Nutrient Delivery and Monitoring

Special Report - Special Report: Smart Nutrients

Find and maintain a balance that’s optimum for your cannabis crop.

December 5, 2017

Allow me to start with a prediction: Within four years, prototype camera systems will be able to assess a crop canopy’s nutritional status, using hyperspectral cameras that analyze thin slivers of the visible spectrum, and adjust fertilizer injectors and irrigation systems practically in real time.

If that seems far-fetched, consider that handheld Soil-Plant Analyses Development (SPAD) meters—that measure leaf greenness, quantifying differences invisible to the naked eye and providing a rough correlation with nitrogen content—have been available for years. Growers will be the ones who calibrate these camera systems—not the engineers who build them—so migrating to data-driven decision making now will provide you a competitive advantage.

In the meantime, this article provides insights into the fertilization of solid root substrates (rockwool, coir, etc.) using inorganic salts, though some growers successfully use organic sources. Many of the concepts apply to all formulation types. Scheduling and fertilizer-application decisions begin with effective monitoring.

What to Monitor

Root substrates should be tested for pH and electrical conductivity (EC) at least every two weeks using a non-destructive “pour-through” technique. Graph these results. You’ll learn the trends that develop over your crop’s growth stages. Also, occasionally track this data every few hours following a fertilization. You’ll be surprised how quickly the plant takes up fertilizer in just 24 hours. Adjust fertilization accordingly to maintain your desired pH and EC, based on crop stage and your experience with the cultivar. The fertilization schedule will vary depending on sunlight and temperature in a greenhouse or outdoor setting, but will maintain more stability in controlled environments.

You can determine a powerful, data-based understanding of your crop’s nutrient status by developing a graph that compares laboratory testing results for individual nutrient levels overlaid with your routine pour-through tests. Substrate testing by an external lab is expensive, with tissue testing even more so. For cost effectiveness, track soil and tissue nutrient content regularly for the first two crops in a new grow system, then annually after that. Tissue and soil samples should be taken every two weeks, minimum. Your end goal is to create a “hospital chart” hanging near the crop for the whole team to refer to, with actual measurements plotted over time and desired ranges clearly indicated. This can effectively facilitate consistent nutrition across crops and multiple growers, and in multiple facilities.

Application Methods

Water-soluble fertilizers are best delivered using a fertilizer injector, which doses the correct proportion of a concentrate into hoses, dripper lines or sprinklers. Of course, injectors can also be used to fill a hydroponic or ebb-and-flood reservoir. They are water-driven, so don’t require electricity. Injectors should be sized based on your anticipated flow rate: Exceeding an injector’s flow capacity causes it to seize up, and a sub-minimum rate results in inaccurate dosing.

Larger, more expensive units measure the flow rate to dose most accurately, have a 15- to 20-year lifespan, and can be integrated into environmental control systems. They can also be integrated with pH and EC probes plumbed directly into the delivery pipes for monitoring, feedback and alarming.

Smaller devices can be mounted near the crop or on a dolly for portability. Their lifespan can be more than five years if protected from direct sunlight and flushed when taken out of use. Whether fixed or portable, it’s effective to have a bypass on or plumbed around the injector for applying domestic water without nutrients.

Some units have a fixed dosing ratio, while more versatile ones have adjustable settings. Electronic solenoids can be integrated for automating the system, if you have an irrigation controller. You also can attach a battery timer to the water spigot that supplies the injector.

A good rule of thumb for watering volume or duration: You can’t overwater containerized plants by applying too much at one time. Once the substrate reaches container capacity, any added solution runs out the drainage holes. You can only overwater by not allowing the substrate to dry properly between irrigation events.

Apply fertilizer solution until water pours out the bottom of the pot. This leachate should be at least 20 percent of what was applied. If you add less, fertilizer salts will accumulate in the pot. This can lead to root damage. Irrigating to a 20-percent leach fraction keeps a proper nutrient balance in the substrate, allowing for consistent availability and optimum nutritional status.

By Robert Eddy, M.S., a hydroponic grower, system/process developer and builder, and controlled environment agriculture (CEA) consultant.