Tomatoes, electrical conductivity and temperature

Expect problems with tomatoes if you are not able to control the temperature and root zone Electrical Conductivity (E.C.). Many Alberta growers buy 30 to 40 day-old tomato seedlings from B.C. These seedlings grown in 4 inch rockwool cubes, will have 5 to 7 set of leaves and first flower buds may be just visible by the time they arrive in late December or early January. These seedlings are grown under artificial light, when they arrive in your greenhouse they are exposed to natural light conditions. Under the less than 8 hours of daylight and low angle of sun, growers use two strategies to control the stretching of tomato seedlings.

1. raise the E.C. of the fertilizer solution over 5 millimhos using potassium sulfate
2. maintain the same day and night temperature, generally around 20°C.

The higher E.C. helps to restrict the uptake of water in the root zone. This is important because of the amount of assimilates manufactured in the leaves is limited due to the low light levels. The combination of limited assimilates and unlimited water and nutrients forces the plant to allocate more food for top growth, therefore the internodes elongate and the leaves are thinner.

"My tomato plants seem to be growing too tall, too fast. The fruit clusters are far apart and the first fruit on the bottom truss is not yet turning. The plants need to be lowered. I am feeding 75 ppm of nitrogen, 125 ppm of phosphorus and 290 ppm of potassium from a ready made fertilizer. Could you offer some advice please?"
This is what is happening with your crop.

• The difference between the day and night temperature is more than 5 to 10 degrees. Either night is too cool or day is too warm.
• Keep temperature around 20°C day and night.
• Raise the E.C. of the nutrient solution between 3.5 and 4.0 millimhos using potassium sulfate fertilizer (0-0-52). The greenhouse grade of this fertilizer is more soluble than the field grade.
• Make sure that you are using calcium nitrate along with the ready-made fertilizer.
• Additional quantities of magnesium sulfate may be needed because most of these fertilizers don't have adequate amounts.
• When fruit sizing begins, reduce the E.C. to around 3 millimhos and start adding additional amounts of potassium nitrate (13-0-46 ) to supply the potassium needed for fruit sizing.

M. Mirza, CDCN
Greenhouse Coverings - December 1999