In onion production in Kenya, you have to know the market requirements and manipulate the production practice to produce the desired quality of product. Planting density and fertilisation are critical in determining the size, weight and general quality of onion bulbs produced.
Onions have a relatively low nutrient uptake efficiency due to their shallow root system, characterised by thick roots with very few and short hairs.
Therefore, it is critical to implement a fertiliser programme adapted to those conditions to help to achieve optimum yields.
It is essential to time the applications of nutrients as close as possible to the rooting zone in frequently and readily available forms.
Major nutrient removal
Nutrient removal figures show that nitrogen and potassium are the minerals needed in greatest quantities in onion crops.
They are fundamental for achieving high marketable yields. However, it is important not to over-apply nitrogen, particularly in bulb onions, as this can delay maturity, soften bulbs and lead to storage rots.
In similar fashion to nitrogen, most phosphorus is required early on in the plant’s development to ensure good root growth and to boost establishment.
Potassium is particularly important where high N rates have been applied, to help maintain yield by increasing dry matter and sugar accumulation in the bulb (better bulb size and weight).
Fertiliser application for onion should, therefore, start by first week after transplanting, with an NPK regime for establishment of the crop. 40 per cent of the N and K as well as calcium requirements should then be applied by the third to fourth week during the first top-dress.
The second top-dressing should then be done at the bulbing stage, supplying the rest of the nutrient needs. Calcium and boron are critical in ensuring a longer shelf life and preventing rots in the bulbs.
Onions prefer a nitrate form of nitrogen as opposed to ammonium forms, and therefore more of nitrate source fertilisers such as in YaraMila Winner and Nitrabor will give better results – better bulb size and weight.
For crop specific fertiliser recommendations, please consult the following agronomists in your area:
Western and Nyanza, Daniel Mui, 0702466343.
Rift Valley, Dennis Nyandaya, 0702466372.
Central, Robert Ngatia, 0702466318.
Yara East Africa Regional Agronomist