Response of Okra to Different Irrigation and Fertilization Methods in the Keta Sand Spit of Southeast Ghana
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Date
2014-07
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Publisher
University of 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.
Description
Thesis (PhD) - University of Ghana, 2014