Structure and Function of Storage Pit, Polota, for Long-Term Storage of Sorghum-A Case Study of Storage Pit in Dirashe Special Worenda, Ethiopia

Abstract

“Storage pits” with bag-like, tubular or flask-like shapes had been utilized for storing grains and nuts all around the world until several centuries BC. Storage pits were, however, mostly replaced with aboveground storehouse once the cultivation of rice, wheat and barley, which were not suitable for underground storage, became widespread. Nevertheless, such storage pits are now still being used locally in some rural villages in Ethiopia and Sudan. Storage pits can prevent losses due to weather, mice, sparrows, fire, water, and theft. However, storage pits are highly humid inside, which leads fungal and bacterial proliferation. Stored grains tend to severely deteriorate within several months, which is often before the next harvest season comes. However, the local people in the Dirashe area in Southern Ethiopia state that underground storehouses called polota with a flask-like shape can store sorghum for a maximum of 20 years. This study investigates the location, structure and storage function of polota in order to understand the reason why polota is capable of long-term storage while such storage pits highly humid inside. First, soil samples were collected from the locations where polota were constructed. The x-ray fluorescence analysis was conducted on these samples to analyze their chemical compositions. Then, the rates of iron (g)/aluminum (g), aluminum (g)/titanium (g), silicon (mol)/aluminum (g) were calculated. The result indicates that polota were built in the areas where basalt layers were chemically weathered. Based on the actual measurement of polota, all the polota are shaped like a flask, about 1.5 m in diameter and about 2 m in depth. The chemically weathered basalt makes it easier to work on while maintaining its dense composition, thus it can be easily formed into a flask-like shape. Also, the airtight characteristic can maintain the temperature and humidity inside stable. The measurement of hygro-thermal properties performed inside polota in which sorghum was stored shows the stable temperature at 31 Celsius and relative humidity at 92%. A low concentration of oxygen (O2), 2.7%, and high concentration of carbon dioxide (CO2), 1,60,000 ppm, were also measured inside polota. While polota is as high in humidity as other storage pits, thus not suitable for storage, it now revealed that a low O2 concentration prevents propagation of noxious insects and a high CO2 concentration induces a state of quiescence to the stored grains, inhibiting deterioration and enabling long-term storage.