Martin P. N. Gent
Dept. of Forestry and Horticulture,
The Connecticut Agricultural Experiment Station
POB 1106, New Haven, CT 06504

Keywords: lettuce, nitrate, electrodes, monitoring

Abstract:  Nitrate concentration was modified in the nutrient solution fed to lettuce seedlings and changes in the nutrient solution and in plant tissues were monitored constantly. A standard nutrient solution was compared to a solution with 1/3 less nitrate. Solution volume, pH, conductivity, and nitrate concentration were monitored. Four hydroponic systems, each feeding about 200 plants, were measured and controlled by a single data-logging computer. Feed-back control of nutrient solution composition used injection of nutrient concentrate based on the resistance of a conductivity cell, and injection of water based on a float switch. Nutrients concentrates were injected through a peristaltic pump in 3 mL increments. Water injection was restricted  to 120 mL/minute, when it was required, by lengths of fine-bore tubing . The volume in the solution reservoir was about 100 Liters. Nutrient solution composition remained stable over a one month period if the following precautions were followed:

1. The nutrient reservoir was stirred to achieve a rapid response from electrodes and conductivity cell.
2. Temperature compensation was used to correct for diurnal variation in solution temperature.
3. A slow, controlled leakage of solution provided a turnover rate of 30% per week.
4. The Potassium to Nitrogen ratio in solution was increased from 1.3 to 1.8 to prevent depletion.
5. Signals from pH and nitrate electrodes were pre-amplified before transmission to the data-logger.

A simple design for a preamplifier is presented.

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