Available Fe in soil
After the rhizosphere soil was air-dried, 10 g of soil was weighed and added to 20 ml of diethylenetriaminepentaacetic acid (DTPA) extracting agent, containing 0.005 M DTPA (CAS No. 67-43-6), 0.01 M CaCl2, and 0.1 M triethanolamine (TEA; CAS No. 102-71-6). Then, the mixture was shaken for 2 h at 180 rpm at 25°C. Subsequently, the solution was filtered and the Fe concentration in the filtrate was measured by ICP–OES.