Five greenhouse experiments were conducted to determine adequate and toxic levels of zinc in upland rice, common bean, corn, soybean, and wheat. The Zn treatments were 0, 5, 10, 20, 40, 80, and 120 mg Zn kg-1 of soil. Relative dry matter yield of 90% was used as a parameter to define the adequate level of Zn applied or the Zn content in the soil and Zn in the plant tissues. Similarly, a 10% reduction in relative dry matter yield was used as a criterion for defining toxic levels in the soil as well as in the plants. An adequate level of applied Zn was 10 mg kg-1 for rice, 1 mg kg-1 for common bean, 3 mg kg-1 for corn, 2 mg kg-1 for soybean and 1 mg kg-1 for wheat. The toxic levels of soil applied Zn were 70, 57, 110, 59, and 40 mg kg-1 for the same crops, respectively. An adequate content of Zn in soil analysis by Mehlich 1 extractant for the rice was 5 mg Zn kg-1, 0.7 mg Zn kg-1 for the common bean, 2 mg Zn kg-1 for corn, 0.8 mg Zn kg-1 for soybean and 0.5 mg Zn kg-1 for wheat, where as by extractant DTPA the contents were 4 mg kg-1 for rice, 1 mg kg-1 for corn and 0.3 mg-1 for bean, soybean and wheat. The toxic levels of Zn in soil, depending on crop species varied from 25 to 94 mg kg-1 by Mehlich 1 and 25 to 60 mg kg-1 with DTPA extractant. Plant tissue analysis showed variation in adequate level from 18 to 67 mg Zn kg-1 and toxic levels varied from 100 to 673 mg kg-1 depending on crop species.
dry matter yield; Zn concentration in plant; Zn concentration in soil
