Determination of Critical Limit for Copper in Soil-Plant and Grain for Predicting Response of Rice to Copper Application in Soil

In order to identify critical limit of copper (Cu) in soil, grain, and straw of rice plant, soil samples were collected from twenty; 12, 5, and 3 soil locations of low, medium, and high Cu status from Madurai district, Tamil Nadu, India for pot culture experiment. Based on the results of pot culture experiment, the critical limits of Cu established for straw and grain of rice both by graphical and statistical methods were 8.07 and 5.22 mg kg(-1) respectively. Similarly, critical limits of soil available Cu as per the graphical and statistical methods was found to be 0.98 and 0.97 mg kg(-1) respectively. Rice plants were highly responded to Cu application in soils below this critical limit whereas soils with Cu greater than 0.98 mg kg(-1) did not respond. The grain and straw yield of rice increased with increasing levels of Cu and application of 1 mg kg(-1) Cu or 8 kg ha(-1) as copper sulfate was sufficient to alleviate the Cu deficiency and also to attain higher yield of rice in the district.