118 temperature [°C], the concentration of CO2 [ppm], and the relative air humidity [%]were measured on a continuous basis from the very start of the growth season. The following parameters were also measured by Growth factors The growth of duckweed is determined by five factors - the photoperiod, the temperature of the water between the roots of the duckweed, the pH of the water, and finally the nitrogen and phosphorus content. The two latter factors were not limiting the duckweed growth in the ECOFERM. The amount of light in PAR [μmol/m2.s-1], the air and water hand: EC value, pH, O2 content of the water, dry matter content and fresh weight of the harvested duckweed. Light According to Van der Top’s growth model, 13 hours of light saturation every day produces the highest growth rate, while too little light light can markedly limit growth. The maximum amount of light saturation measured in the ECOFERM greenhouse was 12 hours. This means that this important growth factor was always at a sub-optimal level. Due to legislation by the local government, which forbids greenhouses in this area, part of the roof surface is nontransparent. As a result, rafters, purlins, and side-walls cast shadows on the duckweed. This has a negative impact on the production. Temperature The water temperature is measured in the area of the roots and leaves of the duckweed. The air temperature is registrated just above the surface of the water. The growth model indicates that a water temperature between 20 and 30°C gives the best growth, with 26°C being the optimum level. The duckweed is liable to die quickly if the temperature exceeds 35°C. Because the water in which the duckweed grows is located in the part of the building above the calves, the warmth they produce provides continuous floor heating. The ventilation air from the stables (humidified and therefore cooled in the biobed) was blown into the greenhouse in order to provide sufficient levels of fresh air. In other words, cool air was supplied, and warm air removed. No additional heating was needed in order to keep the water temperature above 20°C from around April to October. There were occasions during the hottest months (June and July) on which the water temperature reached very high levels. A spray installation was therefore mounted above the pond, which was activated as soon as the air temperature reached 37°C. This prevented the water temperature from rising above 35°C. Fertilization The pH and the Electric Conductivity (EC) values of the water were regularly measured in order to monitor the fertilization of the duckweed. The pH value provides information about the levels of acidity. The EC value shows the total quantity of dissolved fertilizers, based on the conductance of the water. For the optimal solubility of the nutrients and unimpeded growth of the duckweed, the pH should be in between 6 and 7. Humic acid was added at the very start of the Pagina 125
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