Optimizing Nitrogen Application in Onion (Allium cepa L.): Influence of Rate and Time of Topdressing on Growth, Yield and Quality | Chapter 10 | New Perspectives in International Plant and Soil Research Vol. 1
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.
Author(s) Details
M. W. Gateri
National Agricultural
Research Laboratories, Institute of Crops Research, Kenya Agricultural and
Livestock Research Organization, P.O.Box 14733-00800, Nairobi, Kenya.
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