Availability and spatial variability of plant nutrients in paddy fields of Wilgoda Irrigation Scheme in Kurunegala

A study was carried out in the maha season of 2008/2009 to gather information on the status of plant nutrients and their spatial variability in paddy fields in the Wilgoda irrigation scheme, where wastewater is used for irrigation purposes throughout the year. Soil samples were collected from 56 sampling points at upper (Peellawala), middle (Illuppitiya) and lower (Galayaya) parts of the area. Geographic positions of the sampling points were recorded using a Geographical Position System (GPS). Soil samples were analyzed for pH, available phosphorus (P), exchangeable concentrations of potassium (K), calcium (Ca), magnesium (Mg), sodium (Na) and zinc (Zn). In comparison to nutrient contents recorded for paddy growing soils in other parts of Kurunegala by Mapa et al. (2005), this study showed higher levels of P, K, Mg and Na in all study sites. Significant variability in nutrient availability was observed both between sites as well as within a site. Although the inherent K content of paddy soils in Kurunegala is 29-39 mg kg-1, the recorded average K contents at Galayaya and Peellalwala were 59.7 and 67.0 mg kg-1, respectively. Around 20 % of the area had K contents above 78 mg kg-1, which is the critical K level for rice. The reported P content of paddy growing soils in Kurunegala is 7 mg kg-1 but 92 % of the area in Peelalwala had P contents above 12 mg kg-1. The majority of the paddy fields in Peelalwala and Galeyaya had Mg contents above 400 mg kg-1 compared to 89-396 mg kg-1 reported by Mapa et al. (2005). The sodium contents observed in Peellawala and Galayaya were 156 and 167 mg kg-1 respectively, which is significantly higher than the Na contents of 46-69 mg kg-1 reported for other Kurunegala paddy growing soils. These results reveal an increase in all nutrients in the upper part of the irrigation area (Peellawala) where wastewater first enters the paddy fields. Concentrations of highly soluble K, Na and Mg congregate in the lower site while less soluble P and Zn are deposited in the middle site. It can be suggested from these results that regular monitoring of plant nutrient availability in the Wilgoda scheme will help in understanding nutrient supply in wastewater and its movement across irrigated fields. The differences in plant nutrients within a site were particularly evident in the map of spatial variability, suggesting that site-specific fertilizer recommendations should be made to achieve the maximum yield with limited fertilizer costs.