Water Holding Capacity of Soil Mixtures for Green Wall Applications.

Haiti has suffered from severe economic and environmental hardships after the 2010 earthquake near Port-au-Prince. Maintaining an adequate food supply is one of the largest concerns. Recently, a Haitian missionary affiliated with the University of Tennessee wanted to construct a dual-purpose wall for both structural support and plant growth. Consequently, a “green wall” vertical aquaponic system was designed and constructed from materials which would be readily available in Haiti. A primary construction issue was choosing a soil type.  Calibrated HYDRUS models were used to indicate a goal texture. This was optimized for maximum drainage and water holding capacity, two important target features.  Thus, sites were selected for retrieval of two representative soil types designated “loam” and “sand.” Four separate loam/sand mixtures were made with respective ratios ranging from 80:20 to 50:50. After saturation, samples were exposed to increasing levels of suction. A tension table was used to generate low matric potential in order to construct a soil water characteristic curve. At -80 cm H2O pressure the 80:20 mixtures had lost 45% of its initial moisture content, while the 50:50 mixtures lost 54%. The remaining mixtures lost approximately 50% of their moisture content. This knowledge was important in constructing a green wall in such a manner which would not induce excessive ponding and/or drainage through the wall.  Future research will be conducted to extend the drying portion of the curve to a total of -15 bar pressure and also measure a subsequent wetting curve for each soil type.