Understanding the role each element plays in plant growth, and how identifying correct proportions can elevate quality, consistency and yield
By Jennifer Martin
Most cannabis growers own a whole range of nutrients and additives they include in their fertilizer blends. Many just follow advice from manufacturers, their colleagues or online forums, without actually knowing what effects these various substances are having on their plants.
Unfortunately, most channels lack the scientific research for professional results, and the competitive marketplace is now demanding a more sophisticated understanding of individual nutritive elements and their ideal proportions.
Like other plants, cannabis uses hydrogen (H), carbon (C) and oxygen (O) from the water and air, as well as 14 elements absorbed through the root system — nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), chlorine (Cl), iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu), molybdenum (Mo) and nickel (Ni).
Nitrogen, phosphorus and potassium are often referred to as macronutrients. Calcium, magnesium and sulfur are generally considered secondary nutrients, and the rest are classified as micronutrients. As it turns out, almost all plants use more calcium than phosphorus (“Reference Sufficiency Ranges for Plant Analysis in the Southern Region of the United States,” published by North Carolina Department of Agriculture and Consumer Services Agronomic Division, January 2013).
Phosphorus consumption, even during the flowering phase of growth, is more on par with magnesium and sulfur than it is with nitrogen and potassium, so the traditional distinction between macro- and secondary nutrients is not very important. However, it is important to match the ratios of all the elements as closely as possible with the usage requirements of your plants, so as to avoid health-compromising imbalances, unnecessary flushing and overly frequent reservoir changes. This also helps the push toward more consistent, higher quality yields.
How can we know the ratios of elements that cannabis prefers? Through regular lab tests and comparisons of nutrient input, soil and plant tissue “sap.” To date, the bulk of tissue sap testing for cannabis has been sponsored by nutrient companies seeking to further the development of their own products. However, peer-reviewed research on sap samples from a wide variety of other plants show surprisingly similar profiles across the spectrum of photosynthetic species (“Sampling Plant Tissue for Nutrient Analysis,” G.J. Schwab, C.D. Lee, and R. Pearce, University of Kentucky College of Agriculture, 2007). The ratios referenced in university studies are also very similar to those being circulated by cannabis nutrient companies. This lends credence to the reasonable conclusion that, at least in a fundamental way, plants are plants and have the same point of departure when it comes to customizing nutrient formulas.
Cannabis mainly differs from other species in its rate of growth, which requires a higher level of vigilance for optimal feeding throughout the lifecycle. Finding an ideal regimen would follow the same course of nutrient, soil and leaf tissue testing that is already commonly used in agricultural crop production.
Elemental Groups
You might think either soil or tissue sap testing alone would reveal ideal elemental ratios, but research has shown that plants have some capacity to store extra nutrients, possibly showing depletions in soil or nutrient solutions, with simultaneous excesses occurring in the leaves, stems or flowers (“Nutrient Management in Recirculating Hydroponic Culture,” Bruce Bugbee, Crop Physiology Laboratory, University of Utah, 2004). This is particularly true with the primary group of elements — nitrogen, phosphorus, potassium, manganese and chlorine —known as the “mobile elements.” Because these elements can move quickly through the plant tissues, they also easily reach toxic levels before any of the others. Deficiencies of these elements occur first on lower leaves, because when plants can send elements to new top growth, they will.
The second group of nutritive elements is considered “semi-mobile.” These include sulfur, iron, magnesium, zinc, molybdenum and copper, and are taken up more-or-less as needed by root systems, without much concomitant storage (“Nutrient Management in Recirculating Hydroponic Culture,” Bruce Bugbee, Crop Physiology Laboratory, University of Utah, 2004). Deficiencies of these nutrients can occur on various parts of the plants.
The final group includes the “immobile” elements such as calcium and boron. In the case of these nutrients, solubility and uptake are minimal, meaning plant tissues are less likely to store excessive quantities. They might even show deficiencies when adequate amounts are being provided, especially in high humidity environments. Calcium and boron deficiencies show up on new plant growth due to their immobile nature; they cannot easily translocate to newer, more important parts of the plant. Decreased ambient humidity supports better calcium and boron uptake, because, as the plants transpire and release more moisture into the air, all of the elements move more freely from the roots into the leaves and flowers.
Average Proportions
In general, green plants prefer an average ratio of elements (as seen in the graph at right), with each having a unique acceptable range before becoming toxic or deficient. Take note that growth will be inhibited by the most deficient element at any given time.
Almost every nutrient company has a different formula for the flowering phase of growth than for the vegetative phase. Some evidence suggests nutrient needs vary between these phases. However, there isn’t enough evidence to back up the ethos within the cannabis industry of heavily increasing phosphorus quantities during flowering.
This assertion is further underscored by the fact that 95% of all pH down products contain phosphoric acid, which adds almost enough phosphorus for vegetative and flowering growth all by itself, if used on a regular basis.
Commercial Labeling
One important qualifier to N-P-K labeling is that, due to both tradition and quirky governmental regulatory standards, phosphorus and potassium are not actual elemental representations of the quantity of phosphorus and potassium in the bottle or bag. The presence of “P” on a label refers to P2O5, which only contains 44% elemental phosphorus; the “K” indicates K2O, which only contains 83% potassium.
When calculating and choosing ratios for a regimen, this adjustment factor must be taken into account.
In other words, divide the P quantity by about half to discern the amount of elemental phosphorous, and subtract about 20% off of the K quantity to ascertain the amount of elemental potassium. This exception only applies to phosphorus and potassium, and not to the remaining elements. Unfortunately, labeling laws do not require listing quantities of the other 11 elements, and nutrient companies are not known for transparency due to proprietary concerns. This further underscores the need for growers to seek their own scientific data.
Batch variation
As much as it would be nice to find a brand of fertilizer and just stick with it, there’s a chance that batches will vary, which could be significant enough to affect your plants. Too much inconsistency could prove your favorite nutrient line to be entirely unreliable.
To make matters even worse, agricultural testing labs aren’t perfect either. This means commercial cultivation operations that are serious about obtaining the best possible results over time will retest fertilizer batches and send in duplicate samples with different names to find out how consistent both the nutrient company and the lab really are. If that sounds onerous, you should probably start coming to terms with it. Lab testing your inputs, soils, reservoir contents and plant tissues, and cross-referencing all the resulting data over the course of the lifecycle of your crops will become a new way of life for any cultivation business that strives to maintain product consistency and stay ahead of the game.
Continuous Experimentation
Most likely, after a few growth cycles and a few dozen lab tests, you’ll find your ideal formula and method and be able to stick with it. But lab testing should still be a regularly scheduled duty, given the fact that marijuana plants can theoretically always be bigger, faster growing, more potent and higher yielding. Different strains will show slight variations in elemental preferences that can be customized as your testing process develops.
When all the elements finally get balanced for each strain, you will still have a world of growth stimulants, lighting conditions, environmental settings and pruning methods to test in order to reach even higher levels of excellence. At the very least, product consistency will be improved, and flushing and reservoir changes will be minimized, which will save water and labor, and reduce pollution to the environment. Also, bugs and powdery mildew are far more likely to attack unhealthy plants. Nature is designed for pests and pathogens to attack the weakest plants and animals. When your elemental ratios are out of balance, bugs and mildew spores are more attracted to, and able to penetrate, the tissue of the leaves. Plants can appear perfectly healthy and still be surprisingly out of balance, so improvements in yields, quality and pest resistance are almost always possible through these various types of analysis.
You might be thinking to yourself, “I use Brand X, my plants look just fine, and my flowers and yields are good too.” If that’s the case, carry on; but know that motivated competitors will soon be nipping at your heels if they aren’t already.
Conclusion
Ultimately, the data that comes from peer-reviewed research and laboratory testing should be the basis on which the historically subjective cannabis industry will come to depend upon for the assessment of quality product, the same way the agriculture industry has been doing for decades.
A reasonable case can be made that the many remaining mysteries of the natural world are simply scientific formulas that have yet to be discovered.
Jennifer Martin is the winner of the 1998 San Francisco Bay Area Cannabis Cup. She currently speaks at national conferences and consults for the legal cannabis industry. She can be reached through her website, www.MarijuanaPropagation.com.