Onion (Allium Cepa L.) Growth, Yield And Storability As Influenced By Rate And Time Of Nitrogen Application

Abstract

Onion (Allium cepa L.) is an important commercial vegetable crop grown by small-holder farmers in Kenya, for both local and export markets. National average production is low and quality highly compromised due to use of low yielding varieties, low soil fertility and poor agronomic practices. Heavy loses estimated to be over 40% are also incurred in storage, mainly due to sprouting and rotting, further reducing the consumable yield. Nitrogen (N) is an important nutrient affecting onion growth and quality but with an impact on the shelf-life of the crop. This studywas therefore conducted to investigate the effect of nitrogen fertilizer and time of applicationon onion bulb growth, yield, quality and storage. Two field experiments were conducted in 2014 and 2015 with five N rates (0, 26, 52, 78 and 104 kg N/ha), applied as Calcium Ammonium Nitrate at 3, 6, 9 and 12 weeks after transplanting. Two commonly grown varieties of onions, Red Creole and Red Tropicana F1 hybrid were used. The experiments were laid in a randomized complete block design (RCBD), with a split-split plot arrangement and replicated three times. The varieties were the main plots, the fertilizer rates the sub plots and the time of application the sub-sub plots. The experiments were conducted under natural rainfall conditions supplemented with drip irrigation. All other agronomic practices regarding weeding and crop protection were applied as recommended for farmers. Onion bulbs were stored at room temperature conditions for a period of three months. Agronomic data collection commenced three weeks after transplanting while storage data started four weeks after storage. Field parameters included plant height, leaf number, bulb ratios, % bolters, % fallen plants, total yield, marketable yield, bulb weight, bulb diameter, bulb size, bulb neck size and split bulbs. Parameters taken during storage were physiological weight loss (PWL), sprouted bulbs, number and length of sprouts, rotted bulbs and severity of rotting (%). Data were xvi subjected to analysis of variance (ANOVA)and means obtained separated using Fishers protected LSD at 5% probability level. Interaction between N rates and time of application affected plant height in both seasons while number of leaves, bulbing ratios and % fallen tops increased significantly with increasing N rates. Late application of N decreased plant height, reduced the number of leaves and bulbing ratios but stimulated growth late in the season hence delaying crop maturity. Application of 104 kg N/ha increased total yields over the control by 59% in season one and 84% in season two. Marketable yield, average bulb weight and bulb diameteralso increased significantly with increasing N rates in both seasons. Nitrogen fertilizer decreased Size A bulbs (<40 mm in diameter) while increasing the most marketable bulbs in Size B (40 – 80 mm in diameter) and C (>80 mm in diameter). However, late application of N at 12 weeks resulted in low bulb weight and narrow bulb diameters increasing size A bulbs and reducing total yield by 23% in season one and 27% in season two. Marketable yield reduced by 25% in both seasons. The yields increased linearly up to the highest level of N applied hence optimal production was not reached. Application of N significantly (P<0.001) increased thick necked bulbs by 18% over the unfertilized check in season one and 32% in season two. Splitted bulbs increased with increasing N rates recording a 49% in season one. Late application of N at 12 weeks reduced necksizes but increased splitted bulbs by 42% in season one and 87% in season two. Bolted bulbs were not influenced by N or its time of application. Nitrogen increased significantly PWL, the number of rotted bulbs and length of sprouts in both seasons. High early application (3 weeks) increased rotting and physiological loss in weight due to larger bulbs and neck sizes while high late application accelerated sprouting. xvii The Red Tropicana F1 hybrid performed better than the Red Creole variety, recording significant differences in growth and yield parameters. However, the Red Creole variety had a longer shelf life compared to the Red Tropicana F1 hybrid. In conclusion, N fertilizer application improved growth and yield of onion but high rates from 78 kg N/ha had adverse effect on quality and storability of bulbs. Level 104 kg N/ha recorded the best growth and maximum yield for both varieties. Topdressing at 6 weeks after transplanting gave the best yields and quality. The yields increased linearly with N levels, warrantingtrials with higher levels to obtain an optimum and economic yield level. Application of 52 kg N/ha was best for bulbs intended for storage due to reduced rotting and sprouting. The Red Creole variety was a better option for storage. Key words: Plant height, leaf number, crop maturity, marketable sizes, bulb ratios and diameters, bolted bulbs,PWL, accerated sprouting, increased splitting, increased rotting