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Drip irrigation scheduling for optimizing productivity of water use and yield of dry season pepper (Capsicum annuum L) in an inland valley swamp in a humid zone of Nigeria  

S.O. Agele1 , I.A. Agbona1 , B.S. Ewulo1 , A. Y. Anifowose2
1. Department of Crop, Soil & Pest Management, Federal University of Technology, PMB 704, Akure, Nigeria
2. Department of Remote Sensing and Geoinformatics (RSG), Federal University of Technology, PMB 704, Akure, Nigeria
Author    Correspondence author
International Journal of Horticulture, 2014, Vol. 4, No. 14   doi: 10.5376/ijh.2014.04.0014
Received: 23 May, 2014    Accepted: 29 Jul., 2014    Published: 13 Oct., 2014
© 2014 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:

Agele et al., 2014, Drip irrigation scheduling for optimizing productivity of water use and yield of dry season pepper (Capsicum annuum L) in an inland valley swamp in a humid zone of Nigeria, International Journal of Horticulture, 2014, Vol.4, No.14 1-10 (doi: 10.5376/ijh.2014.04.0014)

Abstract

The effects of drip irrigation schedules (weekly and fortnight intervals) on water use, yield and water productivity of dry season pepper grown in inland valley swamp was investigated between December 2009 and May, 2010. The first planting (December, 2009) adequacy of soil moisture from planting to date of first flowering was assumed, thereafter irrigation was imposed during reproductive growth. In the second sowing (Janaury, 2010), pepper seedlings were drip-irrigated weekly and fortnightly from transplanting to fruit harvest. In both experiments, irrigation was imposed using low-head (gravity) drip system weekly and fortnightly and 1.38 litres of water per plant at each irrigation while soil moisture storage ranged from 100 to 50 % of plant available water. Higher root biomass and densities at soil depths were obtained for fortnight irrigation over weekly. Within the crop root zone, and across irrigations, soil moisture contents ranged between 14.7 and 11.8% for the respective surface (0 – 20cm) and lower (30-45 and 45-60 cm) soil depths. Soil moisture tension were - 7 to -10 bar and -10 to -14 bar for the respective seedling establishment and reproductive growth phases. Total fruit yield and water productivity were higher (8.8 and 1.85 kg/ha/mm) in December over January (8.5 t ha-1 and 1.25 kg/ha/mm) sowing. In addition, over weekly (9 t ha-1) irrigation, fruit yield obtained (8.1 t ha-1 ) under fortnight irrigation translated to 24 % water savings.

Keywords
Inland floodplain; Root; Crop water stress index; Moisture depletion

In sub-Saharan Africa, inland wetlands constitutes about 135million ha of land (IWMI, 2002), However, the soil, agriculture and water resources potentials of inland valley swamps had not been fully utilized. The underutilization may be due to inadequacy of technical know-how (technological packages) and strategies for efficient management of water for irrigation (IWMI, 2002; National Fadama Development Project, 2000). Dry season crop production (mostly vegetables) in inland valley swamps (Fadama schemes) characterized by shallow water tables, is a common feature of the farming system of the tropics (National Fadama Development Project, 2000).
Variable water table depths (0.3, 0.5, 0.8, 1.0 and 1.5m) are characteristics of inland valley swamps (fadama ecosystems) of the humid tropics (Ogwu and Babalola, 2002, IWMI, 2002). The variable water table depths would imply differences in the contributions of water tables via capillary rise (upflows) to crop water requirements (evapotranspiration) at different growth stages of the crops. In general, crops grown in the dry season in the humid tropics encounter variable depths of water table. In circumstances of declining water table depths, these crops will encounter increasing intensities of soil moisture deficit in situations where the upper half of the soil is dry and soil water in the lower horizons becomes unavailable to crops. The crops are therefore exposed to temperatures and soil and air moisture deficit stress at different phenological growth stages.
Capillary rise (upflows) can constitute significant component in the root zone water balance of crops grown in inland valley swamps especially those characterized by shallow water table depths (Benz., 1984, IWMI, 2002). Despite the realization that fresh shallow water tables can help to meet crop water requirement, knowledge on how best to incorporate upflow from shallow water tables in irrigation scheduling is limited (Hurst., 2004; Ayars., 2006). Reduced irrigation above water tables does not only results in more efficient use of water resources, it also lowers the risk of water logging and nutrient losses below the root zone (Pitts., 1993). Successful exploitation of ground water resources by a crop depends on several factors that include water table depth, soil water retention and transmission properties, evapotranspiration demand, and plant root system (Chabot., 2002).
Unlike deep water table conditions, a shallow groundwater table maintains elevated soil moistures in the root zone (Chen and Hu, 2004). However, in the presence of shallow groundwater, a crop will use water from both stored soil water and the groundwater provided the groundwater quality does not preclude plant use. For a shallow unconfined aquifer, water can move upward from the water table to relatively drier soil surface layers through capillary rise driven by soil matric potential gradients. Ground water table contribution to root zone moisture and zone of maximum uptake is related to groundwater table depth, soil unsaturated hydraulic conductivity, atmospheric demand, the rooting depth, zone of active root combined with water availability, the soil hydraulic head (Chabot., 2002). Other factors are rainfall, irrigation, root water uptake, soil evaporation, water table depth, soil water retention and transmission properties, evapotranspiration demand (Chabot., 2002; Yeh and Eltahir, 2005; Fan., 2007).
Where water resources from ground water tables are potential contributors to crop water requirements, irrigation can be reduced with no detrimental effect on crop yield (Patel and Joshi, 1986; Ayars., 2006). It is necessary to quantify the effect of reduced irrigation in presence of shallow water tables on water and nutrient use efficiency and water savings without reductions in yields (Hurst., 2004). If upflow can contribute up to 30% of crop water use (evapotranspiration) in well watered crops such as soybean and wheat, in irrigation scheduling, water additions which ignore the potential contribution from upflow from water tables will exacerbate the problem of excessive leaching and thus recharge. In the tropics, there is scanty information on the irrigation requirements of crops grown on inland valley swamps characterized by shallow and variable water table depths.
Irrigation scheduling using concepts of soil water availability, such as readily available or plant available water is unlikely to be useful for systems with shallow water tables (Hurst., 2004). In addition, the concept of replenishing a soil water reservoir as it becomes depleted might also not be able to be applied to root zones where upflow is a significant component in the water balance and depletion of stored soil water is not evident. For example, for a soil under the influence of water table at 1.5m, the upper half of the root zone is extremely dry and water is unlikely to be available to plants. While the lower half of the root zone is close to saturation, the low root densities within a dry upper half of the root zone may prevent extraction of water enough to meet potential transpiration rates (Hurst et al., 2004). Talsma (1963) and Hurst. (2004) opined that near saturated hydraulic conductivity is a better indicator of potential upflow rates, with distances between the root zone and water table that maintain potential transpiration rates increasing with higher near saturated hydraulic conductivities. Inland valley swamps/floodplains, are characterized by variable and shallow water table depths, its contribution via capillary rise (upflow), constitutes significant component in the root zone water balance and can supply important fraction of crop water use (evapotranspiration).
Extensive land areas in Nigeria are characterized by shallow water tables fed by steams and river courses (inland flood plains) and hence seasonally flooded. The contribution to of ground water table via capillary rise to soil moisture storage and crop water use (evapotranspiration) provides a unique opportunity for dry season crop production in inland flood plains.The relevance of the soil and water resources of inland valley swamplands to the attainment of year round production of crops especially vegetables and the attainment of food and nutritional security cannot be over emphasized. The agricultural potentials of tropical inland valley swamps or flood plains (Fadama ecosystem) can be harnessed via the effective management of its soil and water resources. It is imperative therefore, to develop management guidelines for sustainable exploitation of soil and water resources of inland valley swamps to meet year round crop production and for the attainment of food and nutritional security, improved agricultural livelihoods and contributes to productive wetland based farming. Low-cost technologies (low head /bucket gravity drip irrigation system) for small holder farmers are available (IWMI, 2002; FAO, 2005). It is also necessary to evaluate the effectiveness of these systems for crops (vegetable crops) grown in the dry season on inland valley swaps characterized by variable but shallow water table depths. The objective of this study is to examine the value of drip irrigation schedules for optimizing productivity of water use and yield of dry season sweet pepper (Capsicum annuum L ) grown in an inland valley swamp in a humid zone of Nigeria.
Materials and Methods
Experimental layout
Field experiments were conducted on a sandy clay loam Alfisol to determine the effects of irrigation scheduling strategies (wet: weekly and dry: fortnight irrigation intervals) on water use, yield and water productivity of dry season pepper grown in inland valley swamp in a rainforest zone of Nigeria. The experiments were conducted between December 2009 and May, 2010. The sandy loam soil at the site of study is a tropical rainforest Alfisol classified as Clayey Skeletal Oxic-Paleustalf (USDA Soil Taxonomy, 1999). Experiments were conducted between December 2009 and April, 2010 for which pepper seedlings were grown on soil water reserve until peak vegetative growth (date of first flowering) after which irrigation was imposed during reproductive growth (flowering to crop maturity), and between Janaury and May 2010), pepper seedlings were drip-irrigated weekly and fortnightly from transplanting to fruit harvest. Pepper seedlings were grown on the field at 90 by 30 cm spacing and on 10 by 5m field plot per treatment and at three replications per treatment.
Soil characteristics
The soil at the site of the experiments was sampled and analysed for physical (textural class, bulk density, water holding capacity) and chemical (organic matter, N, P, K Ca, Mg, CEC, electrical conductivity) properties using standard laboratory procedure. Five samples were taken within the row and five from the inter-row spaces in each field. The results of the analyses are presented in Table 1.


Table 1 Some physical and chemical properties of soil at site of experiment


Soil Moisture and Root Studies
Soil moisture was estimated by gravimetric method and the root observations at harvest were made by water spray-soil separation method. Core samples were taken at incremental depths of 10 cm to 60 cm depth (5 soil samples/10cm depth) while bulk density was determined for the samples taken at each soil depth and the values were employed in the conversion of gravimetric soil moisture contents (oven-dried moist soil samples at 105 oC for 24 hours) to volumetric (cm3.cm-3). Soil moisture depletion (SWD) was obtained from the differences in soil moisture contents (changes in soil moisture contents:(S) measured between two measurement period. Soil moisture contents were determined weekly at incremental depths of 20 cm of soil and was taken with augers and core samplers.. Soil water pressure heads was measured using tensiometers installed at 20 and 60 cm below soil surface at 10 cm away from pepper plants. Each of the main plots had tensiometers placed at 20 and 60 cm depth for the daily measurement of the hydraulic gradient. The tensiometers placed at soil depths were to indicate the downward water movement following irrigation or if otherwise the hydraulic gradient was reversed. From the experimental field, ten points were sampled weekly starting from transplanting to crop physiological maturity. Five samples were taken within the row and five from the inter-row spaces in each field. Average daily temperature was used to calculate thermal time (TT) for each day; TT (is daily temperature from emergence, E to date of first flowering, HV multiplied by the number of days from E to HV). Cardinal temperatures, namely base temperature (Tb 8℃), optimum temperature (Topt 32℃), and maximum temperature (Tmax 42℃) (Agele., 2002), were assumed in the calculation of heat unit accumulation.
Pepper growth and fruit yield
Data were collected on pattern of soil moisture storage and depletion, and agronomic parameters of root and shoot biomass, leaf area and fruit yield characters of pepper. Agronomic characters of root and shoot biomass, leaf area, fruit yield and yield components were monitored from ten plants per plot randomly sampled from 2 m2 at the center of each plot. Root and shoot biomass were oven-dried at 80 oC for 48 h and dry weights were recorded. The depth of the effective root zone was estimated after full cover by excavating the root system of different plants. The depth of the effective root zone was found to be around 60 cm (Agele., 2002). Pepper plant leaf area was measured at 50% flowering date using a leaf area meter (Delta T, UK). Pepper fruits were harvested weekly from ten plants sampled per plot starting from physiological maturity. Harvested fruits per plant were counted and summed over all fruit harvests while mean fruit weight per plant was computed from the average of ten sample weights of fruits.
Irrigation strategies
In the first experiment (first planting: December, 2009), adequacy of soil moisture from planting to date of first flowering was assumed (growth stage scheduling plus regulated/deficit irrigation). Pepper seedlings were therefore grown on residual soil water (soil water reserve) until peak vegetative growth (date of first flowering; 2 to 7 Weeks after transplanting, WAT). Thereafter irrigation was imposed during reproductive growth (flowering to crop maturity: 8 to 16 WAT). In another set, second planting (Janaury, 2010), pepper seedlings were drip-irrigated weekly and fortnightly from transplanting to fruit harvest. For both experiments, irrigation regimes consisted of water application weekly and fortnight intervals using gravity-drip irrigation system and 1.38 litres of water per plant at each irrigation via point source emitters of 2l/h discharge rate which were installed on laterals per row of crop. The emitters were installed on laterals per row of crop and were spaced 1 x 1 m apart. Irrigation buckets were suspended on 1 – 1.5 m high stakes to provide the required hydraulic heads. Irrigation buckets were suspended on 1 – 1.5 m (hydraulic heads) high stakes (IWMI, 2002; Olufayo, personal communication). Table 2 presents the various growth phases of pepper. For the high to low irrigation regimes soil moisture storage ranged from 100 to 50 % of plant available water. There was two-day pre-irrigation treatment (4.8 mm/plant/day) following pepper seedling transplanting, and thereafter, the weekly and fortnight irrigation treatments were imposed.


Table 2 Soil moisture management strategies for dry season grown-pepper in an inland swamp: growth on residual moisture from planting to days to 50% flowering and supplementary irrigation at reproductive growth, and irrigated crop from transplanting to crop maturity


At weekly interval, soil moisture content was determined using the gravimetric method (5 soil samples/10cm depth) and via tensiometers placed for measurements of the hydraulic gradient. Each of the main plots had tensiometers placed at 20 and 60 cm depth for the daily measurement of the hydraulic gradient. The tensiometers placed at soil depths were to indicate the downward water movement following irrigation or if otherwise the hydraulic gradient was reversed.
Peak evapotranspiration (ETpeak) rate for the crop under drip irrigation treatment was estimated as:
ETpeak = ETo*P/85……………7
where ETpeak is peak evapotranspiration rate for the month or period, ETo is the reference evapotranspiration, for the month/period (e.g. 5.1 mm/ day), P is the proportion of total land area covered by the crop leaf area (cm) which is assumed 80% (after Agele, 1999; Agele., 2011).
ETpeak = 5.1 *80/85 = 4.8 mm/day
The volume of water required per plant (irrigation requirement, IR) was estimated as:
Irrigation requirement (IR ) = ETpeak * area/crop/En
(IR) = ETpeak * area/crop/En …………8
= 4.80 *0.27/0.94 = 1.38 litres/day
where area per crop is 0.18 m2 (90 * 30 crop spacing: 0.18m2)
Effective moisture content within 0-60 cm soil depth at incremental depths of 10 cm, is the sum of moisture contents (4.15cm) in each layer 0-10, 10-20, 20-30, 30-40, 40-50, 50-60 cm) These gave soil moisture content of 0.80, 0.83, 0.88, 0.65 and 0.76 % before irrigation. Moisture contents at site of experiment at field capacity (21g/100 g soil), and permanent wilting point (PWP) (7.8 g/100 g soil) while the depth of crop root zone (RZd) for water extraction is approximated from values obtained for tomato in the study area within 0 - 60 cm (Agele, 1999).
Maximum (management) allowable deficit (MAD) for pepper was set at 50 %.
Net water requirement (NWR) was calculated as:
NWR= (Fc-PWP) * Bd * RZd * MAD………….9
NWR = (21 – 7.8) * 1.26 * 60 * 0.5 ( g/g*g/cm3*cm)
= 498.96 = 49.9 mm = 4.99 cm
Irrigation frequency or irrigation interval (Ir Int.) is calculated as net water requirement (NWR)/ peak consumptive use rate (ETpeak ) by the crop
IrInterval = NWR/ETpeak ……………10
= NWR/ETpeak
= 4.99/4.8 = 1.
The calculated irrigation interval (Ir Int) is once per day.
Weather variables at site of experiment during crop growth cycle (soil and air temperatures, vapour pressure deficit (vpd), solar radiation, wind speed will be monitored from Meteorological Observatory, 500m from site of experiment). Data collected were subjected to analysis of variance (ANOVA) while significant treatment means were separated using the Least Significance Difference (LSD) test at 5% level of probability.
The second year experiments which involved identical treatments as in 2009 were sown on December and January 2009 and 2010 respectively. The results for the two-years experiments were separately analyzed, and were not significantly different from one year to the other. Therefore, data collected for the two-years of study were averaged and means are presented in tables and figures in the text.
Results and Discussion
Weather condition of the site of studysite
The weather conditions at site of study during pepper growth is presented shown in Figure 1. Atmospheric vapour pressure deficits range from 2.7 to 3.7 kPa while solar radiation ranges between 14.21and 15.16 MJ/m²/day. Rainfall at the site of study is characterized by gradual rise from the month of January until it reaches the peak in the month of June. Thereafter, it declined in the month of July and August when a little break in rainfall is experienced. However, the months of September and October are characterized by heavy but infrequent rainfalls and this is the second modal rainfall. November marks the unset of the dry season.


Figure 1 Weather conditions during pepper growth


Wetting Pattern
The patterns of soil moisture redistribution following irrigation (wetting pattern) monitored by scrapping every 10 cm depth of soil. Figure 2a and b present trends on the wetting pattern two and five days following irrigation. The results show that irrigation regimes affected soil moisture replenishment, its storage and depletion by pepper plants across soil depths and sampling dates. There was wider surface wetting and shallower zone of active root water uptake (higher surface width and depth of wetting) under weekly irrigation while wetting and zone of active root water uptake occur at depths with fortnight irrigation interval.


Figure 2a Soil water potential at 2 days after irrigation



Figure 2b Soil water potential at 5 days after irrigation


Effects on soil moisture storage and suction
Typical volumetric water contents during pepper growth for the respective weekly and fortnight irrigation intervals are presented in Figure 3a and b. Irrigation regimes considerably enhanced soil moisture storage within crop root zone depths. Within the crop root zone, and across irrigations, soil moisture contents ranged between 14.7 and 11.8% for the respective surface (0–20cm) and lower (30-45 and 45–60 cm) soil depths. Between transplanting, establishment and mid season (1–7 WAT), available soil water at the surface was higher in weekly compared with fortnight irrigation (14.9 to 14.5% respectively). However, at lower depths and during reproductive growth (8 to 16 WAT), available water was lower (10.8%) for weekly compared with fortnight (11.9%) irrigation (Figure 3a and b). Across sampling dates, soil moisture contents were lowest for plots irrigated at fortnight intervals compared with weekly irrigation. The changes in profile (010 cm depth) soil water storage during pepper growth for the weekly and fortnight irrigation frequency in comparison with field capacity and permanent wilting moisture contents is shown in Figure 3c. In both irrigation treatments, available soil water was above 50% throughout the growing season. In the weekly irrigation treatment plots, soil water contents remained fairly high as compared to fortnight irrigation. However, in the fortnight irrigation treatment, available water fell below 50% after 40 DAT during the growing season and resulted in lower yield presumably due to moisture stress occurring prior to flowering. The period at the be-ginning of the flowering period is most sensitive to water shortage and soil water depletion in the root zone during this period should not exceed 25% percent. For high yields, an adequate water supply and relatively moist soils are required during the total growing period. Reduction in water supply during the growing period in general has adverse effect on yield and the greatest reduction in yield occurs when there is a continuous water shortage until the commencement of fruit harvest (Sezen., 2006). Soil moisture tension varied during pepper growth phases and within the crop root zone depths: surface soil (0–20 cm) and subsoil (20–60 cm) is presented in Figure 4a and 4b. Soil moisture tension also ranged from –7 to –10 and -8 to –14 bar for surface soil and –3 to –8 and –2 to –8 bar at subsoil depths for weekly and fortnight irrigation intervals. In general, soil water potentials (tension) ranged between –7 to –14 bar and –2 to -8 bar at the respective surface and subsoil depths. The values ranged from – 7 to –10 bar and –10 to –14 bar for the respective seedling establishment (1–7 WAT) and reproductive phase to crop maturity (8–15 WAT). Averagely, while soil water suction was low for weekly (–5 to –9 bar), it ranged between –5 to –12 bar for fortnight irrigation interval.


Figure 3a Trends of soil moisture storage during pepper growth (weekly irrigation)



Figure 3b Trends of soil moisture storage during pepper growth (fortnight irrigation)



Figure 3c Changes in soil water content 0-10 cm depth) during pepper growth (weekly and fortnight irrigation)



Figure 4a Trends of soil water potential during pepper growth (weekly irrigation)



Figure 4b Trends of soil water potential during pepper growth (fortnight irrigation)


Effect on rooting pattern and characteristics
Patterns of rooting and root characteristics of pepper as affected by irrigation regimes are shown in Figure 5a. Higher root biomass and densities were obtained for fortnight irrigation over weekly. Root penetration was significantly lower in weekly compared to fortnight irrigation, and lower root densities at surface and subsoil depths were observed for fortnight irrigation. The average length of primary roots raised from the stem base and root biomass ranged from 17.8 cm in to 19.3 cm and 67.5 g to 73.4 g for weekly and fortnight irrigations. Poorer root growth was observed under the more frequent replenishment of depleted moisture by weekly irrigation regime (Figure 5b). The development and characteristics of root in weekly irrigation may be attributed to frequent replenishment of depleted moisture by weekly irrigation regime. Due to insufficient moisture under fortnight irrigation, plants exert maximum pressure for an enhanced root growth and development (ramification of surface and sub-surface horizons) to improve access to the available soil moisture. Irrigation at fortnight intervals enhanced root growth through greater root biomass and densities even at deeper depths compared with weekly irrigation. This trend suggests greater plant access to water during mid-season and reproductive growth phases when increasing intensities of soil and air moisture and heat stresses obtain.


Figure 5a Pattern of rooting depth in weekly and fortnight irrigation at 9 weeks after transplanting (WAT)



Figure5b Depth-wise pattern of soil moisture storage in weekly and fortnight irrigation at 9 weeks after transplanting (WAT)


The two sowings (December and January: experiments 1 and 2) were characterized by situations in which the lower half of the root zone is close to saturation (near saturated hydraulic conductivities). This situation coincides with the establishment and mid season growth phases of pepper growth. However, the reproductive growth fell within periods when the upper half of the root zone was dry, soil moisture availability may not be sufficient to meet crop water requirements. In a situation where upper half of the root zone was dry and lower half moist, the development of the root systems (root densities) into lower half of the root zone will enhance extraction of soil moisture to meet significant fraction of crop evapotranspiration. Groundwater contribution is optimized when roots are fully developed. While the lower half of the root zone is close to saturation, low root densities within this half may prevent extraction of enough moisture to meet potential transpiration rates (Hurst., 2004: McFadyen & Grieve,2012.).
From the patterns of rooting characteristics of pepper as affected by irrigation regimes (Figure 5a), it appears that a large amount of the water that originated either from irrigation or from ground water was added to the top zone when the crop was not fully developed. Therefore this amount might not contribute much to crop water use. This is supported by the fact that the average moisture content within crop root zone was 201 mm while the estimated evapotranspiration was 164 mm.In both experiments, a large amount of water was added to the top of the root zone before the time the crop was able to use for dry matter accumulation (the establishment phase of pepper growth). Actively extracting roots can increase capillary rise fluxes (Thorburn, 1997), therefore, guidelines for irrigation requirements in some situations will be dependent on the rooting depth and root length density found in the soil profile. Thorburn (1997) recommended more frequent irrigation events with small amounts of water during the period when root length is small. The interval between irrigation events can be increased when roots have been fully developed taking advantage of the presence of the groundwater.
Effects of planting date and irrigation interval on growth, yield and water Productivity
The effects of planting date and irrigation regimes were profound on some growth and fruit yield characters of pepper (Table 3 & 4).


Table 3 Growth and yield characters of pepper grown on residual soil moisture to days to 50% flowering and supplementary irrigation during reproductive growth phase



Table 4 Growth and yield characters of pepper grown under weekly and fortnight irrigation regimes from planting to crop maturity


Effects of planting date
Fruit yields varied significantly between December and January sowings (experiments 1 and 2) and irrigation treatments (Table 5). Differences were obtained in pepper fruit yields between the December and January sowings (experiments 1 and 2) across the irrigation treatments. Total fruit yield was higher in December (8.8 t ha-1) over January (8.5 t ha-1) sowing. During the first sowing (December), the upper half of the root zone is close to saturation and the short distances between the root zone and water table would have enhanced the maintenance of potential transpiration rates under the near saturated hydraulic conductivities and at low soil moisture suctions (-3 to -6 bar).


Table 5 Growth and yield characters of pepper grown as affected by planting date (supplementary and full irrigation), and weekly and fortnight irrigation regimes


Effects of irrigation interval
Irrigation regime affected shoot biomass and fruit yield, In addition to higher roots and shoot dry weights, leaf area and fruit yield were higher in weekly irrigation (Table 5). Across the sowing dates (December and January), fruit yields were lower under fortnight irrigation (averaging 8.1 t ha-1) compared to weekly (averaging 9 t ha-1) interval. About 6.4 and 8.2 % yield reductions were obtained under fortnight compared with weekly irrigation. Irrigation regimes (weekly and fortnight intervals) affected soil moisture storage. The increases in yield under weekly irrigation is attributable to improvements in fruit yield components like number and weight of fruits per plant and mean fruit weight (g.plant-1), these fruit yield components were lower under fortnight irrigation. Weekly irrigation provided more frequent replenishment of depleted moisture from crop root zone, this might have promoted uptake and use of moisture required for fruit initiation and filling. Although, low to mild moisture stress was obtained during establishment and mid-season (1 – 7 WAT), imposition of weekly and fortnight irrigation during the reproductive growth was successful at alleviating greater soil moisture deficit stress and sustained fruit yield. Irrigation interval intervals, soil moisture depletion over two sampling periods were summed to determine seasonal water use by pepper and the results were presented in Table 3 and 4. The results showed that application of between 60 and 40 mm (41.4 and 20.7 litres) for the weekly and fortnight irrigation intervals produced seasonal moisture contents of 201 mm within crop root zone was 164 mm. Although, the dry irrigation treatment (fortnight interval) brought about yield reductions however, about 24 % water savings (reduced crop evapotranspiration) were obtained under fortnight compared with weekly irrigation (60 mm). Water productivity values were 1.85 and 1.25 kg/ha/mm for the December and January sowings.
Planting date by irrigation interval interaction
The interaction of sowing date by irrigation interval is significant on some growth and fruit yield characters of pepper measured in this study (Table 5). This is an indication that the two sets of treatments were interdependence on pepper. The results of this study demonstrated that irrigation regimes imposed (fortnight and weekly) produced differences in water use and fruit yields and the effects of irrigation will be dependent on local weather conditions and crop rooting characteristics such as rooting depth and density. As sowing was delayed from December to January, stressful growing environmental conditions elicited by increasing intensities of soil moisture and vapour pressure deficits and high air temperatures could have lead to reductions in fruit yields across the irrigation treatments. It is concluded that as precipitation reduces over time especially during crop growing seasons, the development of water-saving management practices for sustainable agriculture now and in the future is imperative.
Conclusion
The results of the field study conducted on sandy loam Alfisol in an inland valley swamp (flood plain) showed that irrigation regimes imposed (fortnight and weekly) produced differences in water use and fruit yields of dry season pepper. Irrigation regimes imposed (fortnight and weekly) substantially enhanced water use and fruit yields of dry season pepper grown in an inland valley swamp/floodplain in a humid tropical environment. These results suggest that alleviating soil and air moisture and heat stresses in the early to mid-season and during the flowering and fruit filling periods via application of irrigation has the potential to maintain pepper fruit yields. The trends in soil water balance indicated that reduced irrigation was accompanied by over 20 % water savings and did not result in significant fruit yield reductions in pepper. Differences were obtained in pepper fruit yields between the December and January sowings (experiments 1 and 2) across the irrigation treatments. Total fruit yield was higher in December (8.8 t ha-1) over January (8.5 t ha-1) sowing. In both experiments, fruit yields were lower under fortnight irrigation (averaging 8.1 t ha-1) compared to weekly (averaging 9 t ha-1) interval. Weekly irrigation offered the best compromise in terms of fruit yields and water productivity in the circumstance of declining water table depths and high climatic demand of the dry season in the site of study. Application of water to crop root zone via the use of irrigation improved soil moisture storage, evapotranspiration, growth, fruit yield and water productivity of pepper. Establishing the optimal irrigation scheduling is important in the development of water-saving management practices for sustainable agriculture in the wake of the hydrothermal (extreme heat and water deficits) stress envisage for the future.
Acknowledgement
The authors gratefully acknowledge the Senate of the Federal University of Technology, Akure, Nigeria, for the opportunity of a University Senate Research Grant Number URGC/MAJOR/2007/179.
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