Dialing in the nutrient profile of a marijuana fertilizer regimen is almost as complicated as optimizing the health of the human body. For that reason, the more we can understand and minimize the variables, the better we will get at growing consistently high quality cannabis.
The first major decision you will face is whether or not to use soil or some kind of circulating water system. Traditionally, this has been referred to as the choice between soil and hydroponics, but the distinction between the two is not as black and white as it might seem. Technically speaking, any grow medium that lacks decomposed plant or animal material (humus) is hydroponic, even if it appears like soil. On the flip side, compost tea can be added to inert media such as coconut fiber (aka “coir”) to create a planting mix that behaves like soil, with a thriving population of beneficial microorganisms.
Soil mixes of various kinds continue to gain popularity over the supposedly high-performance water culture systems, even as the commercial marijuana cultivation industry explodes. This is because soil is more forgiving and thus has a higher probability of achieving consistent professional results, where water culture systems can kill your plants overnight if a timer, pump, dripper or dosing system breaks down. In an era of million-dollar investors counting on growers to deliver, the risk of an equipment malfunction isn’t usually worth taking.
Even if you are just a home grower, managing a circulating water system adds layers of complexity that might not be worth the investment. You will have to maintain a nutrient reservoir within a narrow temperature range in order to keep your pH and oxygen levels stable and, under normal circumstances, the reservoir will have to be cleaned and refilled every 10 days. It’s not impossible, and it might in fact be the wave of the future, but it certainly takes more skill and practice than just watering your potted plants every few days.
As for fertilizers, we all want to be as organic as possible these days. We have seen enough evidence at this point that it’s good for humans and good for the earth. Plus, the organic label carries a premium in a competitive marketplace.
The main scientific distinction between organic and synthetic fertilizers is their solubility. Organic fertilizers rely on microbial activity in the soil to become available to the plant, where synthetic fertilizers are immediately available as mineral salts making direct contact with the roots. Think of it as being analogous to the difference between a human body absorbing food through normal digestion vs. through an IV directly into the bloodstream. Both methods have their advantages. Microbes in soil produce all kinds of helpful bio-stimulants for plants, and they have a certain kind of “intelligence” that keeps nutrient availability within reasonable parameters, just as your intestinal flora mediate the nutrients that enter your bloodstream.
With synthetic fertilizers, the plants react more immediately to being fed. There is no delay posed by the need for microbial action, and there is no concern that certain nutrients will remain unavailable due to incomplete biological activity. There is, however, a greater risk that the plants will get overdosed and suffer crippling damage, just as a human body would if high amounts of potassium or some other nutrient were pumped directly into the bloodstream. In addition, synthetic fertilizers are far more likely to leach out of the plants and pollute the environment.
Is it possible to reap the benefits from both forms of fertilizer? Yes, and you probably already do. If, for example, you buy a soil mix containing worm castings and bat guano, and then you add some kind of bottled liquid fertilizer, you are most likely working with both organic and synthetic fertilizers at the same time. It’s worth calling the fertilizer company to ask them about possible nutrient lockouts that can occur when you start combining formulas, but the most important thing to know is that too much mineral salt content is a risk to the health of your soil microbes. Keep the synthetic fertilizer quantity low in relation to the organic content. You can add certain bio-stimulants like kelp, humic acid, and amino acids to make a smaller amount of synthetic fertilizer go a long way.
Always test the pH and ppm (parts per million) of your water runoff to make sure the salts aren’t getting above 1500ppm and that the pH is staying between 5.7-6.3. This ensures happy microbes and effective nutrient availability. If your runoff is out of range, don’t waste a day fixing it. Soil is a very dynamic environment, and cannabis is a very dynamic plant. Even a few hours of extreme pH or nutrient excess can have a permanent effect on your crop, especially once you are in the flowering cycle.
Jennifer Martin is a pioneer in American cannabis cloning. Having supplied Bay Area marijuana dispensaries since the passage of Prop 215 in 1996, she helped bring over 2 million marijuana plants into the world. She also won the 1998 San Francisco Bay Area Cannabis Cup with the strain Bubbleberry, by a 32-point spread on a scale of 200. She has never entered a Cannabis Cup since due to her focus on cloning.