Filtration increased lettuce yield of the control, gravel, and sand treatments by 87, 63, and 52%, respectively over unfiltered treatments and it enhanced the nutrient uptake. Significantly higher (P<0.05) lettuce yield observed in filtered water treatments should have resulted from adequate root respiration facilitated by removal of suspended solids through filtration. A good substrate in a hydroponic system maintains a nutrient reservoir in the root zone and provides adequate air pore space for respiration (Harris, 1978). It has been shown that sand media provide relatively better growth of lettuce compared to rice hull and a mixture of sand and rice hull. Furthermore, the sand media can act as a biofilter to reduce suspended solids from pond water. As a result, it can enhance the cation exchange capacity and increase the availability of nutrients to the plants. Sand culture of hydroponics also reduces plant pathogenic risks. The study showed relatively higher yields of lettuce observed with the sand treatment and this should have resulted from a combination of the above factors.
Chaves et al. (2000) reported improvement of water quality through integration of hydroponically grown lettuce in which nitrate concentration was reduced by 20–27%. Therefore nutrient uptake by lettuce plants in this study also did contribute to the depletion of nutrients in the pond water.
The major constraints to use of pond water are found to be low level of macronutrients, low dissolved oxygen and high suspended solids in catfish pond water.
The critical NO3-N, PO4-P, K and Ca concentrations for lettuce nutrient absorption were 1.0, 0.5, 0.5 and 0.8mgL−1 (14, 16, 19.5, 16mgL−1), respectively. Nutrient contents of catfish pond water in this study were below these critical concentrations. The average concentration of K, Ca, Mg, Mn, Fe, Zn and Cu in the catfish pond water was 12.96±0.12, 5.14±0.14, 0.66±0.02, 0.02±0.01, 0.31±0.14, 0.05±0.01, and 0.03±0.00mgL−1, respectively. These nutrients levels are much lower than the nutrient content of hydroponic solutions used by several researchers. K and Ca contents of nutrients solutions used in hydroponics production of lettuce are as highas400and197mgL−1,respectively.The observed lettuce yield in this experiment was relatively lower than the values reported by Rakocy et al. (1992), Karam et al. (2002) and Karimaei et al. (2004) who grew lettuce plants hydroponically using nutrient solutions and Seawright et al. (1998) who cultured lettuce using recirculatedwaterinanintegratedtilapia-lettuceproductionunits,in which effluent was enriched with a nutrient solution. Other studies have shown that increase in fertigation frequency induced significant increase in yield of lettuce, chilli and tomato (Al-Ghawas and Al-Mazidi, 2004) especially at low nutrient concentration levels indicating that high irrigation frequency could compensate for nutrient deficiency. Further studies to assess the effect of increased irrigation time and frequency on growth and nutrient uptake of lettuce are warranted.
Climatic conditions could have impacted on the results, however, treatment effects were not masked because the conditions were standard to all treatments. Tindall (1983) reported that lettuce varieties that a readapted to tropical environment sare to lerant to high day temperatures. Models to characterize leaf and total dry matter production of hydroponics-grown lettuce (Lactuca sativa L.) as a function of solar radiation and temperature have been demonstrated (Lopes et al., 2004).
Lennard and Leonard (2006) showed that Lettuce grown in gravel as a hydroponic media was efficient at removing nutrients from fish culture water. This phenomenon was not observed in this study. Adequate Ca content in lettuce grown on sand media might have resulted from either the ability of sand to accumulate Ca from pond water or the presence of an adequate Ca reservoir in sand. Catfish pond water can supply adequate quantity of micronutrients Fe, MnandCu but deficient in Zninall treatments except lettuce grown on sand media using filtered water.
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