Fertilization Management

During the first 4 weeks after transplanting, the dry matter accumulation, and therefore nutrient requirement is relatively low. Afterwards, there is a rapid vegetative growth, and subsequent bulb initiation and formation with a strong nutrient demand.

In addition to moment of nutrient demand, it has to be considered that onions in general have low nutrient uptake efficiency, especially for phosphorus compared to most crops. This is due of their shallow root system characterized by thick roots with very few and short root hairs. As a consequence, onion’s roots have an overall low surface area for nutrient absorption. Therefore, the strategies of fertilization should focus on delivering the nutrients as close as possible to the rooting zone in a frequent, and in readily available forms.

Nutrient application strategies can be carried out as follow:

Dry applied fertilizers, a fraction incorporated prior planting, and the rest side-dressed according to crop growth and demand. In dry applied fertilizer programs, higher nutrient rates are usually needed to cover inefficiencies due to low nutrient uptake efficiency (related to root morphology), leaching losses, and nutrient reactions (fixation) in the soil.

Fertigation, by providing nutrients though the irrigation system, delivers the fertilizer direct to the root zone and by this improving nutrient uptake efficiency by roots. Localised irrigation provides a limited zone of moisture where roots can thrive. This practice also ensures better application uniformity for every plant, and allows the right nutrients to be applied at the right time, and at the right amount according to plant needs.

Foliar application is used to address an immediate nutritional need or where soils conditions restrict availability of specific nutrients. Micronutrients are most commonly applied in this way.