USE OF HIGH ACCURACY GPS FOR SOIL GEOCHEMICAL MAPPING IN PADDY FIELDS; A CASE STUDY IN KALUTARA, SRI LANKA

Abstract

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Commonly, paddy fields in Sri Lanka are nearly flat grounds and no much geochemical variation is expected within a single field. Paddy fields in the wet zone along the western coastal areas show remarkable decrease in land areas used for rice cultivation mainly due to low productivity. Soil geochemical parameters in paddy fields are governing factors in rice production. Fertilizer dose is recommended for areas or fields and formers apply the same amount o f fertilizer to all paddy fields without considering local variations o f the soil fertility levels o f fields. Present study aimed to analyze local variation o f cultivated paddy fields including topography and soil geochemistry using high accuracy GPS data by developing Digital Elevation Model (DEM) and GIS models. Present research was carried out in paddy fields in Kalutara. The land was surveyed using GPS-RTK method with the aid o f high accuracy GPS. Soil samples were taken in a grid at two different depths (15 cm and 30 cm) and analyzed for P, K, Mn, Fe, Cu, Zn and organic matter and pH was also measured. Results showed greater variation in all these parameters spatially as well as vertically in the soil profilethough this single field generally appeared as flat ground. Constructed maps demonstrated that certain parts o f the field contained higher level o f nutrient elements than standards while certain areas lesser than standards for rice cultivation. The DEM showed 3.8 m elevation difference from west to east direction within the same field. The observed variation in geochemistry o f the paddy fields was closely related to the slope/aspect o f the terrain. This study revealed that topography o f the terrain and existing geochemical parameters should be taken into consideration when recommending fertilizer dose to die field. It is recommended to develop a local topographic model showing elevation and spatial variation o f soil chemical parameters to select optimal level o f fertilizer.