THE DECLINE IN THE AVAILABILITY OF COPPER FERTILIZER APPLIED TO YELLOW BROWN LATERITIC SAND FOR THE GROWTH AND UPTAKE OF SUBTERRANEAN CLOVER

Abstract

In a glasshouse experiment, subterranean clover (clover, Trifolium subterraneum L. cv. ‘Dalkeith’) was grown on a yellow brown lateritic sandy earth where three levels of copper (Cu) had been applied in a field experiment. The soil was collected from the Cu application of 0.69 (Cu1), 1.38 (Cu2) and 2.06 (Cu3) kg Cu ha−1, originally applied as Cu sulfate (25% Cu), from the field site where the Cu had been in contact with the soil for 3, 6, 9, 12, 15, 18, 21, 24, 27, and 30 years. The Cu2 level was the recommended Cu application when the soil was originally cleared of indigenous vegetation. The availability of Cu for uptake by clover (cv. ‘Dalkeith’) plants declined linearly with time over 30 years of contact between the soil and the applied Cu for a yellow brown lateritic sandy earth in a semiarid Mediterranean environment. A steady decline in both the Cu concentrations of the youngest open blade (YOB), and the Cu concentration in dry whole shoots of clover resulted with increasing time of soil contact. The Cu content in shoots of clover plants increased with Cu application and declined with the length of time since the Cu had been applied. For the Cu1 applied, and the Cu2, the dry weight of shoots (DWS) declined with the length of contact between Cu and the soil, particularly where Cu was in contact with the soil greater than 18 years. For all Cu additions, a linear model was fitted to the decline Cu concentrations in YOB and dried shoots with time. This model predicts that Cu deficiency would be observed about 19, 28 and 40 years after the application of the 0.69, 1.38, and 2.06 kg Cu ha−1 at this site. The YOB and the remainder of shoots (ROS) of clover were used to define critical concentrations of Cu for the yield of dried shoots. The critical concentrations of Cu in the YOB at the 45 days after sowing were about 3 mg kg−1 for DWS and about 4 mg kg−1 in the ROS of clover. The critical DTPA extractable Cu for 90% of the maximum clover yield was 0.21 mg kg −1 soil.