Response of Okra to Different Irrigation and Fertilization Methods in the Keta Sand Spit of Southeast Ghana

Abstract

The sandy soil in the Keta Sand Spit, Southeast Ghana is infertile but is used for intensive vegetable cultivation. The vegetable production systems are managed with large amounts of irrigation water and fertilizers on sandy soils with low inherent water and nutrient retention capacities. The long term sustainability of a shallow groundwater lens which is used for irrigation in the area is threatened by several consecutive years of over withdrawal. Also, the shallow groundwater which is the primary irrigation water resource is prone to salinization from the Keta Lagoon, the Atlantic Ocean and brackish water underneath. There is excess input of phosphorus into the soil, through the continuous application of animal manure as the sole plant nutrient over the years. To ensure the sustainability of vegetable production at the Keta Spit, introduction of water saving irrigation systems and improved irrigation and fertilizer management schemes are important. Thus, the main aim of the study was to explore the productivity of drip irrigation compared with sprinkler and the traditional bucket irrigation in the Keta Sand Spit. Okra (Abelmoschus esculentum L) is widely grown in Southeast Ghana including the Keta Sand Spit area and was therefore used as the model crop. The study was conducted to evaluate the effect of different irrigation and fertilization methods on crop evapotranspiration, yield, nitrogen uptake, crop intercepted solar radiation, radiation use efficiency and water productivity of okra grown in a sandy soil. The basal crop coefficient (K cb ) was related to spectral reflectance measurements. The basal crop coefficient and soil evaporation coefficient (K e ) for drip and sprinkler irrigation were also estimated using FAO-56 methods. There were four seasons of study to determine the okra crop response to the following treatments: 1. sprinkler irrigation with manure spread on the soil (SSM); 2. sprinkler irrigation with manure placed around the plants (SPM); 3. Bucket irrigation with manure spread on the soil (BSM) 4. drip irrigation with manure placed around the plants (DPM) and 5. drip irrigation with fertigation (DFT); i.e. nutrient solution added to irrigation water. Fertigation was done only two times (two weeks after germination and immediately after flowering) during the first and second experiments while weekly fertigation (8 times from two weeks after germination) was done during the third and fourth experiments. Results from the experiments showed that the okra crop did not respond well when fertigation was done only twice (two weeks after germination and immediately after flowering) in the first and second experiments, probably due to nitrogen lost through leaching on the extremely sandy soil. However, a significant improvement on the yield response (P≤0.05) in the fertigated treatment compared to SSM, SPM, BSM and DPM was obtained when fertigation was done weekly for eight weeks. In the third experiment, with similar nitrogen application (89 kg N/ha) for all treatments, the highest yield, N uptake, water productivity (WP) and radiation use efficiency (RUE) were obtained under DFT and these parameters were significantly higher than the other treatments (SSM, SPM, BSM and DPM). Increase in fertilization in the fourth experiment (from 89 kg N ha -1 to 140 kg N ha -1 ) increased yield and WP compared to the three previous seasons, WP for DFT and DPM being significantly higher than for SSM, SPM and BSM. In the four seasons under sprinkler irrigation, yield was higher with manure placed around the plants (SPM) compared with manure evenly spread on the soil (SSM) even though the difference was not significant during the first and last experiments. The optimal crop N-uptake was 125 kg N/ha independent of season and adequate N-supply seemed especially important for ensuring sufficient light interception and radiation use efficiency during the fruiting stage of growth. Simple linear regression equations (R 2 > 0.9) were developed to provide an estimate of K cb during any time of the growth period with the fraction of the photosynthetically active radiation (f PAR ) data from spectrosense measurements. The dual K c values derived from FAO-56 methodology for sprinkler irrigation were K c and K c end : 1.05 whilst for K e 0.85 and K cb end : ini : 0.95, K e mid : 0.05 and K e end ini : 1.10, K c mid : 0.9 0.29, and for K cb ini : 0.15, K cb mid : 0.76 respectively. For drip irrigation the initial, mid and late season K c were 0.48, 0.89 and 0.98 with K e having 0.33, 0.04 and 0.22, and for K cb 0.15, 0.85 and 0.76 respectively. The K c data showed that the evaporative component was high during the initial stage of the growing season, due to the high frequency of irrigation. Seasonal crop water use (ET c ) for sprinkler irrigation were 346 mm, 339 mm, 379 mm, and 346 mm for the four experiments. For drip irrigation these were 233 mm, 236 mm, 269 mm and 233 mm. By adopting drip irrigation for okra, the seasonal crop water use could be reduced by nearly 30 %. From the results it is concluded that: 1. on coarse textured sandy soil drip irrigation with weekly fertigation resulted in significant water savings, yield increase and improvement in WP, RUE and nitrogen uptake compared with sprinkler irrigation combined with farm manure 2. The dual crop coefficient which allowed differentiation between crop transpiration (basal crop coefficient, K cb ) and evaporation from the soil (evaporation coefficient, K e ) should be used for irrigation scheduling in the Keta Sand Spit.