When it comes to the age-old debate over whether it’s better to grow cannabis in soil, or to grow the plant hydroponically, what we’re really talking about in 2017 is outdoor versus indoor growing. Why, you might ask? Simply put, it’s because no one actually grows indoors using real topsoil anymore, as our indoor “soils” are really soilless mixes that are coco-, peat- or sphagnum-based. And, technically speaking, these soilless mixes are, by definition, hydroponic mediums; today, the only real earth soil used is in outdoor gardens. So let’s examine the pros and cons associated with both growing outdoors in soil and growing indoors in hydroponic mediums.

Indoor hydro growers like this NorCal operation use “soilless” mediums. (Photo by Nico Escondido)

Yield vs. Quality

Aside from the actual medium, the heart of this debate boils down to quality versus quantity. Granted, some of the best outdoor marijuana can surely hold its own against indoor marijuana, but more often than not, the level of quality in indoor weed easily trumps outdoor ganja.

Still, when it comes to yield per plant, the outdoor farmer will usually win hands down. The deciding factor here is obviously space: Outdoor gardens are virtually unlimited when it comes to plant heights, and it is not uncommon to see marijuana trees ranging in size from 10 to 15 feet tall. Plants on this scale can yield anywhere from 5 to 10 pounds apiece.

Of course, to pull this off, outdoor growers need to start as early as possible: Right after the last frost of winter, when the ground begins to thaw and spring is in the air, growers need to be ready to get their plants in the soil. This requires seedlings or clones to be started indoors four to six weeks prior, which means that the process begins for many outdoor growers as early as February.

Outdoor grows produce heavy colas and big yields. (Photo by Nico Escondido)

Once the plants are in the ground, outdoor farmers have up to seven months of hard labor before harvest time begins. Irrigation, pruning, training and trellising consume hours upon hours each day. At the peak of flowering, each outdoor plant can require an hour of maintenance per day—and with dozens, if not hundreds, of plants in the garden, the man-hours and labor costs can really add up. While the yields come fall may more than make up for these costs, if the time isn’t properly spent, the quality of the harvest will suffer.

Conversely, indoor growers tend to grow squat, bushier plants, most often in automated hydroponic systems. These systems lend themselves to smaller plant sizes but can easily accommodate hundreds, even thousands, of plants in a commercial-garden setup. Hydroponic systems are well-suited for what is typically known as a “sea of green” (SOG)—or, more commonly, a “screen of green” (ScrOG)—setup, in which smaller plants are packed tightly together and grown through a screen trellis rolled over the garden canopy during the vegetative stage. Similar in theory to outdoor trellis systems, indoor screen trellises are usually horizontal and consist of string netting in 4-inch squares. Outdoor trellis systems, by contrast, are often vertical and wrap around the entire exterior of the plant; they can be made from wood, metal or wire screening that surrounds the plant like a cylinder, allowing the branches to grow laterally through the support structure.

By growing many smaller plants indoors, growers can attempt to boost their yields, but these still don’t compare to outdoor yields on a per-square-foot basis. However, it’s with the quality of the herb that indoor growers make up the difference. Because indoor growers can control every aspect of their plants’ environment, they are better situated to maximize each strain’s genetic potential.

Whereas outdoor growers have some control over watering and nutrient-feeding schedules, indoor growers can not only control irrigation and nutrient programs, but also the garden’s temperature and humidity, CO2 saturation, light cycles, and pest and disease controls. These factors can be crucial in determining whether or not a plant achieves its full potential in terms of cannabinoid and terpene production.

Indoor vs. Outdoor Mediums

As previously stated, the original debate over soil versus hydro has become a bit antiquated due to the fact that no one uses real earth topsoil indoors anymore. That being said, topsoil is still used in outdoor gardens today; however, even this is declining in frequency, as more outdoor farmers turn to composted mediums that combine smaller amounts of topsoil with decomposing organic material to create a fuller, nutrient-rich medium.

Outdoor container boxes hold composted soil for huge plants. (Photo by Nico Escondido)

Composting is a deliberate activity that growers undertake with the goal of accelerating what occurs as a matter of course in nature—i.e., rot, or the decomposition of organic matter. This affects both the physical and chemical properties of the soil, improving the mineral content as well as aeration and moisture retention within the medium. In soil, compost acts as a slow-release fertilizer, adding both major and minor mineral elements to the medium throughout the plant’s life cycle.

While composted soils are the more popular choice for outdoor growers these days, indoor growers are relying more and more on amended soilless mediums. These soilless mixes look, feel and act very similar to traditional soil, but they are coco-, peat- or sphagnum-based substrates that have added amendments like perlite, wood chips and vermiculite. As we know, hydroponics (or more accurately, water culture) is the method of cultivating plants without soil. So even though these mediums look and feel like soil, they are technically soilless—which, by definition, makes them hydroponic substrates.

Of course, soilless mixes are not right for every type of indoor hydroponic system. For example, the popular flood-and-drain setups that use flood tables and drain the nutrient solution back into a reservoir are not ideal for loose, granular soilless mixes. Instead, a flood-and-drain setup needs a more stable, immobile medium like rockwool or hardened, expanded clay (HEC) pellets. Soilless mixes are ideally suited for another popular type of hydroponic setup: top-feed grow systems.

Mineral wool has been used for decades in hydroponic systems. (Photo by Nico Escondido)

Top-feed hydro systems rely on drip or spray emitters that are staked directly into the soilless medium at each plant site. These emitters are fed via thin spaghetti tubes running off the main water lines from the reservoir. Emitters drip or spray either pure water or nutrient solution into the medium at regular intervals controlled by timers on the reservoir pumps. These types of top-feed hydroponic gardens are among the most popular systems in use by indoor growers today, because the soilless mediums are very forgiving in terms of the mistakes that can be made in feeding nutrients to plants (more on this later). The more traditional hydro mediums, such as rockwool, HEC, and stand-alones like coco fiber or perlite, offer little to no protection for delicate roots when overfeeding occurs. This is why soilless mediums are often recommended for beginners and for smaller hydroponic setups outside of commercial gardens.

Mediums & Plant Nutrients

The plant nutrients that we deliver into our medium are simply mineral powders or dissolved minerals in water (i.e., the nutrient solution) that contain three basic elements: nitrogen (N), phosphorous (P) and potassium (K). Other soluble trace amounts of minor elements like calcium, magnesium, iron and manganese are also included in fertilizer products, but the main three are always listed as an NPK ratio on the front label.

HEC holds down rockwool plugs in a flood-and-drain system. (Photo by Nico Escondido)

When it comes to nutrient feeding, more is definitely not better. After the first month of growth, a plant’s uptake of nutrients declines drastically, especially in terms of nitrogen. This is why, for cannabis growers, there are usually two-part formulas (A and B) for the vegetative stage and the flowering stage of the plant’s life cycle. The medium used in the garden plays a big role in the plants’ ability to absorb nutrients. At the root level, this ability is governed by the cation-exchange capacity, or CEC (which is similar to—but not the same—as electrical conductivity, or EC, which is measured by a ppm meter).

The CEC dictates the ability of a medium to hold moisture and nutrients for the roots to use when needed. Because of this, the CEC also affects the pH levels of both the medium and the nutrient solution being applied. The pH level is a critical factor in hydro systems, because it governs the outcome of an electrical battle between the roots and the medium. Mediums such as rockwool or HEC have an electrical charge of zero, making them electrically neutral and allowing them to very easily release water and nutrients.

Soil helps provide needed nutrients for supercharged plant growth. (Photo by Nico Escondido)

However, a low or neutral CEC can be very dangerous to a plant, as it allows the roots to attract strong minerals directly into the root zone. This easy access to the roots is known as a loss of buffering and can lead to nutrient burn in the roots. Most hydroponic growers use nutrient solutions derived from concentrated minerals, so it’s recommended that in gardens with a low-buffering medium, growers use mild nutrient formulas to start—especially if you’re a beginner grower.

Additionally, when using hearty mediums such as a soilless mix that offers much better buffering, there are still dangers in overfeeding. An abundance of salts in the medium can lead to nutrient lock-up in the root zone, prohibiting plants from any nutrient uptake. Most synthetic fertilizers have higher salt contents than organic fertilizers, and these salts can build up residually over time and change the pH of the medium, preventing nutrients from being absorbed by the root system and causing the plant to dehydrate due to a lack of moisture uptake. This is another reason why mild organic-nutrient lines are often recommended for beginner growers.

Organic and “veganic” nutrients are gaining popularity both indoors and out.

Understanding EC in Soil & Hydro

As mentioned previously, a ppm (parts-per-million) meter actually measures the electrical conductivity (EC) of a nutrient solution or medium. The ppm meter works by measuring how quickly electrons work their way from one probe to the other within a solution or medium. In nutrient solutions or grow mediums, salts equate to “stepping stones” for the electrons to travel faster, so solutions or mediums with higher salt concentrations (build-up) will have higher EC readings. Conversely, distilled water with no salts will have an EC of 0.

Soil probes test for moisture content, EC (Photo by Nico Escondido)

Since all nutrients contain salts, the more nutes that are present in a solution, the higher the ppm level will be. And the higher the ppm level, the more the salts will steal water away from the plant. This can lead to plant dehydration and nutrient lock-up. A solution of 1,500 ppm has high salt concentrations; at 2,000 ppm, roots will actually struggle to absorb enough water as the osmotic pressure increases. The optimal ppm levels for healthy growth range from 600 to 1,200 ppm, or an EC range from 0.9 to 2.4.

Indoor vs. Outdoor

When all is said and done, the advantage an outdoor garden offers over an indoor one really comes down to yield. However, the advantages that indoor growing offers are twofold: First, the quality can be much higher if done properly, and second, the grow cycles are usually slightly faster.

This latter advantage comes down to system type (hydro) and the medium used. This is because roots prioritize their intake in the following order: oxygen, nutrients and water. With oxygen being the surprise winner of the three necessary elements, we are reminded that roots, in fact, breathe in oxygen while the rest of the plant aboveground breathes in CO2.

The quality of this indoor Cheesecake comes from higher cannabinoid and terpene content. (Photo by South Bay Ray)

When you consider this fact, it makes more sense that hydroponic mediums that are less dense and more “airy” can aid in faster development. This is also why many hydroponic growers aerate their reservoirs and amend their soilless mediums with additives like wood chips or vermiculite, as they help air penetrate the medium and latch onto other particles for absorption by the roots.

Still, there is plenty to be said in favor of sun-grown outdoor cannabis, cultivated deep in organic mediums with composted nutrients. These plants may not hit 28% THC, but they’ll be close enough and might even taste a tad better. So whichever way you decide to grow, remember to grow the right way and help the world grow, too.

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