Horticultural commercial farming is increasing in Kenya with farmers expanding their acreage to increase production. Bulb onion is the third most farmed horticultural crop in Kenya behind tomatoes and cabbages.
Profitability in onion farming depends on producing quality onions of a medium-small size grade and above.
Under good management, at least 75% of the yield should be of medium size grade and above while medium small and small take 15% and 10% respectively.
However, this is not the case in most farms. 60% of what is harvested in most farms is of below medium size grade. This is why most farms produce less than 5 tons per acre. A huge loss.
Causes of bulb size variation
Crop spacing
Crop spacing refers to inter and intra distance between two crops. Bulb onion size increase or decreases with spacing. Intra-spacing is directly proportional to bulb size. The wider the spacing the bigger the bulb size and vice versa.
A spacing of 7-8 cm by 15 cm is the most ideal.
Underfeeding
Like animals, crops need “food” (nutrients-Nitrogen, phosphorus and Potassium) for growth, tuber and fruit formation.
Soil test analysis determines the quantity of manure and fertilizer applied. Well, decomposed manure should be applied in furrows before planting for better results.
Topdressing with 150 kilograms of DAP, NPK 17.17.17 and CAN fertilizers on the 14th, 45th and 60th day after transplanting is recommended.
Applying fewer rates of these fertilizers and manure or late application results in undersized onions.
Water stress
Insufficient soil moisture for crops leads to a condition known as water stress. Water is the most essential component for crop growth and tuber development. Water stress inhibits root development and crops nutrients uptake.
Bulb onions require water most in the second and third months when bulb formation and expansion occur respectively.
However, most farmers do not irrigate sufficiently, especially in furrow irrigation systems. In farms with steep elevations, the rate of soil evaporation is high leading to water stress. Water stress is rampant in dry seasons when there is no sufficient water for irrigation.
Always ensure your bulb onions are well irrigated at all times for optimal bulb formation and expansion.
Variation in soil fertility
The majority of farms do not have uniform soil fertility. There are some sections of the farm with extremely low soil fertility. This lack of uniformity can only be corrected through soil test analysis which most farmers do not carry out.
The variation in soil fertility is the main cause of some patches of the farm producing small undersized onions despite the uniform application of fertilizers.
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Mixed-up onion varieties
This is a common problem for farmers who outsource onion seedlings in the late season. At this point, the demand for seedlings is often low leaving seedling raisers confused about where to take the excess.
Outsourcing at this point there is always a risk of getting mixed up seedlings. The mix-up is always between early maturity and late maturity seedlings. When planted together the late maturity seedling are often in a small size while early maturing seedlings are ready for harvesting.
Raising your seedlings is always the best option.
Finally, frequent soil test analysis and early scouting can help identify the growth variation early and introduce correction measures such as additional topdressing and watering in affected areas.
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