Effect of N uptake concentration on nitrate leaching from tomato grown in free-draining soilless culture under Mediterranean conditions

Free-draining soilless culture is commonly used in greenhouses in the Mediterranean Basin, frequently with tomato. Crop N uptake concentration has been proposed as a tool to assist with N management of crops in soilless culture. Three tomato crops were grown in free-draining rockwool to evaluate the relationship of crop N uptake concentration to crop N uptake and NO3− leaching. The three crops were 2005 spring, 2005 autumn–winter, and 2006 spring crops. Drainage fractions were 24% in each crop. Irrigation and drainage volumes were determined daily. N applied and leached were determined daily in the 2005 spring crop and otherwise weekly. Crop N uptake was determined by difference. Average N concentrations (NO3−–N plus NH4+–N) in nutrient solution were 9.6, 11.5 and 9.2 mmol L−1, and in drainage (N leached) were 14.8, 8.5 and 11.8 mmol L−1. The amounts of N leached were 178, 69 and 145 kg N ha−1. Crop N recoveries were 63, 82 and 69%; unrecovered N was lost by NO3− leaching. Average N uptake concentrations were 8.6, 12.5 and 8.4 mmol L−1. Maintaining the applied N concentration at slightly less than N uptake concentration for much of the crop was associated with a very high crop N recovery (82%) by the 2005 autumn–winter crop. In the two spring crops with lower crop N recoveries (63 and 69%), the average N uptake concentration was higher than the average applied N concentration. The highest N concentrations in drainage occurred when applied N concentration clearly exceeded N uptake concentration. These data demonstrated the potential for using crop N uptake concentration to assist with the formulation of nutrient solutions for soilless crops.