Nutrient solution flowing through the root zone picks up heat that may have to be removed for crops such as lettuce.
Photo: John W. Bartok Jr.

Root zone temperature is one of the major factors that affect root health and plant growth. The temperature of the nutrient solution is generally optimum between 60° F and 75° F. Some plants such as lettuce, alstroemeria and strawberries like it near the lower end, whereas tomatoes, cucumbers and cannabis do best near 75° F. Determining the optimum temperature for your crops is an important first step.

Problem: Nutrient solution is too cool

Cool solution temperature usually occurs in northern climates during the winter. Its major effect is that it slows growth. Research has shown that leaf numbers, leaf length and total fresh and dry weight can be affected. Low temperature can affect dissolved nutrients and physiological properties.

A cool solution usually occurs for several reasons. If the tank is located in the greenhouse or headhouse, the room temperature may be too low. Tanks that are in contact with the ground or buried can be cooled by contact with the 50° F soil. Also, supply/return piping will lose heat to the air in the growing area.

A first step is to insulate the tank and pipes. Spray-on foam, flexible sheet insulation and 1-inch thick pipe insulation will slow heat loss. On above-ground tanks, double bubble insulation available from home centers is a low-cost material that works well.

There are several methods of heating the solution to maintain the desired temperature. For small tanks, electric resistance heaters are inexpensive to purchase but more expensive to operate than a fossil fuel type heater. Natural gas or propane boilers are a good choice for larger tanks as they are available in many sizes and easy to install. As the nutrient solution is corrosive, a heat exchanger is needed to prevent damage to the boiler. PEX tubing or EPDM rubber tubing works well as a heat exchanger in larger tanks. An alternative is to use a swimming pool heater with a corrosion-resistant copper-nickel heat exchanger. Environmentally friendly heaters such as a heat pump or solar collector system have also been used but are generally more expensive to install.

To size the heater, first determine the tank volume in gallons (cubic feet x 7.48 gal/cu ft = gallons). Then calculate the quantity of heat needed (tank volume (gal) x 8.33 pounds/gal x temperature rise in degrees F = Btu of heat required). The difference between the desired nutrient solution temperature and the solution returning to the tank from the growing beds can be used as a guide. The output from the heater/boiler (Btu/hour) should be at least this large.

Maintaining the optimum air temperature with evaporative cooling, shade screens and tank insulation can help to prevent overheating of the nutrient solution.
Photo: John W. Bartok Jr.

Problem: Nutrient solution is too warm

A nutrient solution that is too warm is more likely to be the problem, especially in the summer and in warmer climates. Conditions that occur from warm solution include plant heat stress, wilting, fruit abortion and buildup of harmful bacteria, fungi and rootborne insects. At temperatures above 75° F, dissolved oxygen levels dip below 8 percent and evaporation of the solution can occur, resulting in a higher concentration of nutrients.

Again, the first step is to insulate the tank and pipes. Also, maintain an air temperature in the growing area close to the desired level. This can be done through external shading, internal shade screens, fan and pad evaporative cooling and fog. If supplemental lighting is being used, operate it only at night when the air is cooler. When adding water to the nutrient solution, use cold water.

Water chillers that remove heat from the solution are the most common equipment used. The excess heat in the solution is transferred to water or glycol in the heat exchanger in the tank. The heat exchanger can be a stainless steel fin pipe, PEX or rubber tubing. The solution is pumped to the chiller, where the heat is removed and sent back to the nutrient tank to get warm again. The refrigeration system expels the heat to the outside air. It requires about 1-1/2 horsepower to provide one ton of cooling (12,000 Btu/hour).

An alternate system is to use geothermal energy with buried heat exchanger pipes transferring heat from the solution to the cool 50-55°F soil 6-8 feet below ground. This saves energy as only a pump is needed. Another system is to circulate cool ground water from a well or municipal system through PEX, rubber or poly pipe placed in the tank. The water is returned to a well or discharged to a drainage area. The heat exchangers in these systems need to be sized for the quantity of water in the tank and the temperature difference.

Nutrient temperature is important for good plant growth. You can avoid failures or poor yields by maintaining the temperature between acepted limits.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com