Input and output of minerals for a small watercress bed fed by chalk water

(1) An outline balance sheet for water and the elements sodium, potassium, calcium, phosphorus and nitrogen was constructed for a small watercress bed at Doddings Farm, Bere Regis, Dorset. The bed had an area of 45 m2and was fed by borehole water. The input in borehole water, soil, precipitation, fertilizer and cress material was measured and also the outputs in evaporation, water outflow, cress, animal material and soil. (2) During the period 20 March 1967-4 July 1968 109 kg dry weight of cress material was taken from the bed and 37.5 kg was marketable. (3) Evaporation and precipitation were negligible components of the water and element budgets. The input borehole water and the water flowing out of the bed were the most important means of entry and departure of sodium, potassium, calcium and nitrogen. About 75% of the phosphorus input was as fertilizer and more than 10% of the output was as cress material. (4) Disturbance to the bed through fertilizing or harvesting caused large fluctuations in the concentrations of elements in the out-flowing water over periods of 10-24 h following the disturbance. The phosphorus content of the output water reached peak values of up to 100 ppm after fertilizer application. (5) The total outputs of sodium, potassium, calcium and nitrogen in solution during the period of observation were similar to the inputs in solution but the output of phosphorus in solution was about fourteen times the input as borehole water. Water from watercress beds might therefore have a significant effect upon the phosphorus economy of the streams into which it is discharged.