EFFECTS OF DRAIN DEPTH OF VERTISOLS , NITROGEN SOURCE AND TIME OF APPLICATION ON NUTRIENT UPTAKE BY MAIZE ( Zea mays L . ) IN WESTERN ETHIOPIA

Nitrogen is the most limiting plant nutrient in Vertisols in western Ethiopia. Vertisols properties and management factors as well as fertilizer source and time of application could influence nutrient uptake by crops. With this view, a field study was conducted at Ambo Agriculture Research Centre experimental site during the main cropping season of 20132014, with the aim to determine interactive effects of drain depth of Vertisols, N source and time of N application on nutrient uptake by maize. Treatments comprised five drain depths (0, 15, 30, 45 and 60 cm), two N sources (urea and ammonium sulfate) and two timings of N applications (twice and thrice). The experiment was laid out in Randomized Complete Block Design (RCBD) with three replications. Result showed that there was significant interactions effect of drain depths, N source and time of application on the concentration of nutrients in the grain and stover. The NH4-N uptake by maize was found to be significantly better than NO3-N utilization by maize. Grain and stover uptake of N, P and K by maize increased with drain depth and thrice split application of ammonium sulfate. It is concluded that draining off excess soil water from the rhizosphere is the key factor in improving nutrient uptake by maize in Vertisols in Ambo area.


Introduction
Ethiopia is ranked 3 rd in terms of acreage of deep, black, cracking, clay soils (Vertisols) in Africa, after Sudan and Chad, estimated at 12.6 million ha with 7.6 million ha in the highlands (1500 meter above sea level) (Berhanu, 1985).The ecology of these plateaus is characterized by high annual rainfall, in excess of 900 mm, and moderate temperatures, which leads to relatively low evaporation, particularly during the growing period.Moderate to severe waterlogging and serious water-borne soil erosion are common features in these Vertisol areas.About 2.21 million ha of the highland Vertisols are cropped and some 6 million ha are left under native pasture because of severe drainage problems in the main rainy season (Berhanu, 1985;Jutzi et al., 1987).Bull (1988) estimated that Ethiopia Vertisols can produce about 12 million tons of food grain if improved management practices are widely adopted.Despite the big potential of the Vertisols, except, the 25%, which is cultivated, the rest are mostly in bottom lands, get flooded, remain uncultivated and are mainly used for dry season grazing.The productivity of Vertisols can be increased by surface drainage.According to Kanwar et al. (1982), the key to improved Vertisols utilization for human food production is effective surface drainage.Removal of excess water during the wet season is one of the most crucial management practices for Vertisols, which differentiates them from most other soils.
In Ethiopian Vertisols, N is the most universally deficient plant nutrient element (Engdawork, 2002;Mohammed, 2003).The crop response to N applications in Vertisols is closely linked to soil moisture variations and, hence to rainfall pattern (IBSRAM, 1989;ICRISAT, 1989).According to Deckers et al. (2001), total N is universally deficient in tropical Vertisols, usually varied from 0.02 to 0.08% and rarely exceeds 1%.Likewise, Berhanu (1985) also reported the total N contents of Vertisols of the central highlands and eastern lowlands of Ethiopia varied from 0.08 to 0.22%.The low N content caused by denitrification losses resulting from poor drainage, leaching losses and less OM input.In this respect, management measure that can alleviate waterlogging conditions could be a possible means of reducing denitrification losses.Int.J. Agril.Res.Innov.& Tech.7 (1): 1-7, June, 2017 The use of drainage furrows for excess surface water drainage had been reported to improve maize yields and higher maize yields were also observed when the drainage furrows were combined with fertilizer N (Ikitoo et al., 2003).Ahmed (1988), reporting on Vertisols management under humid tropical conditions, and Muchena and Ikitoo (1992), working in Kenya, observed increases in yields of field crops grown in Vertisols because of draining excess water.Draining excess water with furrows to a depth of 40 cm was found essential for successful maize production in Vertisols under sub-humid conditions (Sigunga et al., 2002).Thus, management technologies for crop production in Vertisols should be geared towards the control of water dynamics and improving soil fertility (Probert et al., 1987;Ahmed, 1988).
In Ambo, drainage is a serious constraint to maize production in Vertisols during rainy seasons.Although many works have been done on Vertisols management, no attention has been given on drainage conditions of Vertisols in relation to nutrient uptake by maize.Therefore, the objectives of this study were to determine the effect of drain depth and N source and time of application on nutrient uptake by rain-fed maize grown on Vertisols in Ambo.

Description of the study area
The experiment was conducted at Ambo Agriculture Research Centre experimental site during the main cropping season in 2013.Ambo Agriculture Research Centre is located at 8°55' north and 38°07' east at an altitude of 2225 meter above sea level.Climatic data of the experimental site as obtained from nearby weather stations is given in Table 1.The soil of the study site was classified as Vertisols (Morton, 1977) with low soil N status (Engdawork, 2002).The site is characterized by a slope of less than 2%.

Field experiments
A field experiment was conducted on waterlogged Vertisols to test the effect of drain depth on nutrient uptake by maize during the main cropping seasons.The experiment was laid out in a Randomised Complete Block Design (RCBD) with three replications.The treatments comprised of five drain depths (0, 15, 30, 45 and 60 cm), two N sources and time of applications.The two N sources (urea and ammonium sulphate) were combined in a complete factorial arrangement.Nitrogen from urea and ammonium sulphate (AS) was applied at the recommended rate of 92 kg N ha -1 for maize, and all treatments received the recommended phosphorus rate of 20 kg P ha -1 in the form of triple super phosphate (TSP).Urea and AS fertilizers were applied in split, viz.half at sowing and half at 35 days after sowing when maize was at knee-height stage, and 1/3 at sowing, 1/3 at 35 days after sowing when maize is at knee-height stage and 1/3 at flowering stage of maize crop, respectively.All TSP was applied in a band at sowing.Prior to treatment application, soil samples from the site were collected for physicochemical analysis.

Cultural practices and planting materials
The cultivation of land in the study area starts in the months March and April during the short rainy seasons when workability of Vertisols is relatively good.Land cultivation is almost exclusively done using oxen-drawn implements.For maize, two or three passes are considered sufficient.Open drains were formed manually using hoes and spade, measured and adjusted to the required drain depths.Highland maize hybrid Jibat (AMH851) released for highland agro-Int.J. Agril.Res.Innov.& Tech.7 (1): 1-7, June, 2017 ecologies, widely grown the study area.The maize hybrid was planted in 6 rows of 5.0 m long with inter-and intra-row spacing of 0.75 and 0.25 m, respectively.Plots within a block were separated by 1 m space and blocks separated by a 2 m path.One plant hill -1 was maintained after thinning, giving plant population of 53,333 plants ha -1 .The recommended weed control practice for maize i.e., twice hand weeding at 30 and 55 days after sowing followed by slashing at milk stage was adopted.

Plant sampling and analysis
Samples for plant tissue analysis were taken from each of the plots, dissected into grains and stover.Dry matter (DM) was determined by drying subsamples of grains and stover in the oven at 70 0 C for two to three days to attain constant weight.The sub-samples were ground and analyzed for N, P, and K contents.Total nitrogen concentration was determined by the micro-Kjeldahl method (Bremner, 1965).For the determination of the remaining elements, plant samples were first subjected to wet digestion (Mehlich, 1984).P content was determined colorimetrically using a spectrophotometer.The procedure involved the use of Vanado-molybdate yellow method (Murphy and Riley, 1962).A flame photometer was used for the determination of K (Heald, 1965).
The total N, P and K uptake in stover and grains were calculated by multiplying the N, P and K contents by the respective stover and grain yields ha -1 .Total N, P and K uptake, by the whole plant were determined by summation of the respective grain and stover N, P and K uptakes on hectare basis.

Soil sampling and analysis
A total of five representative composite soil samples were randomly collected from each block from 0-30 cm soil profile before drainage depth was made.An auger was used to sample five randomly selected spots per each soil depth.The soil of these sub-samples was mixed thoroughly, dried at room temperature, ground and sieved through a 2 mm screen for physico-chemical analysis; whereas for OC and total N determination soil samples were passed through 0.5 mm sieve.
Laboratory analysis was carried out to characterize the soil physical and chemical properties that are considered important for crop management.Soil particle size was analyzed by the Bouyoucos hydrometer method following the procedure described by Day (1965).After harvesting of maize crop, soil samples were collected from all the plots from 0-15, 15-30, 30-45 and 45-60 cm depth and analyzed for the aforementioned chemical properties.
Soil pH was measured potentiometriclly using a pH meter with combined glass electrode at soil: water ratio of 1:2.5 as described by Carter (1993).Organic carbon (OC) content of the soil samples was determined using the wet oxidation method (Walkley and Black, 1934) where the carbon is oxidized under standard condition with potassium dichromate in sulfuric acid solution.Finally, the organic matter (OM) content of the soil was calculated by multiplying the percent OC by 1.724.The total N content of the soil samples was determined by the Kjeldahl method using micro-Kjeldahl distillation unit and Kjeldahl digestion stand (Jackson, 1958).Available P of soil samples were determined by the Olsen method using NaHCO3 as extracting solution (Olsen et al., 1954).Exchangeable K in soil samples was determined from the leachate of ammonium acetate (NH4OAc) solution at pH 7.0 by flame photometer (Rowell, 1994).

Statistical analysis
All the data collected were managed properly using the Excel computer software.The collected data was subjected to the analysis of variance using the SAS program version 8.2 (SAS Institute, 2001).Treatment means for each parameter were separated by Duncan's Multiple Range Test (DMRT) and Least Significant Difference (LSD) test at P = 0.05.

Results and Discussion
Some physical and chemical topsoil characteristics of these Vertisols before commencement of the trials are presented in Table 2.

Effect of drain depth
The analyses of variance showed that drain depths, nitrogen fertilizer source and time of application had significant effect (P < 0.05) on grain, stover and total biomass N, P and K uptake of maize.The interaction between drain depths and nitrogen source was also significant (P<0.05)influencing maize grain, stover and total biomass N, P and K uptake of maize, while the interaction between drainage and time of N application should a non-significant effect on N, P and K uptake of maize (Table 3, 4 and 5).
Table 3. Interaction effect of drain depth and nitrogen source on grain and stover N uptake of maize grown on Vertisols.
Drain depth (cm) N uptake (kg ha   The grain and stover, N, P, K, and total uptake parameters increased with furrow depths where the maximum uptakes as recorded at 60 cm deep drain depth and the minimum in undrained plots.Under both urea and AS fertilizer application, the N, P and K uptake of maize increased progressively with increased drainage depth.But, application of AS showed more pronounced N, P and K uptake than urea application (Table 3, 4  and 5).Urea is susceptible to enormous losses if drainage depths were not provided.These results are corroborated by Sigunga et al. (2002) who reported significantly higher N, P and K uptake in plants with 40 cm and 60 cm deep furrows with fertilizer AS suggests a more favorable root environment in the drained plots that lead to general crop vigor and higher nutrient demand by the crop.They reported also that drain depths of Vertisols created favorable conditions for growth and development of maize by removing excess water from the soil and making good aeration to the soil thereby considerably increased the N, P and K uptake of maize as compared to maize grown without drainage.

Effect of nitrogen source
Application of different source of fertilizer N increased nitrogen content and the uptake of other nutrients.The highest grain N (75.18 kg ha - 1 ) and stover N (62.45 kg ha -1 ) uptakes were obtained with AS fertilizer application at a drain depth of 60 cm and the least uptake recorded from the undrained plots with urea application (Table 3).The results clearly showed the positive effects of drainage depths on maize grain and stover yields and the improvement of grain and stover N uptakes with drain depths.Similarly, the highest grain P (17.48) and stover P (14.10) uptakes were observed with AS application at drain depth of 60 cm while the lowest grain and stover P were recorded from undrained plot with urea application.Furthermore, the grain and stover P uptakes exhibited positive responses to drain depths where the maximum plant P uptake was observed at drain depths of 45 and 60 cm.Maize grain assimilated much of P than stover.This result is in agreement with Skowronska and Filipek (2010) who reported that maize grain was the main accumulation pool for element P. The uptake of potassium by maize plant was also influence by drain depth.On the other hand, the highest grain K (65.89 kg ha -1 ) and stover K (70.32 kg ha -1 ) uptake was observed with fertilizer AS application at deep drain depth of 60 cm, followed by 45 cm drain depth.The total K uptake also increased with drain depth and the maximum was observed at deep drain depth of 60 cm.This result is in line with Sigunga et al. (2002) who reported that total K uptake was highest at deep drain depth of 60 cm coupled with fertilizer AS.

Effect of time of nitrogen application
Thrice split application of N source resulted in higher grain, stover and total biomass N, P and K uptakes than twice application (Fig. 1).The grain, stover and total biomass N uptake of maize rose from 51.40, 43.72 and 95.12 kg with twice split application of nitrogen fertilizers source to 61.23, 50.58 and 111.81 kg ha -1 with thrice split application, respectively.Similarly, the grain, stover and total biomass P uptake increased from 9.25, 7.10 and 16.35 kg with twice split application of nitrogen fertilizers to 13.96, 11.62 and 25.58 kg ha -1 with thrice split application, respectively.Similar result is reported by Sigunga et al. (1997); thrice split application of N sources enhanced uptakes of N and P by grain and total dry matter of maize grown on a Vertisols.In case of maize grain, stover and total biomass K uptake rose from 45.0, 53.95 and 98.95 kg ha -1 with twice split application of nitrogen fertilizers sources to 52.54, 62.30 and 114.84 kg ha -1 with thrice split application, respectively.These results are in agreement with Sigunga et al. (1997) who reported that thrice split application of N sources improved uptake of K.

Conclusion
Nutrient uptake by crops is influenced by interacting soil, climatic and management factors as well as fertilizer characteristics.The grain and stover N, P, and K uptake of maize were significantly improved by drain depth.Ammonium-N source was found to be significantly superior to the NO3-N source in terms of grain and stover uptake of N, P, and K of maize in situation where drain depth was provided.Nitrate-N source is susceptible to enormous losses if drains of at least 45 cm depth are not provided.It is concluded that provision of drains 45 and 60 cm deep with thrice split application of ammonium sulfate is essential for successful maize production in waterlogged Vertisols in Ambo area.

Fig 1 .
Fig 1. Effects of nitrogen time of application on grain, stover and total biomass N, P and K uptake of maize grown on Vertisols.Bars for each parameter with different letters are significantly different at 5% probability.

Table 1 .
Monthly rainfall (mm), minimum, maximum and avergae air temperature of Ambo Agriculture Research Centre for 2013 cropping season.

Table 2 .
Selected physico-chemical properties of Vertisols of the experimental site before the formation of drainage depth in 0-30 cm of the soil profile.

Table 4 .
Interaction effect of drain depth and nitrogen source on grain and stover P uptake of maize grown on Vertisols.
Means within a column followed by the same letter(s) are not significantly different at 5% probability

Table 5 .
Interaction effects of drain depth and nitrogen source on grain and stover K uptake of maize grown on Vertisols.