Identification of soil fertility status of subak rice fields based on soil chemical properties in Denpasar city, Bali, Indonesia

Soil functions as a storehouse of nutrients that are needed by plants for growth and production. Decreased soil fertility will cause a decrease in productivity due to reduced nutrients in the soil. To determine the state of nutrients, it is necessary to evaluate the status of soil fertility. This study was carried out in the Subak rice fields in Denpasar City, Bali Province, Indonesia. Denpasar City, is the capital of Bali Province which continues to grow, relying on the rice production from its rice fields. The main objective of the study was to evaluate the fertility status of soils in Subak rice fields in Denpasar City based soil chemical properties and to map their fertility status using Geographic Information System (GIS). The approach for this study included surveying and soil sample analysis. The parameters observed were soil chemical properties including: CEC, Base Saturation, C-Organic, P2O5, K2O and pH. The study delineated three categories of soil fertility status, namely high, moderate and low. The total area with high soil fertility was 1312.76 ha (63.77 %), while for the medium and low, the respective total areas were 560.83 ha (27.24 %) and 184.89 ha (8.98 %). The Subak condition with low soil fertility has deficiency in soil organic Carbon (C) content and low total Phosphorus (P). Recommendations for managing low soil fertility status are addition of organic matter and P fertilizer of 100 kgha-1. The spatial distribution of soil fertility status in Denpasar City was mapped using GIS. The distribution of fertility status in these rice fields shows that the majority of the fields classified as high fertility are located in East and South Denpasar. The different fertilization regimes followed between the Subaks were the cause of the observed different soil nutrient statuses. Therefore, it is necessary to give the same treatment in order to maintain soil fertility. Soil fertility and maintenance of agricultural land productivity were factors that supported food security in Denpasar City. Key words: Subak, Rice fields, Soil Fertility, Chemical Properties, Geographic Information System