COMPOSITION CHANGES IN THE NUTRIENT SOLUTION DURING THE GROWTH OF PLANTS IN RECIRCULATING NUTRIENT CULTURE

The decrease in concentration of the three nutrients was very rapid in tomatoes (with 2 l of nutrient solution available per plant), particularly for potassium, which usually decreased from more than 200 ppm to 0 ppm within two or three days. The decreases in the nutrient solution of cucumbers were much slower (even though their daily uptake per plant was higher) because of the larger volume of nutrient solution per plant (8 l). It took about 5 days for potassium to disappear completely. In the experiments with lettuce, with 1.15 l per plant, concentrations changed very slowly, particularly in the middle of the winter.

The average uptake by tomatoes during the main period of growth was around 100 mg nitrogen, 35 mg phosphorus and 180 mg potassium per plant per day. For cucumbers these figures were approximately 350 mg N, 100 mg P and 400 mg K. The average figures for lettuce varied with the time of year and were in the following ranges: 8–16 mg N, 2.5–5 mg P and 11.5–23 mg K per plant per day.

The total uptake per plant over the whole growing period, as calculated from the amounts of fertilizer applied, was 10–12 g N, 4–5 g P and 18–22 g K for tomatoes, about 40 g N, 12 g P and 48 g K for cucumbers and approximately 0.45 g N, 0.15 g P and 0.65 g K for lettuce.

Figures such as the above, but adapted to local conditions, should be used in determining the size of the receiving basin relative to the number of plants. The larger this size is, the easier it will be for the grower to keep sufficient nutrients in the basin at all times. One litre of nutrient solution per lettuce plant would be more than adequate, but for crops such as tomatoes and cucumbers at least four litres per plant would be advisable.

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