Níveis adequados e tóxicos de zinco na produção de arroz, feijão, milho, soja e trigo em solo de cerrado Adequate and toxic levels of zinc for rice, common bean, corn, soybean and wheat production in cerrado soil

Foram conduzidos cinco experimentos em casa de vegetação, para determinação das doses e dos teores adequados e tóxicos de Zn no solo e na planta para as culturas de arroz de terras altas, feijão, milho, soja e trigo. Os tratamentos com Zn foram de 0, 5, 10, 20, 40, 80 e 120 mg de Zn kg-1 de solo. Considerando 90% da produção relativa de matéria seca como parâmetro de determinação das dosagens adequadas de Zn no solo e na planta, recomenda-se para arroz, feijão, milho, soja e trigo aplicação de 10, 1, 3, 2 e 1 mg de Zn kg-1 de solo, respectivamente. Os níveis tóxicos de Zn no solo obtidos com base na redução de 10% na produção foram de 70, 57, 110, 59 e 40 mg Zn kg-1 de solo, respectivamente para arroz, feijão, milho, soja e trigo e conforme a análise do solo pelo extrator Mehlich 1, os níveis adequados de Zn no solo para as citadas culturas foram de 5, 0,7, 2, 0,8 e 0,5 mg Zn kg-1, respectivamente. Pelo extrator DTPA, os níveis adequados de Zn no solo foram de 4 mg kg-1 para o arroz, 1 mg kg-1 para o milho e de 0,3 mg kg-1 para soja, feijão e trigo. A toxidez de zinco no solo com Extrator Mehlich 1 variou de 25 a 94 mg Zn kg-1 e, com extrator DTPA, de 25 a 60 mg Zn kg-1, dependendo da cultura. Os níveis adequados na planta variaram de 18 a 67 mg Zn kg-1 e os tóxicos variaram de 100 a 673 mg Zn kg-1, dependendo da cultura.<br>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.