64 MARCH 2015 • WORLD AQUACULTURE • WWW.WAS.ORG trials and testing of available rope materials, three 25-40 cm long wicks were used per pot, which were effective in providing plants with sufficient water. For an experiment conducted in La Virgen with four crops, crops irrigated using wicks yielded more than those that were hand watered (bean and tomato were significantly greater). Although this difference was attributed to inconsistent hand-irrigation (because personnel often preferred not to enter the water), irrigation through wicks was highly efficient and reliable. Although potted plant growth on floating rafts was adequate, even simple rafts were relatively expensive and heavy. Thus we thought it necessary to try simpler, lower-cost approaches. The Winning Design The next design for holding potted land crops consisted of simple 3-4 m × 3-4 m structures made of wood or bamboo poles (25-100 mm in diameter), similar to a trellis or lattice, with spacings between them established according to area needed per plant and with flotation provided by empty jugs (Fig. 5). To lower costs, and because they seemed ideal for the application, used plastic bottles (2-3 L capacity) were used as pots, with their capped mouth-end dipped 50 mm to 100 mm in the water and a few holes in the cap allowing for water to enter the potting mix and rise through it by capillarity (Fig. 6). A limited submersion depth of bottles was important to avoid water saturation and hypoxic conditions in the potting mix. To confront this we conducted an experiment with beans, combining potting mix with 33 percent sand to increase aeration. However, yields with only potting mix were significantly greater (7.8 t/ha) than with potting mix and sand (4.5 t/ha), emphasizing the importance of minimal submersion depth and a growing medium with high capillarity. The floating trellis design, using the trellis and the bottles as pots and a watering system using bottom holes for water entry, was very efficient and was used thereafter in all sites excepting La Virgen. Figure 7 shows the type of growth that was consistently obtained. Other Cropping Designs In attempts to further lower costs, several other designs were tested. Among them, small rafts made of only reused bottles securely tied together proved very efficient (Fig. 8). Although simpler and less expensive rafts are made from water hyacinth in flood plains of Bangladesh (Irfanullah et al. 2011), those FIGURE 4. Floating rafts installed in Lake Arenal, Costa Rica (top) and La Virgen Lake, Nicaragua (bottom). FIGURE 5. Examples of floating trellises implemented as the ‘winning’ design. FIGURE 6. Bottles used as pots (left, with bean plants) with comparison between holes vs. wicks for water uptake (right). Bottles were painted white to reflect radiation and shade the root zone.
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