STRATEGIES FOR SOIL WATER MANAGEMENT FOR OPTIMIZING WATER PRODUCTIVITY OF DRY SEASON FADAMA–GROWN PEPPER (Capsicum annuum L.) IN AKURE, NIGERIA

Abstract

Inland valley swamps (floodplains) are characterized by variable and shallow ground water table depths, capillary rise (upflows) from the water table may constitute important component in the root zone water balance and supply important fraction of pepper water use. Three years of field experiments involving four sets of experiments were conducted in the dry seasons of 2009 to 2011, to examine the effects of some strategies for soil water management for optimizing water productivity of dry season fadama-grown pepper (Capsicum annuum L) in Akure, Nigeria. The four sets of experiments which were separately conducted were: i. Irrigation strategies for optimizing water table contribution to soil moisture storage and water use of dry season fadama-grown pepper; ii. Mulching effects on pepper water use, growth and yield of dry season fadama-grown pepper; iii. Effects of drip irrigation frequencies on growth, yield and water use efficiency of dry season fadama-grown pepper, and iv. Effects of differential irrigation on water productivity and water-yield functions of dry season pepper. The soil of the site of experiment is sandy-clay-loam with relatively high water holding capacity. Available soil water in the upper 0.60 m of the soil depth is 187 mm. the percent and volumetric soil water contents at field capacity and permanent wilting point are 21 and 10 % respectively. Mean bulk density was 1.25 g cm-3. The first experiment (effects of irrigation scheduling on the components of soil water balance) in which the contribution of water table via capillary rise (upflows) and irrigation, to soil moisture storage was quantified where capillary rise was taken as the difference between estimated evapotranspiration (ET) and measured soil water depletion (SWD). Capillary rise ranged from 2.3 to 5.2 mm which amount to 81 and 124 % of pepper evapotranspitaion (ETa) across the sampling periods. The results showed that weekly and fortnight irrigation intervals produced seasonal ETa values of 109 and 83 mm and moisture contents of 201 and 164 mm within crop root zone for the respective weekly and fortnight irrigation. Shoot biomass and fruit yields were higher in treatments involving weekly (153 g plant-1; 8.6 t ha-1) irrigation in addition to enhanced water use efficiency compared to fortnight (141 g plant-1; 7.9 t ha-1). The values of water use efficiencies were 0.14 and 0.19 t/ha/mm for the respective 7-day and 14-day irrigation intervals. Soil moisture storage and its depletion, Cg, crop evapotranspiration (ETa) and the relative water use (ETa/Eo) differed in the growth stages of pepper and were influenced by irrigation regimes and the prevailing weather conditions (vapour pressure deficits/vpd, temperature, thermal time) during pepper growth. The magnitudes of seasonal relative water use obtained, indicates the inability of soil moisture storage to satisfy pepper water requirements (ETa). Results show that irrigation regimes imposed optimized the contribution of groundwater to soil moisture storage and water use of pepper. Weekly irrigation offered the best compromise in the circumstance of declining water table depths and high climatic demand of the dry season. It is concluded that irrigation management for crops grown in soils under the influence of shallow water tables should be modified to optimize the contribution from groundwater to soil moisture storage and crop evapotranspiration. The effects of mulching on soil moisture extraction, growth and yield of pepper was examined between December and May of 2009 and 2011. The mulching materials evaluated were transparent and black polythene sheets and a mixture of dry grass species. Data were collected on soil moisture and temperatures and root and shoot biomass, leaf area, flowering and fruiting characters of pepper. Mulching significantly reduced soil temperatures and enhanced soil moisture storage which under grass and polythene mulch were lower compared with unmulched plots. Application of mulch significantly (P < 0.05) increased root and shoot biomass, fruit yield and water use efficiency over bare ground. The effects of drip irrigation schedules on water use, yield and water productivity of pepper were examined. Two planting dates were considered, in 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 from transplanting to fruit harvest. In both experiments, irrigation regimes consisted of water application weekly (7-day) and fortnight (14-day) intervals using gravity-drip irrigation system. Higher biomass and densities of roots were obtained for fortnight irrigation over weekly. Root penetration was deeper in fortnight which also had higher root densities in subsoil 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. Mean surface soil moisture contents was higher in weekly compared with fortnight irrigation (14.9 to 14.1% respectively). However, at lower depths and during reproductive growth (8 to 16 WAT), soil moisture was lower (10.8 %) for weekly compared with fortnight (11.9%) irrigation. Soil moisture tension 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 were - 7 to -10 bar and -10 to -14 bar for the respective seedling establishment and reproductive growth phases. Differences were obtained in pepper fruit yields between the December and January sowings 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, however, the improvement in water use efficiency under fortnight irrigation, and about 8.2 % yield reductions were obtained under fortnight compared with weekly irrigation which translated to 24 % water savings (reduced water use). Seasonal soil moisture storage were 201 and 164 mm within crop root zone was 164 mm, and water use efficiencies were 0.14 and 0.19 t/ha/mm for weekly and fortnight irrigation regimes. The effects of differential irrigation on the growth, stress development and water- yield functions of dry season pepper was examined. The aims were to characterize the water productivity of pepper (water- yield or water production functions) and develop the drought response factor Ky in relation to irrigation regimes. The experiment was conducted between January to May, 2011 using four irrigation treatments based on the restoration of depleted soil moisture via evapotranspiration (ETa). The irrigation regimes based on levels of cumulative pan evaporation (Epan) of 100%, 80%, 60%, and 40% of EPan which indicate relative water deficit of 0, 0.2, 0.4, 0.6 and 0.8 respectively in order to attain maximum and minimum plant water stress conditions. Irrigation was fixed at 5 days-interval at EPan coefficient (Kcp) of 0.70 and accumulated pan evaporation. Pepper plants were therefore drip-irrigated based on levels of cumulative pan evaporation (Epan) values of 1.0 Epan (DI1), 0.8 Epan (DI2), 0.60 Epan (DI3) and 0.40 Epan (DI4). The amount of irrigation at each event and seasonal sum of irrigation amount were 4.82 l/day; 127500 mm, 3.86 l/day; 81600, 2.90 l/day;45900 mm and 1.93 l/day; 20400 mm for the respective well irrigated control (DI1), (DI2), (DI3) and (DI4) the more stressfull treatment. Irrigation levels significantly affected crop evapotranspiration, the growth, number of fruit, mean fruit weight, and water yield function of pepper. The well irrigated control (DI1) produced highest fruit yields of 9.2 t/ha while the highest deficit irrigation (DI4) produced 7.1 t/ha. Fruit yields from droughted pepper plants declined with increasing soil moisture deficits ranging from 8.7 in DI2 .8.1 in DI3 and 7.1 t/ha in DI4. Higher efficiencies of irrigation (IWUE) and crop evapotarnspiration (WUE) were obtained under DI1 (0.72 and 1.34 t/ha/mm ) and minimum in DI4 treatments (2.73 and 1.37 t/ha/mm ) while the maximum crop water sensitivity indices (Ky) were 1.79 and 2.30 for the respective DI2 and DI4 treatments. The seasonal evapotranspiration varied from a low 516 mm in the more stressfull treatment (DI4) to a high 612 mm in the well irrigated control (DI1). Regression equations were worked out between fruit yield and seasonal evapotranspiration and between irrigation levels (the efficiencies of crop water (WUE) and irrigation use (IWUE). The relationships were characterized by high correlation coefficients (R2 > 0.85). The relationships between yield and ET (y = 87x + 376, R2 = 0.85) and for the respective 100EPan: y = 66x + 49379, R2 = 0.93; 80EPan: y = 120x + 17055, R2 = 0.92; 60EPan: y = 77x +14065, R2 = 0.99 and 40EPan: y =65x + 43379, R2 = 0.87). The relationships between irrigation level and the efficiencies of crop water use (WUE/ETa) and irrigation use (IWUE) were: IWUE/ETa : y = -0.02x + 1.41, R2 = 0.98 while WUE/ETa : y = -0.07x + 3.96, R2 = 0.97). The moisture stress sensitivity indices (drought response factor, Ky) were computed from (a) the relative yield (Ya/Ym) and the relative evapotranspiration (ETa/ETm) and (b) relative yield (Ya/Ym) and the relative soil moisture deficit (SAWa/SAWm) relationships of the water production models using multiple linear regression technique. The values of ky were 0.84, 0.93 and 0.98 and 1.79, 2.30 and 2.60 for the DI2 to DI4 and mean ky values were 0.92 and 2.25 for the respective a and b water production models.