Evaluation of ceramic cup samplers to measure nitrate leaching in the field

Three types of ceramic samplers were used to study solute transport in the root zone of a wheat crop. Soil solution was collected by applying either a suction (Coinda and Soilmoisture) or by diffusion through the ceramic wall (Tensionic). The Tensionic instrument allowed sample collection to -60 kPa, whereas the suction cups could sample only to -40 kPa. The hydraulic conductivity of the ceramic had little influence on the volume of sample collected by suction. The Tensionic provided an integrated estimate of soil solution composition over several days, whereas the suction cups effectively characterized the daily composition. No significant differences in nitrate concentrations were observed between the two suction samplers. The Tensionic instrument gave smaller nitrate concentrations than those collected by suction because it integrated over time. However, the change in soil nitrate was similar from the two types of samples to that extracted from soil by KCl, thus making them both suitable for monitoring nitrate leaching under field conditions with acceptable accuracy. The amount of nitrate lost below 25 and 90 cm was determined as 29.3 and 3.3 kg NO3-N ha(-1) respectively when 75% of the annual leaching occurred.