There is no golden rule or one-size-fits-all approach to fertilizing onions, as each field has unique needs and characteristics. All onion farmers must understand that each field is different, and a farmer always needs to perform soil analysis tests before applying any products. Key factors such as soil type, crop history, plant density, onion variety, and anticipated yield should guide the fertilization plan. Adjustments to fertilizer amounts should also be made depending on whether the crop is grown from seed, sets, or transplants (and when the cultivation started), as each goal requires different nutrient support at various growth stages.
Understanding the nutrients needed at each developmental stage is essential to prevent deficiencies. Soil and leaf analysis can help determine nutrient levels and ensure adequate fertilization.
Onion Requirements for Phosphorus (P), Potassium (K) and Nitrogen (N)
For phosphorus (P), soil with levels above 15 ppm is generally sufficient, while potassium (K) should be above 120 ppm. If these levels are met, additional P and K fertilization may not be necessary or can be significantly reduced. However, if soil P is below 9 ppm and K below 80 ppm, these are considered deficient, and corrective fertilization is required.
Regarding nitrogen (N), the nutrient levels in plant tissues should be above:
- 4% during the first growth stages,
- 3% in the bulb development stage and
- 2.5% during the post-bulbing (late season) stages
Furthermore, it is interesting to know that according to studies, for the production of 50 tons of dry bulbs per hectare (20.3 tons/acre) (average yield), farmers - through the harvested onions- remove from the soil: 180kg N, 25kg P, and 110kg K (or 400 lb N, 55 lb P, and 243 lb K).
Fertilization Guidelines for Onion Cultivation - Critical Fertilization Stages
Below, we will mention some general guidelines regarding onion plant nutrition for commercial cultivation.
Onion is considered a heavy feeder and, compared to other vegetables, needs more nutrients to produce high yields. To decrease the risk of nutrient losses (runoff, etc.) and to supply the plants with the amounts of nutrients in the most critical periods, it is best to split the total amount of nutrients into more than 1-2 doses. There is more than one fertilization program a farmer can apply depending on the type of fertilizers used and their combination (e.g., only base and side dressing, base dressing and fertigation + foliar nutrition, etc.).
For an expected yield of 60-65 tons of dry bulbs per hectare (5353-57992 lbs per acre), many growers apply:
- 200-240 kg of nitrogen (N) (178-214 lb/acre)
- 100-130 kg of phosphorus (P₂O₅) (89-116 lb/acre)
- 180-220 kg of potassium (K₂O) (160-196 lb/acre)
- 60-66 kg of calcium oxide (CaO) (54-59 lb/acre)
- 60 kg of magnesium oxide (MgO) (54 lb/acre)
In some cases, 40 kg per hectare (36 lb per acre) of sulfur (S) is also applied. Fertilizer amounts may need to increase if the crop is grown from seed rather than sets or transplants.
Knowing that the nutrient demand peaks during the bulb growth stage (after the 7th leaf has emerged) is essential. As a result, the farmer should have applied the necessary amount (and type) of nutrients needed soon before that stage. Excessive and delayed N fertilization should be avoided since they can delay bulb formation and lead to softer bulbs (quality reduction and storability). Whichever fertilization scheme you follow, you are advised to stop applying when the plant’s neck starts feeling soft (around 4-5 weeks before harvest).
Fertilizer Application Methods - Fertigation Program for Onions
Nowadays, commercial growers use both liquids and granular fertilizers for onion cultivation. Many farmers start by applying 30-70 tonnes per hectare (26765-62453 lbs/acre) of well-rotted manure about 1 month before planting. Alternatively, if synthetic fertilizers are used, we can add 30-50% of the total N and 50% of P and K before planting. The rest of N will be added in the bulb initiation stage. Until the emergence of the 5th leaf, the farmer can apply ⅓ of N, ¼ of P, and the total amount of Ca, if needed. The rest of the amount is added at the onset of the bulb enlargement stage to cover the increased needs of the onion plants.
If fertigation is used, the farmers are advised to divide the total amount of fertilizers needed into four doses: one during plant establishment, one during vegetative growth, one during bulb initiation, and one during the bulb development stage. This program can be combined with pre-planting base soil fertilization. Microelements such as Zn, B, and Cu may also be required.
However, these are just some examples of fertilization patterns that no one should follow without doing their own research. Every field is different and has different needs. Checking the soil condition and pH is vital before applying any fertilization method. You have to consult your local licensed agronomist.
References
- https://vric.ucdavis.edu/veg_info/onion_leaf_analysis.htm
- https://content.ces.ncsu.edu/bulb-onions
- https://www.jica.go.jp/project/english/kenya/015/materials/c8h0vm0000f7o8cj-att/materials_20.pdf
- https://www.cdfa.ca.gov/is/ffldrs/frep/FertilizationGuidelines/Onion.html
- https://dogr.icar.gov.in/index.php?option=com_content&view=article&id=116&Itemid=162&lang=en
- https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0017/126305/Onion-growing-Agfact-H8.1.22.pdf
- https://ohioline.osu.edu/factsheet/hyg-1616
- https://aggie-horticulture.tamu.edu/archives/parsons/publications/onions/ONIONGRO.html
- https://extension.uga.edu/publications/detail.html?number=B1198&title=onion-production-guide
Further information
- Interesting Facts about Onions
- Onion: Info, Nutritional Value, and Health Benefits
- Onion Plant Information and Variety Selection
- How to Grow Onions At Home: Onion Growing Guide for Beginners
- Commercial Onion Farming - How to Grow Onions for Profit
- Onion Soil Requirements, Soil Preparation, and Planting
- Which Factors Can Affect Bulbing in Onions?
- Onion Water Requirements and Irrigation Systems
- Onion Nutrient Needs and Fertilization Requirements
- Major Onion Pests and Diseases and Sustainable Management Practices
- Onion Weed Management: How to Control Weeds in an Onion Field
- Onion Harvest, Yields, and Storage