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Available soil-phosphorus (P), exchangeable soil-potassium (K), soil pH and soil Electrical Conductivity (EC) affect rice production and are highly variable in nature mainly due to the variability in water availability. Therefore, experiments were conducted to find out the variability of some important soil chemical properties of rainfed lowland paddy fields and their effect on land suitability for rice cultivation in the Dry (Mahananeriya), Intermediate (Ibbagamuwa) and Wet (Alawwa) regions in the Kurunegala District of Sri Lanka. Randomly collected soil samples from the above three areas were analyzed for available soil-P, exchangeable soil-K, soil pH (1:2.5) and EC (1:5). Thematic maps for each parameter were prepared using Inverse Distance Weighted interpolation technique. Weighted sum technique was used to overlay maps to prepare the suitability map as Marginally Suitable, Moderately Suitable, Suitable and Highly Suitable. Results showed that the available soil-P and EC levels are preferable for rice cultivation in the rainfed lowland paddy fields in all three regions. Even with this situation, the potential productivity of the lands cannot be obtained due to variability of soil pH and exchangeable Potassium contents. Soil pH and the exchangeable soil Potassium in all rainfed paddy areas of Alawwa is far below the optimum levels for paddy cultivation. In terms of these soil chemical properties, 42% and 32% of rainfed paddy fields in Ibbagamuwa and Mahananeriya regions, respectively, are either highly suitable or suitable but 79% land extent of the rainfed paddy in Alawwa is marginally suitable for rice cultivation. Therefore, a blanket recommendation of remedial measures cannot be introduced to improve productivity of the rainfed paddy fields in these regions while site-specific remedial measures would help to overcome the problems of specific soil chemical properties.

Assessment of crop responses under varying levels of soil available nitrogen (N), phosphorus (P) and potassium (K) is important to develop site-specific nutrient management (SSNM) strategies. This study was conducted to investigate the spatial variability of soil N, P and K levels in paddy grown soils in Anuradhapura, Kurunegala and Polonnaruwa districts and corresponding yield response for nutrient application. Seventy-one paddy fields were selected and four plots (3×6 m) were established at each site comprised of a treatment applied with ample amount of N, P and K and corresponding omission treatments. At harvesting, total yield of each plot was recorded. Considerable heterogeneity of soil N (12.5-43.9 mg/kg), P (2.7-34.2 mg/kg) and K (14.6-221.9 mg/kg) was observed in paddy grown soils of the three districts. A significantly higher average yield of N applied treatment (5.8 Mg/ha) than that of N omitted treatment (3.9 Mg/ha) indicated a yield response for N application.However, nonsignificant differences of average yields of P (5.7 Mg/ha) and K (5.6 Mg/ha) omitted and ample treatments (5.8 Mg/ha) revealed a poor yield response for the application of these nutrients. Stepwise multiple regression analysis revealed that neither the initial soil P and K levels nor the other studied factors (growing season, irrigation scheme, soil texture, pH, EC, available Fe and Mg) have an impact on yield responses observed for P and K applications. Our results indicated large spatial heterogeneity in soil P and K levels in paddy grown soils. Absence of a significant response for the applied P and K and a considerable variability of yield responses highlighted the need of further studies on P and K reserves of paddy growing soils and factors controlling the variability in yield response. Further, site-specific application of N is shown as a priority to improve the productivity of paddy lands.