or 3 years at about one-third the rate recommended for new muck.

Manganese

Manganese deficiency shows up as a yellowing between leaf veins. See the section Manganese, on page 18 , and Table 1-13, Application Rates for Magnesium, Calcium and Micronutrients, on page 16 .

Molybdenum

Molybdenum deficiency may occur when onions are grown on moderately acid to strongly acid muck soils. A seed treatment has proven beneficial. The treatment is accomplished by dissolving 15 gm of sodium molybdate in 45 mL of water. Spray this solution from an atomizer bottle on 2.3 kg seed (5 lb) spread thinly on a plastic sheet. Do not use excessive water, as this can cause the chemical to penetrate the seed embryo and cause injury. Mix seed thoroughly and let dry. For foliar application rates, see Table 1-13, Application Rates for Magnesium, Calcium and Micronutrients, on page 16.

Zinc

Zinc-deficient plants develop yellow-striped foliage. They become stunted, twisted and bent. Deficiencies occur mainly with shallow muck soils or where the calcareous subsoils are mixed with the muck. See the section Zinc, on page 19, and Table 1-13, Application Rates for Magnesium, Calcium and Micronutrients, on page 16.

Plant Analysis

When used in conjunction with a soil analysis, plant analysis is a useful tool for diagnosing crop problems or for evaluating a fertilizer program. The objective of a good fertilizer program is to maintain tissue nutrient concentrations on the lower side of the sufficiency range. Attempting to bring the nutrient analysis up to the higher end of the range could possibly result in over-fertilization and may not be economical. For more information see the section, Plant Analysis, on page 3.

Only use sufficiency ranges as a reference and a guideline. Plant analysis does not replace soil testing or a sound soil fertility program. See Table 9-51, Sweet Onion Nutrient Sufficiency Ranges.

Table 9-51. Sweet Onion Nutrient Sufficiency Ranges

Adapted from Maynard, D.N., and G.J. Hochmuth. 1997. Knott's Handbook for Vegetable Growers, 4^th Edition. John Wiley & Sons, Inc. New York.

Plant Part: Most recently mature leaf

ppm = parts per million

Related Links

• Publication 363 - Vegetable Production Recommendations

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