Latent vs Sensible Heat for Controlled Environment Agriculture

Proper control of temperature and humidity inside a grow room can be the difference between success and failure. Grow room temperature directly impacts leaf surface temperature, CO2 supplementation, relative humidity, transpiration rates and nutrient uptake – to name a few. Temperature control may appear as simple as setting a programmable thermostat; but of course, it’s more complicated than that.

Overheating forces plants to take up more water and then quickly transpire it, which can cause humidity overload when the lights go off. And a high relative humidity can lead to devastating problems like powdery mildew. During nighttime periods of plant respiration, lights are off, which typically reduces the grow room ambient temperature by 5-10 degrees. The temperature drop and high relative humidity frequently create a wet environment that approaches the dew point.

The leading strategies for controlling temperature and humidity in a grow room include ventilation and dehumidification, or some combination of the two.  Determining which strategy to implement depends on the prevailing type of heat in the room. When plants transpire, stomata open, releasing water vapor through an evaporative process that cools the leaf. Water molecules in the plant absorb heat and are converted to a gas – water vapor. Since there is a phase change during this process, the heat absorbed by the water molecules is defined a latent heat. The other type of heat is called sensible heat, which is heat that is either added or subtracted without a phase change.

For the case of an overheated grow room dominated by sensible heat, a simple method for lowering the temperature is ventilation. Ventilation can be used to lower both temperature and humidity. Passive ventilation techniques have been utilized for thousands of years, so the technology is proven and so are its limitations. The efficacy of ventilation can be subject to local climate conditions and can be challenging during CO2 supplementation.

Dehumidification should be considered when cooling a grow room with excessive latent heat and corresponding high humidity. Latent heat converters (LHC) transform excess water vapor into liquid, which dehumidifies the air and converts the heat of condensation (latent heat) into sensible heat that can be used to heat the environment when needed. Ventilation can be reduced, which lowers heating costs and allows atmospheric CO2 enrichment. In addition to converting “wet” heat to “dry” heat, an LHC can be a good source of clean, readily accessible water. Moisture from inside the grow room can be recycled and used again.

Electric lighting can be the dominant source of heat in a grow room. As we know, LEDs are more efficient than traditional lighting technologies – they convert more electrical energy to light and less to heat. Energy supplied to an LED that isn’t converted to light becomes heat that gets radiated into the air. In contrast, much of the heat generated by an HPS fixture is contained in the light beam in the form of infrared (IR) energy. The IR energy, which is absorbed by the plants, raises the leaf surface temperature and induces higher rates of transpiration. So, HPS lighting can lead to higher amounts of latent heat (via plant transpiration), while LED lighting creates more sensible heat by radiative heating. These differences can influence the strategies used to optimize temperature and humidity in a grow room.

Nutrient Strategies with LED Lighting

In order to produce healthy and vigorous plants, it’s important to understand how nutrient efficacy can be affected by upgrading to LED lighting. Growers are frequently surprised to discover the light intensity and uniformity on their canopies aren’t what they thought. High-pressure sodium (HPS) and ceramic metal halide (CMH) lights degrade in intensity fairly quickly. And in many cases, the intensity and uniformity aren’t up to par even with new bulbs. In contrast, a properly designed and installed LED solution should provide the right light intensity with excellent uniformity across the canopy. Further, the spectral content of LEDs is different from traditional lighting technologies, and the change in spectrum may result in a variance in nutrient uptake by the plant. So how does this impact your nutrient strategy?

Drip irrigation using water-soluble fertilizers, otherwise known as fertigation, is a common practice in controlled environment agriculture (CEA) – see figure below. The primary goal of fertilizing is to augment the nitrogen uptake in the plant, which should enhance fertility and productivity. Since nitrogen is a gas, ammonium nitrate (NH4NO3) is used in powder/crystal form and combined with water to supply nitrogen to the crop.

When upgrading to LED lighting with different spectrum, intensity and uniformity profiles, fertigation strategies may need to be modified. We know that as light intensity (PPFD) increases, transpiration typically increases. Higher transpiration rates will pull more water out of the fertigation solution – leaving behind a relatively high concentration of mineral ions from the ammonium nitrate (nitrogen salts) in the media or root zone. This can make it difficult for the plant to take in water and nutrients, thereby creating nutrient imbalances.

The best way to ensure the ideal fertilizer concentration is to measure the pH and electrical conductivity (EC) of the nutrient solution and in the growing media. Since mineral ions in the fertilizer solution conduct electricity, measuring the EC is a perfect way to determine if the right amount of nutrients are being used to meet the needs of the plants without over fertilizing. It’s important to measure both the nutrient solution and the soil/media near the root zone to determine the salinity level of each. An elevated EC reading means a high concentration of mineral ions (fertilizer). If the EC reading is too high in the nutrient solution, the solution should be diluted with additional water. If the EC is too high in the soil/media, it is best to perform a leaching operation to bring it back into balance. The figure below shows a typical meter that measures pH and EC.

Likewise, measuring pH of the soil/media is an indicator of salt build up. Ammonium-based fertilizers will acidify soil, meaning the pH will increase. For most commonly grown hydroponic crops, an optimal pH range is between 5.5 and 6.5.

There are a number of factors that determine the ideal EC value for a grow operation including, crop species, light intensity, temperature and humidity.  However, some general guidelines for CEA grown cannabis are shown in the chart below. From the chart, the electrical conductivity, and hence the nutrient concentration, increases with increasing maturity of the plant. An EC meter measures electrical conductivity in units of millisiemens per centimeter(mS/cm).

An incorrect nutrient concentration during fertigation can cause plant leaves to discolor, which is the result of a toxic effect commonly referred to as “fertilizer burn.” In the extreme, over fertilizing leading to a salt build up at the root zone can pull water out of the plant and cause it to wilt and die. To ensure an optimal harvest, the nutrient concentration may need to be adjusted to accommodate changes in light spectrum, intensity and uniformity from a lighting upgrade.