Effect of different rate of nitrogen fertilizer on the growth and yield of cabbage (Brassica Oleraceae) at Debre Markos, North West Ethiopia

A research was conducted at Debre Markos, North West Ethiopia in 2015. The objective of the study was to evaluate different rate of nitrogen fertilizer on growth and yield of cabbage. The experment was laidout in randomized complete block design with four treatments and three replication. Nitrogen fertilizer have no significant effect on plant height and number of outer leaves, while leaf length, leaf width, head diameter, head fresh weight and head dry weight had significant effect on cabbage. The widest leaf width (21.86 cm) was recorded from 150 kg/ ha and followed by 100 kg/ ha, while the narrowest leaf width (16.93 cm) was from 0 kg/ ha N. The highest leaf length (20.1 cm) obtained was from 150 kg/ ha and the lowest leaf length (16.166 cm) was obtained from 0 kg/ ha. The highest head diameter (11.043 cm) was obtained from 150 kg/ ha, while the lowest head diameter was 8.696 cm noticed from 0 kg/ha. The highest fresh weight (0.771 kg/plant) was recorded from 150 kg/ ha and the lowest (0.442 kg/plant) was from 0 kg/ ha. The highest dry weight (0.114 kg/plant) was recorded from 150 kg/ ha while the lowest (0.0437 kg/plant) dry weight was obtained from 0 kg/ha of Nitrogen.


INTRODUCTION
Cabbage (Brassica oleracea L. var.capitata) belongs to the family cruciferae and it is a biennial crop with a very short stem supporting a mass of overlapping leaves from a compact head.It originated from wild non-headed type 'colewart' (crambecordifolias) from Western Europe and northern shore of Mediterranean (Semuli, 2005).It has been domesticated and used for human consumption since the earliest antiquity.It is a cool season crop that is popular with gardeners and commercial producers.
Cabbage is known for its nutritional importance and it E-mail: yebirzaf80@yahoo.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License is rich in mineral and vitamins like A, B1, B2 and C. It is also known for its cooling effect.Being an appetizer, it aides digestion thereby helps to prevent constipation.It also protects against cancers.Cabbage can grow easily under wide range of environmental condition in both temperate and tropical, but cool moist climate is most suitable (Kibar et al., 2014).Optimum growth occurs at a mean daily temperature of about 17°C with daily mean maximum of 24°C and minimum of 10°C.Mean relative humidity should be in the range of 60 and 90% (FAO, 2012).
Cabbage is grown for its head in more than ninety countries throughout the world (Meena et al., 2010).The major cabbage growing countries of the world are China, India, South Korea, Germany, Japan and South Africa (Sarker, 2002).Cabbage ranks fifth among the vegetable crops of the world.The area planted with headed cabbage worldwide in 2011 was estimated at about 68.584 million hectare (FAO, 2008).In 2008, area planted by cabbage was about 2.5 million hectare in Asia, 0.5 million hectares in Europe, 80,000 hectare in America and 120,000 hectare in Africa.In Africa, a total of 2 million tons were produced in 2008 and it has shown an increase by 20% over the 10-year period between 1998 and 2008.The five cabbage producers in African countries are Kenya, Egypt, Ethiopia, Niger and South Africa and these five countries have maintained the dominance of the sector throughout this period.Ethiopia accounted for 12% of the total production in Africa (Nicolas et al., 2012).Area, production and yield of head cabbage in Ethiopia 2007/2008 were 1989 hectares, 11,765 tons and 5.9 t/ha, respectively.In 2008/2009 it grew to 3,399 hectares, 24,133.4tons and 7 t/ha, respectively.Within these two years, the area has increased by 70% whereas the production has increased by 105%.
Ethiopia production of cabbage in 2005 is about 15,208 tones in an area of 2120 hectare with 7.2 tone/hectare (CSA, 2005).An overview of export data for 2013 showed that Ethiopia exported about 7.00 mt of cabbage, cucumber and eggplant to Djibouti and Somalia.Amhara region has a strong potential for production of cabbage.The region produced over 20.00 mt in 2013.The world average yield is 10-40 t/ha (Ogbodo, 2009).Production of cabbage could be enhanced through efficient use of applied nitrogen through adoption of good management of strategies capable of promoting effective and efficient use of applied water as insured by drip irrigation technology (Asare et al., 2010).
Different cultural practices and growing environments are known to influence growth and yield of the cabbage.So far, research in the country was mainly focused on the identification of superior cultivars of onions and adopting improved management practices.Mineral nutrition is a main factor that affects growth and yield of cabbage.Nitrogen Yeshiwas 277 is often referred to as the primary macronutrients because of the probability of plants being deficient in this nutrient and the large quantities taken up by plants from the soil relative to other essential nutrients.Nitrogen comprises 7% of total dry matter of plants and is a constituent of many fundamental cell components (Brady and Weil, 2002).It is one of the most complexes in behavior, occurring in soil, air and water in organic and inorganic forms.For this reason, it poses the most difficult problem in making fertilizer recommendations.
Plant demand for nitrogen can be satisfied from a combination of soil and fertilizer to ensure optimum growth and yield of cabbage.
The major essential plant nutrient nitrogen was found increasing in short supply in the soils of Eastern, Western and Southern Africa (Rao et al., 1998).Nitrogen is required in much greater quantities than most other nutrients.It is an important component of proteins, enzymes and vitamins in plants, and is a central part of the essential photosynthetic molecules; chlorophyll (Marschner, 1995).Plant demand for nitrogen can be satisfied from a combination of soil and fertilizer nitrogen to ensure optimum growth.
Thus, it requires applying of appropriate rate of fertilizer for the enhanced cabbage productivity and sustainable yield.Many experiments show that nitrogen application increases the total yield of cabbage (Patrick et al., 2012).But this is possible as long as it is managed properly in terms of rate and time of application.
Nitrogen levels have to be regulated to obtain high yield from cabbage.Thus, knowledge on this factor is helpful to optimize cabbage yield through efficient use of rate of fertilizer.In Western highlands of Ethiopia, particularly around Debre Markos, there is a need by farmers to increase productivity of cash crops like cabbage to maximize their profit from the small plot of farm they have.However, farmers of this area who grow cabbage frequently give less attention to nitrogen fertilizer rate.Moreover, information on cabbage nitrogen application rate for optimum yield and other agronomic practices are limited; since most of the farmers in this area has small plot of land and the yield obtained from this is very low.Due to this, their income is less and they unable to improve their livelihood.So it is important to increase the yield of cabbage from this small plot of land to improve the income of farmers around this area.The main objective of this present study was to evaluate the effect of different concentrations of nitrogen and to determine its optimum level for cabbage yield and growth.

Description of experimental site
The experiment was conducted at Debre Markos University College of Agriculture and Natural Resources in 2015 cropping season using furrow irrigation.Debre Markos University is geographically located at 300 Km North West of Addis Ababa at about 10 0 18'10"north latitude and 37 0 44'53" East longitudes at an altitude of about 2450 meter above sea level (m.a.s.l).The minimum and maximum temperature were 10.6 and 22.30°C, respectively.The mean annual rain fall of the area is about 1100 mm (Planning and Economic Development of East Gojjam, 2004).

Experimental materials
Cabbage seed variety Copenhagen market was used; Nitrogen fertilizer was used as experimental material during the study.There were four levels of nitrogen rates (0 kg/ha, 50 kg/ha, 100 kg/ha and 150 kg/ha).Urea was used as a source of nitrogen fertilizer.

Experimental design
The experiment was conducted in a Randomized complete block design with three replications.Cabbage seeds were sown in the nursery.The seeds were sown at the first week of February 2015 in the seed bed.After sowing, the seeds were covered by straw mulch.Irrigation was conducted frequently until the seedlings fully emerged.The permanent bed was prepared and the layout was arranged appropriately.The beds were irrigated sufficiently before seedlings were transplanted to make the soil suitable for them.After these, the seedlings were transplanted to the bed when they were two to three pairs of leaves.In the bed, plants were transplanted 40 cm between rows and 30 cm between plants.The total number of plant in each plot was 24 plants with 4 row and 6 plants with each row.Seedlings were transplanted in the permanent field at April, 2015 and fertilizer (UREA) was applied two times per growing season.The first application was 10 days after transplanting and the second application was one month after the first application.

Plant height (cm)
This was measured by using ruler starting from ground level to the tip of the outer longest leaf of individual plant.The mean of five selected plants from a plot was recorded.

Number of leaves/ plant
Total numbers of fully developed outer leave from each sample plant was counted at time of harvesting.

Leaf length (cm)
This was measured by placing a ruler from leaf base to the tip of the leaf of an individual plant and then recorded.The average of the selected five plants per plot was recorded.

Leaf width (cm)
This refers to the maximum diameter of the longest leaf measured using ruler at the widest point of the leaf.

Head diameter (cm)
At harvest, randomly taken samples of cabbage heads from the center were taken and the head diameter (HD) was measured using caliper (model LEG ilex-250 mm, US patent) and was expressed in centimeter.

Fresh weight (kg/P)
This was recorded from eight plants per plot (two central rows) resulting total yields per net plot.The whole plant parts were measured using the beam balance (Model WA310 rev-B aeadam equipment made in China).

Dry weight (g)
A homogenate (100 g) was prepared for determination of percent dry weight from each plot of head samples and oven (DP 203A: P/N 2123LST (A24) China) dried at a temperature of 120°C for 48 h.Then the weight was measured using digital balance.

Data analysis
The parameters considered in this study were subjected to Analysis of Variance (ANOVA) by using SAS Computer Software version 9.2 (SAS Institute Inc., 2008).When ANOVA showed significant differences, mean separation was carried out using LSD (Least Significant difference) test at 5% level of significance.Interpretations were made according to Gomez and Gomez (1984).

Plant height (cm)
The level of nitrogen had revealed non -significant (P> 0.05) effect on mean plant height (Table 1).This is because of the fact that nitrogen is responsible for vegetative growth of plants and laterit increases the head diameter through increasing head diameter rather than plant length.
This experiment was in contrary with that of Easmine et al. ( 2009) which states that increased nitrogen from 0 to 250 kg/ha increases plant height from 47.72 to 36.16 cm, respectively.

Outer leaf number
Mean number of leaves per plant at physiological maturity was not significantly (P>0.05)affected by Nitrogen fertilizer (Table 1).This is due to the fact that at maturity, cabbage leaves were folded and number of unfolded leaves was decreased.This result is in agreement with the findings of Pankaj (2006).N=nitrogen and Ns = Non significant at 5% level.Means followed by the same letter(s) in the same column are not statistically significantly at 5% level of significancy.

Leaf length (cm)
Application of nitrogen fertilizer significantly (P<0.05)influenced leaf length of cabbage (Table 2).The longest leaf length (20.10 cm) was obtained from the plot that received 150 kg of nitrogen per hectare when compared to the control (0 kg nitrogen per hectare) which was 16.16 cm.The positive effect of nitrogen (N) on leaf length might be due to its key role in the synthesis of chlorophyll, enzymes and proteins.The result was in agreement with that of Hadfield (1995) who reported that adequate application of nitrogen promotes vigorous growth and dark green color of cabbage and also, nitrogen is important in formation of chlorophyll and is also a component of protein.Similarly, Souza et al. (2008) reported that application of 200 kg N ha -1 significantly enhanced the length of cabbage leaves.Singh and Chaure (1999) also indicated that application of N at 150 kg ha -1 gave the best result with regards to cabbage leaf length.

Leaf width
The level of nitrogen had reveald significant (p<0.05)effect on the leaf width of cabbage (Table 2).The widest leaf was 21.81 cm and was obtained from 150 kg N/ha.However, it was not significantly different from that of 100 kg N/ha (18.7) while the narrowest leaf was (16.93 cm) obtained from 0 kg of nitrogen per hactare.

Head diameter
Head diameter was significantlly (p<0.05)affected with the application of different rate of nitrogen fertilizer (Table 3).The highest head diameter (11.04 cm) was obtained at 150 kg N/ha while the lowest head diameter (8.69 cm) was obtined with application of 0 kg/ha of nitrogen.This finding is in agreement with those of Keteseeman (2006) who reported, head diameter increased from 98 to 218 mm when the nitrogen level increased from 0 to 120 kg/ha, respectively.This was possibly due to higher synthesis of carbohydrate and their translocation to the sink, that is; cabbage head which subsequently helped in the formation of larger and comparatively broader head of the cabbage.

Fresh head weight /plant
Application of nitrogen fertilizer at different rate also showed a very highly significant (P <0.01) effect on fresh head weight per plant (Table 3).Increasing nitrogen level from 0 to 150 kg ha -1 resulted in progressive increase in head weight of cabbage.Cabbage grown at 150 kg ha -1 of nitrogen rate had the highest head weight per plant (0.771 kg /plant ), however the result is simillar with 100 kg/ha while cabbage grown without nitrogen fertilizer had the lowest (0.442 kg/plant) fresh head weight.This is due to nitrogen that increases the vegetative growth and produces good quality foliage and promotes carbohydrate synthesis through photosynthesis and ultimately increased yield of plants (Mengel and Kirkby, 1987).

Dry weight
Regarding to the dry matter contents, nitrogen fertilizer highly significantly (P<0.01)influenced the mean head dry weight (Table 4).The increasing levels of nitrogen encouraged head with a significantly higher dry weigh as compared to the control plot.The maximum dry weight of cabbage head (0.114 kg per plant) were recorded with application of 150 kg ha -1 of N whereas, the minimum dry weight (0.043 kg per plant), were detected in the controls plot.There were no significant difference between plots that received 100 and 50 kg N/ha.

Conclusion
Nitrogen rate has a significant effect on leaf length of cabbage.The highest leaf length was recorded at plot receiving 150 kg /ha which was 20.1 cm while the lowest leaf length was recorded at plot recieving 0kg/ha of nitrogen which was 16.166 cm due to the role of nitrogen on photosyntsis and protein formation.Similarly, leaf width of cabbage had siginficant difference between different level of nitrogen.The largest leaf width was obtained from 21.86 cm while the lowest was 16.93 cm)from 150 and 0 kg/ha, respectively.
Nitrogen rate had significant effect on yield of cabbage.The highest head diameter (11.04 cm) was recorded at 150 kg/ha, while the lowest head diameter (8.69 cm) with 0kg/ha.Fresh head weight (0.771 kg/plant) was registerd at 150 kg/ha.On the other hand, the lowest fresh head wieght (0.442 kg per plant) 0kg N/ha.Similar result was obtained for dry head weight, that is; (0.114 kg/plant) with 150 kg N/ha and 0.043kg with 0 kg N/ha.In general, nitrogen level had significant effect on cabbage leaf length, leaf width, head diameter, head fresh weight and dry weight.Thus, we suggest to the growers and stakeholders in the study area to use 150 kg of N per hectare because of the yield and its components and definitely will become profitable.

Table 1 .
Effect of nitrogen fertilizer on plant height and leaf number of cabbage.

Table 2 .
Effect of nitrogen fertilizer on leaf length and leaf width of cabbage.

Table 3 .
Effect of nitrogen fertilizer on Head diameter and fresh head weight /plant of cabbage.

Table 4 .
Effect of nitrogen fertilizer on head dry weight of cabbage.