Soil Erosion and Conservation Measure in Northern Part of Okinawa Island

Abstract

Okinawa prefecture is located in the monsoon region of Asia and has a humid subtropical climate. Therefore the properties of the soil in this district reflect the climatic factors. One particular kind of soil, called Kunigami Maji because of the locality and the local dialect, is distributed mainly in the northern parts of Okinawa island, Ishigaki island, and Iriomote island. Kunigami Maji soil in red-yellow in color and is regarded as a problem soil from the point of view of erosion. This study was made in order to estimate and to establish controling soil erosion on reclaimed fields in the northern parts of Okinawa island. The relations among the factors of USLE (Universal Soil Loss Equation) were analized on the basis on rainfall energy and soil loss from the fields. The method of analiysis adopted in the current study is indicated with reference cited. The results obtained from the investigation are summerized as follows: 1) According to 9 samples obtained from the field plots tested, K value of USLE factors is higher in silty soil than in clayey soil in each plot of inclination of 3° 5° 8° 2) R value changes from 438 (m2 tonf/ha • hr.) to 1073( m2 tonf/ha . hr.) by the year test was conducted. 3) C value of pineapple was estimatled at 0.56 4) Countermeasures against soil erosion and areas for further study were proposed as follows: (1) The field lot like puddy is recomended to protect soil loss from ruclaimed area. (2) Mulching with layers of grass or leaves to decrease rainfall enery and runoff. (3) Stabilization of soil particle by using compost or humus. (4) Reasonable layout of terrace and drainage channel. (5) Decreasing muddiness of runoff to minimize river and ocean contamination.