Optimizing Nitrogen Application in Onion (Allium cepa L.): Influence of Rate and Time of Topdressing on Growth, Yield and Quality

Onion (Allium cepa L.) is an important commercial vegetable crop grown by small-holder farmers in
Kenya for both local and export markets. The national average production is low and quality is highly
compromised due to use of low yielding varieties and poor agronomic practices. Field experiments on
the influence of nitrogen and time of application on growth, yield, and quality of onion bulbs were
conducted in 2014 and 2015 at the National Agricultural Research Laboratories. The experiments
were laid out in a Randomized Complete Block Design (RCBD) with a split-split arrangement and
replicated three times. Nitrogen (N) was applied as Calcium Ammonium Nitrate at five levels
including, 0 (control), 26, 52, 78 and 104 kg N ha-1. These were applied at four different times of
applications at three, six, nine and twelve weeks after transplanting. Two onion varieties popularly
grown in Kenya were used, the Red Creole and Red Tropicana F1 hybrid. Nitrogen and time of
application showed significant differences in all parameters studied except bolting. Nitrogen at 104 kg
N ha-1 applied at 6 weeks gave the best results with regard to plant height, number of leaves, bulb
ratios, bulb diameter, average bulb weight, yield and marketable yield. Six weeks after transplanting
was the best application time with regard to most parameters and maturity of the crop. Yields
increased linearly with increased N rates but declined by over 23% with late application at 12 weeks.
High rates resulted to thick necks and increased split bulbs especially with late application at 9 and 12
weeks. Red Tropicana F1 hybrid was the best performing variety with regard to most parameters
especially total and marketable yield. Nitrogen applied at the right time improves growth, increases
yield and improves quality. Since the yield response was linear in both seasons, higher rates should
be evaluated to get the optimal rate. Time of application equally affected growth, crop maturation and
yield as well as yield components with late application negatively affecting these parameters. From
this observation it is apparent that sufficient N is required early in the season. When it is deficient in
the juvenile stage, rapid growth is restricted, resulting to loss of yield and poor quality bulbs. Thus it is
essential that an optimum level of N is supplied early for maximum yield and improved bulb quality.
The predicted optimum time of N application from this study was six weeks after transplanting.
Nitrogen at 104 kg/ha applied at 6 weeks after transplanting gave the best growth, yield and quality of
bulbs. Application of too much N late in the season (9 and 12 weeks) increased split bulbs and neck
sizes. Excessive application late in the season (as farmers do) should be discouraged and avoided in
the regime for best results. This shows that an optimal rate applied at the right time (4R’s of nutrient
management) optimizes the efficiency of fertilizer use for good yields and hence profitability. Although
hybrid seed was expensive, the yield obtained was high and quality was fairly good. The Red
Tropicana F1 hybrid obtained maximum yield of 30,533 kg/ha at 104 kg N/ha applied at 3 weeks while
the Red Creole obtained a maximum yield of 24,674 kg/ha with the same level applied at the same
time. To improve production and marketability, Kenyan farmers should adopt the hybrids.