Input and output of minerals for a small watercress bed fed by chalk water.
The input and output of water, Na, K, Ca, P and N were measured over a 14 month period in a 45 m2 watercress bed containing 6927 kg of a soil/gravel mixture and fertilized with 69.2 g/m-2 of "Lawes Complete" fertilizer applied at sowing and with 4 applications each of 138.4 g/m-2 superphosphate, 1 application of 69.2 g/m-2 superphosphate and with 5 applications of 138.4 g/m-2 of Fisons "High K" fertilizer. Evaporation and transpiration were negligible components of the water and nutrient budgets. The input borehole water and the water flowing out of the bed were the most important means of entry and departure of Na, K, Ca and N. About 75% of the P input was as fertilizer and >10% of the output as cress material. Disturbance of the bed through fertilizing and harvesting caused large fluctuations in the concentrations of elements in the put-flowing water over periods of 10-24 hours following the disturbance. P content of output water reached 100 ppm after fertilizer application. The total outputs of Na, K, Ca and N in solution were similar to the input in solution but the P output in solution was about 14 times the input as borehole water.