Abstract:
Potassium (K) is an essential macro nutrient for growth and production of rice (Oryza
sativa L.). Exchangeable K fraction is the available form of K for plants. However,
exchangeable K concentration in rice growing soils can vary with climatic factors,
water management practices, cropping systems and soil properties. Hence, the
objective of this study was to determine the variation in lowland soil exchangeable K
concentration among six agro-climatic zones (ACZs), water supply systems (major
irrigation, minor irrigation and rainfed), rice-based cropping systems (banana,
vegetable, other filed crops and fallow) and soil orders (Vertisol, Alfisol, Entisol,
Histosol, Inceptis and Ultisol). Total of 1200 paddy soil samples were collected using a
stratified random sampling approach. Each sample consisted of six soil cores obtained
from the top 15 cm of the soil profile of a paddy track (Yaya) considering topographical
heterogenities. Exchangeable K in soil was extracted using 1 M ammonium acetate
and determined using flame photometer. Soil exchangeable K concentration was
ranged between 26.4 – 1133.8 mg kg−1 with a mean concentration of 240.1 mg kg−1
.
Exchangeable K concentration was similar among different ACZs (p >0.05). When
compare water sources, the highest and the lowest exchangeable K concentrations
were recorded in minor irrigation and rainfed areas, respectively (p <0.05). Rice banana cropping system had higher exchangeable K concentration than continuously
paddy cultivated lands (p <0.05). Further, the highest exchangeable K was observed in
Vertisol followed by Alfisol and Entisol (p <0.05). Among all the soil orders tested,
Ultisol reported the lowest exchangeable K concentration (p <0.05). In summary, most
of the paddy growing soils in Sri Lanka have sufficient amount of soil exchangeable K
(i.e., optimum K level in soil is 40 - 80 mg/kg) without causing K deficiencies to rice
plants. Exchangeable K concentration varied among water sources used, rice-based
cropping systems adopted and soil orders.
