Determination of limiting values of optimum copper supply of oat and barley plants.In order to determine the copper content required by oat and barley plants (limiting values), pot experiments with increased copper supply as well as plants of high yielding fields were analyzed. Because of the great influence of the nitrogen supply on the Cu‐requirement, the limiting values for Cu should always be related to the range of optimum N‐contents (30–40o N at the beginning of tillering and 12–20o at the appearance of the panicle).From the pot experiments the following limiting values in the dry matter of the aerial parts of oats can bei deduced: 5 ppm Cu at the beginning of tillering and 3 ppm at the appearance of the panicle. For (spring sown) barley the corresponding values are 5.5 and 3.5 ppm Cu.The limiting values obtained from the field samples are 4 ppm for oats and 5 ppm für barley (both valid for a yield level of 5 t/ha).In view of their higher yield level, the data of the pot experiments are considered the ‘theoretical’ lim
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