Copper Critical Levels and Supplying Powers of Takhli, Loei, and Khorat Soil Series for Corn Production

Abstract

Pot experiments were conducted to examine critical levels and supplying powers of Cu in Takhli, Loei and Khorat soil series, as well as reliabilities of two methods of soil analysis, i.e., extraction with 0.1 NHCI and extraction with DTPA. Samples of the three soils were applied with different rates of CuCl₂ and then repeatedly cropped with corn in small pots until a difference in corn dry matter was found between Treatments receiving different rates of Cu. Soil samples were taken before each cropping for analyses of available Cu by the two analytical methods mentioned above. Results showed that the three soils were not deficient in Cu. However, copper status of Takhli and Khorat soils were close to the critical levels. It was estimated that annual corn cropping could be practiced for at least 5, 37 and 2 years on Takhli, Loei, and Khorat soils, respectively before response of corn to Cu application could be obtained. Narrowest ranges of extracted Cu that covered critical levels for Takhli, Loei and Khorat soils were 0.40 - 0.45, 1.05 - 1.30 and 0.25 - 0.35 ppm, respectively when extracted with 0.1 N HC1 and were 3.0 - 3.3, 2.4 - 2.7 and 0.8 - 0.9 ppm, respectively when extracted with DTPA. Considering the three soils as one group, the narrowest ranges were 0.35 - 1.05 and 0.9 - 3.0 ppm for extractions with 0.1 NHCI and with DTPA, respectively. Narrowest ranges of Cu concentrations in corn plants were 5.6 - 6.8, 4.7 - 4.7 and 3.5 - 3.7 ppm for Takhli, Loei, and Khorat soils, respectively. Considering the three soils as one group, the narrowest range of Cu in plants was 3.7 - 5.6 ppm. It was concluded that extraction with 0.1 NHCI gave more reliable prediction than extraction with DTPA. However, differences in critical levels found for each soil suggested that Cu concentration in plants would provide more reliable prediction than amounts of Cu extracted from soils by the two methods.