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.