Plants use mineral nutrients, including nitrogen (N), to build and supply their cellular chemistry factories. The weight of dried flower is made up largely of these nutrients, plus oxygen, hydrogen and carbon. This gives rise to a prime directive for the flowering stage: Pour on the nutrients.
In addition to being a building-block material, nitrogen is also a constituent of amino acids, which are at the epicenter of organic processes. Whether amino acids are used to create form-giving proteins for new growth, or being moved to the site of the new growth to deliver their nitrogen cargo, the more amino acids produced (and the faster they are produced), the faster biomass is created. Growers, it seems, are really cultivating amino acids.
Driver or Shepherd?
The direct link between nutrients that are given and weight added makes it clear the grower’s goal is to drive as much nutrient uptake as possible. Anything that impedes nutrient delivery to flowers is counter to that goal. And while many growers are timid when it comes to nutrients, the best-yielding growers we’ve seen are consistently those whose media is crusted with dried salts in late flower.
Nutrients, water and time are the grower’s main tools. But, in a business context, time weighs nothing and costs a great deal, while nutrients add weight and are cheap. Remembering that calcium nitrate in a 50-pound bag ends up as 50 pounds of plant and flower biomass, growers don’t want their plant’s grazing - they want to drive every atom of weight-adding nutrients they can into the plant.
Fast and Slow, Long and Short
When managing nitrogen, it’s important to note that feeding with a lot of nitrogen produces faster-growing plants, often with dense vegetative growth, and a little nitrogen produces a smaller, more compact plant with less foliage. Armed with just this knowledge, growers can develop a nitrogen program that delivers plants with a shape, size and potential to deliver so much more yield in a given flower period.
Take, for example, the vertical-rack grower’s challenge to produce high-yielding plants small enough to fit inside the racks; or the grower who likes long branches for trellising and wants them to be supple; or the team that wants to eliminate netting hassles and wants stronger branches. Low nitrogen and precision pruning, and maybe more time, are possibilities for the vertical grow; higher levels rich in ammoniacal nitrogen during transition can deliver suppleness; and more compact plants can be on lower nitrogen doses during the vegetative period to produce self-supporting plants.
Unique situations aside, nitrogen’s control of growth rate affects every cannabis grower, every day. Rapid growth delivers longer internodes that push the flowering sites at each node farther apart, making it more difficult for buds to coalesce into large colas. The same can be said for bud density, as each flower site sprouts not a fruit, but the miniature version of the larger plant structure, and those buds react to nitrogen and growth rate in the same way the veg plant does.
A grower looking for dense buds and big colas should dial back the nitrogen a bit to shrink the internodes and draw the buds closer together. On the other hand, growing in a region where molds and yeasts can pop up at the drop of a hat, fast growth may be helpful by preventing big colas from forming, thus helping to prevent bud rot.
A grower looking for dense buds and big colas should dial back the nitrogen a bit to shrink the internodes and draw the buds closer together.
A Weighty Supporting Cast
Plants depend on more than nitrogen, and every nutrient they utilize plays a key role in the plant’s biological enterprise. Calcium aids in the growth of cell walls, magnesium completes chlorophyll, sulfur participates in numerous organo-chemical reactions, and potassium regulates internal cellular pressure. These minor elements act as catalysts for chemical reactions essential to plant biology. Along the way, all of these nutrients add up to a lot of weight, and phosphorus (P) and potassium (K) are the heaviest.
The obvious question is: How much of each of these nutrients are needed at each phase? A practical place to start is to look at successful nutrient products.
One popular premix product line’s recipes are represented in the chart to the left. Think of it as a feed schedule that has been translated into parts per million (ppm) of each nutrient.
The chart shows the ppm content of the major nutrients in each of the recipes we have tested. The veg recipe is used throughout the veg period; flower weeks 1 through 3 (F1-3) are the transition period; and the F6 recipe is used from week 6 until harvest or flush. Volume is measured in ppm, and the mix is expressed as the ratios of each nutrient to nitrogen. The veg formula mix is 1-.9-1.7. For every 100 ppm of nitrogen, the recipe delivers 90 ppm of phosphorus and 170 ppm of potassium. The veg recipe has high phosphorus levels compared to many recipes we have seen, but this aligns with phosphorus’s association with good root development. While the veg phosphorous and potassium levels are clearly elevated over the early-flower recipes, remember that veg light levels are as little as 50 percent of those in flower. So relative to the photosynthetic potential, the phosphorous and potassium levels in veg are very high.
Instead of varying nitrogen in different phases, this manufacturer uses a relatively constant nitrogen level across the plant’s life. This has to do with calcium, but the effect is that the nitrogen throttle has been opened up wide, and the plant is always capable of rapid growth. (Editor’s Note: Background on this and all aspects of cannabis nutrient management can be found in “Cannabis for Capitalists,” written by the columnists and downloadable from Amazon.) But nitrogen needs help for growth to happen, so the manufacturer in this example manipulates the supply of nutrients to match general needs in each phase. The 175-ppm nitrogen level supports rapid veg growth, super-rapid growth in transition and high yields. Phosphorous and potassium levels in the veg formula support root development and rapid veg growth, and the late flower recipe is where those minerals are poured into the plants. The plumping that happens when high levels of phosphorous and potassium are added shows just how much biomass nitrogen can drive when given access to unlimited supplies. That should inspire experimentation to try to find where the nutrient/yield limit is.
In addition to our suggestion that more is better, we also recommend raising phosphorous and potassium levels in week 4, or even 3, to start adding weight sooner and letting time work its magic. The plants are revved up and ready to go, so why wait? The phosphorous and potassium levels to use depend on the growing scheme. Media growers may want to start low and adjust the levels up until salt crusting appears on the media surface, but hydro growers can blast away.
Finding the exact amount of nitrogen and other nutrients that optimize yield, quality and cost remains a task for growers to perform on their plants under their own circumstances.
Mary, Mary Quite Contrary, How Do Your Nutrients Flow?
Regardless of the nutrient-delivery system used, yields track to nutrient levels. And the inescapable truth is that all cannabis nutrient systems need to deliver the same nutrient levels to produce the same yields. The ability to understand nutrient uptake allows growers to understand whether or not they can turn up the volume any more.
Finding the exact amount of nitrogen and other nutrients that optimize yield, quality and cost remains a task for growers to perform on their plants under their own circumstances. In finding that answer, growers learn how to get the most out of their nutrient dollar. Both container (media) and ebb-and-flow growers can use labs to understand the uptake of all nutrients or see nutrient buildup that may warrant a change in input formulations. Or when reservoir batches no longer deliver the appropriate nutrient levels, this signals replenishment or a fresh start. Once labs have established details of a system, quick electrical conductivity (EC) measurements and/or on-site tissue analysis are available for day-to-day feedback.
We recommend organic growers develop their systems with the help of media nutrient testing. Insight into nutrient activity in their media is very helpful because the gas pedal for organic growers is bio-activity. The more microbes available to convert organic compounds into plant-available nutrients, the more nutrients can be delivered to the plant. Growers can add or limit enzymes, sugars and other additives that affect the rate of bio-activity, and lab analysis of the media shows them how bio-activity affects nutrient delivery rates. Organic growers are amino acid farmers too; they just start a little further back in the process.
You can’t hide low or high nitrogen. Plants reliably reflect the grower’s timidity or boldness. A veg fertilizer test we monitored included sending media samples to the lab, and the results showed nitrogen levels running at 30 ppm, and magnesium and potassium levels grossly exceeded typical levels for that growth phase. Compare 30 ppm of nitrogen to the 175-ppm veg level we just examined, and you can guess how well the test plants did. After the recipes were tweaked, the numbers fell in line with typical veg levels and growth increased.
Without nitrogen, plants would have everything they need to exist except the life to assemble them. Use it wisely and be rewarded with increased control and higher yields.