Most agriculturally important crops’ nutrient requirements and optimal environmental conditions have been examined extensively over hundreds of years, and possibly even since some of the earliest civilizations. (The book “Soil Fertility and Fertilizers: 4th Edition” (Tisdale SL, Nelson WL, Beaton JD., 1985) suggests that early civilizations of the Tigris-Euphrates basin in Western Asia recognized the richness of alluvial soils, or river deposits.) Resulting from a long history of human-agronomic research is a food system that supports billions of humans.
Cannabis sativa’s nutritional requirements have not been explored like many other food crops such as corn, wheat and rice. Many people can and do grow C. sativa; however, their experiences have not been recorded in a manner consistent with optimization of an agricultural system due to the plant’s complex legal history. Cannabis sativa cultivation is unique, as it is partially a botanical system, which aims to optimize flower production, and partially an agronomic system, which targets food/fiber/chemical production.
By most accounts, C. sativa is a plant that has numerous services to offer humans (e.g., medicinal, nutritional, textile, etc.). And like other crops, it is important to investigate the way C. sativa obtains nutrients from its environment, including specific nutrient ratios. This is particularly important because its cultivation is no longer relegated to the shadows; instead, it is now a growing industry, and cultivation refinements must be made to meet the reality of competitive business environments where efficiency and profit drive success.
Phosphorus is a plant nutrient of great concern in the agricultural and environmental sectors because it is limited in agricultural soils and overly abundant in freshwater ecosystems. Furthermore, the level of phosphorus recommended in cannabis cultivation also is out of line with other agricultural sectors. So let’s look at the physiological function of phosphorus, its interactions with other nutrients in growth media, and a proposed estimate of phosphorus fertilization appropriate for the growth of Cannabis sativa.
Physiological Roles of Phosphorus
Phosphorus is considered a macronutrient because it is a component of phosphate, which is an important molecule in many physiological plant functions. For starters, phosphate plays a major role in plant energetics, where it is a structural constituent of nicotinamide adenine dinucleotide phosphate (NADP+). NADP+ acts as a shuttle that moves electrons released from the light-energy harvesting centers of the chloroplast to the sites of sugar formation (i.e., energy storage reactions). NADP+ is also an energy source for nitrogen assimilation where it is used to convert nitrate to ammonia for use in protein synthesis.
The bottom line:Essentially, phosphorus is part of the photosynthetic system where it plays a role in the transfer of energy from light to the areas where that energy is stored as sugars. These reactions happen in the presence of light.
Another energetically important molecule that contains phosphate is nicotinamide adenine dinucleotide (NAD?), which differs from NADP? structurally by one phosphate molecule, and functionally because it is an important molecule in glycolysis (energy use). NAD? shuttles electrons liberated during glycolysis and the Krebs cycle (aerobic metabolism) to the membranes of the mitochondria where adenosine triphosphate (ATP) is formed.
The bottom line:In short, phosphorus is also part of the energy-use processes in plants where it moves energy from sugars to areas of growth. These reactions happen in the absence of light.
This brings up the most notable molecule of which phosphate is a constituent: ATP, the most readily available source of energy in all organisms, including plants. Its energy, which is held in phosphate-phosphate bonds, is used in a variety of metabolic processes. The many roles ATP plays in plant metabolism are beyond the scope of this column, but its most important role is short-term storage, translocation and transfer of energy.
The bottom line:Simply stated, phosphate-phosphate bonds hold a large amount of potential energy that is easily used during numerous physiological processes; ATP is the preferred energy source for most metabolic reactions. Almost all energy-creating/use processes result in the formation of ATP, which is used for important cellular processes.
Phosphate is also a major constituent of nucleic acids. In short, DNA (deoxyribonucleic acid) is the molecule that is used by organisms—including plants—to store information. Information stored in DNA is then transcribed to RNA (ribonucleic acid), which acts as both courier and translator in the protein-building process. Phosphorus’s importance in these processes is twofold: One, phosphate molecules make up the structural backbones of both DNA and RNA; and two, ATP’s stored energy is used to undertake the reactions associated with the conversion of genetic material to proteins, including the proof-reading mechanisms (which possess the ability to recognize, and match or reject nucleotides).
The bottom line:Essentially, phosphate makes up the structural components of DNA and RNA; in addition, the energy stored in ATP’s phosphate-phosphate bond is used during the reactions were the cell replicates and “reads” DNA.
Finally, phosphorus is a part of important cell-membrane molecules called phospholipids. These cellular constituents are composed of fatty acids attached to a sugar backbone that contains a phosphate group. Phospholipids aggregate in a specific manner; their orientation results in a “barrier” called the phospholipid bilayer. In short, these molecules allow the compartmentalization of cellular structures (for example, the nucleus, mitochondria, chloroplasts), as well as the cell itself, thus separating the internal cellular environment, where metabolic functions are tightly controlled, from the external environment. These molecules are very important in maintaining homeostasis (stable conditions).
Phosphate is a very reactive molecule with a negative charge. Depending on environmental conditions, it will chemically bind with numerous other fertilizer constituents. Therefore, it is important to apply phosphorus nutrition in a manner where it is separate from certain fertilizer constituents and under a narrow range of pH conditions.
Plants do not take up elemental phosphorus; instead, they show a preference for certain species of phosphate. For example, Henry D. Foth and Boyd G. Ellis, both Professors of Soil Science at Michigan State University, reported in their book “Soil Fertility” (1988) findings that suggested that plant roots preferred H2PO4- (dihydrogen phosphate) to the HPO42- (hydrogen phosphate) ion by a factor of 10, alluding to the important role of media pH in phosphate uptake.
The bottom line:Evidence suggests that plants have a preference for dihydrogen phosphate, which is the most common phosphate species at a pH between 5.5 and 6.5.
The hydrogen ion concentration (pH) also plays an important role in phosphorus solubility and availability. For phosphorus to be available to plants, it must be in solution, not chemically bound to other soil/fertilizer constituents. Fertilizer/soil pH controls much of the phosphorus availability to plants where high pH conditions promote phosphate/calcium binding, which forms a solid (i.e., apatite mineral) that is not available to plants. Under low pH conditions, phosphate will react with aluminum (berlinite mineral) and/or iron, which will also precipitate from solution and be unavailable to the plant.
The bottom line:These phosphorus-metal interactions exemplify the need to manage media pH and plan specific nutrient applications accordingly. For this reason, ideal media pH for most terrestrial plants is in the range of 5.5-6.5 because it maximizes phosphate availability.
The next phenomenon to consider regarding phosphorus nutrition is that of concentration-mediated adsorption/immobilization mechanisms. Total phosphorus concentration influences two processes: media adsorption (not chemically bound-available) and chemical reactivity (chemically bound-unavailable). Because phosphate is a charged molecule, it is capable of both adsorption to the growing medium and chemical reactions with other ionic species (i.e., metals). Which of those processes is favored depends, to a large extent, on the concentration of phosphate, according to the 1999 research paper, “Surface Charge and Solute Interactions in Soils.”
The authors of the 1983 research paper “Multifactor Kinetics of Phosphate Reactions With Minerals in Acidic Soils,” among others, hypothesize that adsorption mechanisms prevail at low phosphate concentrations, while reaction processes are dominant at high phosphate concentrations.
The bottom line:These studies suggest that over-use of phosphate fertilizer is inefficient and potentially counterproductive; lower phosphorus concentrations will likely result in better plant health outcomes.
Phosphorus Interactions With Other Nutrients
Nutrient interactions can be broadly classified into two groups: synergistic and antagonistic. Research has shown that nitrogen and phosphorous are synergistic in their effects on growth rate and biomass. That synergism appears to be principally driven by nitrogen-induced uptake of phosphorus. Magnesium has also been widely reported to have a synergistic relationship with phosphorus; higher levels of magnesium are believed to increase the rate of phosphorus uptake because magnesium is an integral part of the phosphorus-uptake system.
Conversely, phosphorus interacts negatively with other nutrients in media, thus also participating in antagonistic interactions. We have already discussed concentration and pH effects on the interactions among phosphate and aluminum, calcium and iron. Phosphorus and zinc also have been reported to interact antagonistically under high-phosphorus conditions. Though the mechanism has not been fully described, numerous authors’ findings point to slow zinc transmission from root to shoot under high phosphorus conditions, zinc-tissue-concentration dilution due to increased rates of growth, and metabolic deficits due to slow enzyme activation/activity resulting from insufficient zinc-tissue-concentration. Ultimately, the result is a buildup of unprocessed phosphorus in plant tissues (i.e., phosphorus toxicity) and leaf characteristics that mimic zinc deficiency.
The bottom line:These interactions further exemplify the need to apply phosphorus independent of most other plant nutrients (i.e., metals) and with temperance.
The Take-Home Message
Phosphorus is an important component of a plant’s physiology, and it is a principal constituent of all fertilizer programs. Many cannabis-specific fertilization programs promote excessive phosphorus application, which, due to antagonistic interactions, can lead to poor nutrient-uptake efficiency, nutrient shortages, lower-than-optimal yields, and excessive fertilizer-based capital expenditures.
Evidence from other cropping systems suggests that phosphate fertilizers should not be applied with a “more is better” philosophy. Most systems recommend an available concentration of ~30ppm phosphorus and a balanced application of other nutrients in inert media (i.e., peat mixes, etc.) or hydroponic solutions.
Some examples of balanced nutrient ratios that are used in other cropping systems include those published for phosphorus to copper (750:1), iron (100:1), potassium (0.15-0.20:1) and zinc (100-125:1).
Finally, almost all systems recommend that phosphorus fertilizers not be applied in mixture with calcium or trace metal fertilizers due to immobilization interactions.
All research in the field of agriculture suggests that a balanced fertilizing approach is always better than over-application. This assertion has been shown to be important both fiscally and biologically. Therefore, the approach to Cannabis sativa nutrient programs should not differ; balance should be used as the guiding principle. However, the real question remains: What is the proper nutrient balance for Cannabis sativa?