As a crop, cannabis stands out due to its ability to produce a dizzying array of useful compounds, its fast growth rate, and its large appetite for light, water, and nutrition. Despite this, too much weight has been placed on the belief that cannabis has very particular nutritional requirements. Many fertilizer companies have profited off the perception that cannabis needs a dozen different bottles of secret sauces to flourish. In reality, cannabis requires the same 17 nutrient elements that all plants require.
Nutrients are essential for all biological processes in plants. Although they are all vital for growth and development, each nutrient is required in different quantities at different stages of growth. Nutrients are generally split into four categories: macronutrients, secondary nutrients, micronutrients, and the elements taken from the air and water.
We produced a deep dive on the chemistry behind cannabis fertilizers and nutrition to give growers the information they need to find the best quality and value for their specific growing needs.
Nutrient Availability and Deficiency
Nutrients limit growth if they are not available to plants in sufficient quantities. In other words, plant growth is limited by the most limiting nutrient. Nutrient deficiencies directly impact growth and yield before there are any visible symptoms, so it is important to have a well-designed fertilizer regime that is closely monitored through testing irrigation water, runoff, and leaf tissue nutrient content.
Plants require different quantities and ratios of nutrients at different stages of growth and the growth cycle of cannabis is so fast and dynamic that feeds should change every couple of weeks to meet the plants changing needs.
Plants need nitrogen (N), phosphorus (P) and potassium (K) in larger quantities than other nutrients and they are the most commonly deficient nutrients. In cannabis, the need for these nutrients changes throughout the growth cycle with N needed more in the vegetative and early flower stage, and P and K needed more as flowering progresses. Many fertilizers marketed toward cannabis contain an excessive amount of P, often many times more than any other crops used, and more than any plant could absorb.
Secondary Nutrients and Micronutrients
The secondary nutrients, calcium (CA), magnesium (Mg), and sulfur (S) are still required in significant quantities and are frequently deficient. In fertilizers marketed to cannabis, calcium and magnesium are often strangely grouped together as “Cal-Mag.” Because of this, the application of fertilizer and diagnosis of deficiencies are wrongfully combined. Ca and Mg are, in fact, distinct elements which are taken up differently, required in different quantities, perform different roles in plant biology, and display different deficiency symptoms.
Plants absorb nutrients as charged molecules called ions. Generally, plants always absorb nutrients as the same ion and all plants, including cannabis, take up the same nutrients. With that said, the main differences in fertilizer formulation are:
- The ratios of nutrients to one another.
- The quality of the sourcing
- How it is applied.
- How it affects the chemistry and biology of the root zone.
- How readily nutrients are available across different conditions (ph, temp, EC, etc.)
Chemical fertilizers are blended from highly soluble salts with known chemical compositions. When compared with organic sources, chemical fertilizers are more predictable because they supply nutrients in ionic forms that are readily available to plants. Additionally, they are often less expensive than organics and less likely to clog irrigation equipment.
Premixed chemical fertilizer blends are often marketed to the cannabis industry as “nutrient lines” with various component fertilizers added at different ratios throughout the grow cycle.
Alternatively, growers can blend their own fertilizer mixes from raw mineral salts. The main advantages to blending your own salts are reduced cost and being able to customize and fine-tune the mixture. Creating a custom nutrient formula requires expertise and may call for over a dozen components but many fertilizer companies can also make custom mixes for a minimal fee.
Rates and Ratios
Once flowering is triggered, cannabis grows and develops very quickly. To meet the changing needs of a crop, nutrition must match each developmental stage. While the science behind the precise nutritional needs of cannabis is still in its infancy and is cultivar dependent, we can look to a few different trends from reputable commercial formulas and the available research.
In the vegetative stage, plants build structure and size and require steady supply of N, and calcium balanced with the other essential nutrients. During the transition from vegetative to flowering, plants begin to stretch and grow rapidly, requiring a higher rate of balanced fertilizer. As flowering progresses, P and K become more important for flower development.
As plants finish their lifecycle toward the end of flower, metabolism slows and nutritional needs decline. There is an ongoing debate about the need for flushing at the end of flowers. Some argue nitrates or other compounds negatively affect the smoke and flavor of flower and flushing the media at the end of flower reduces those compounds. Alternatively, plant metabolism continues to harvest and requires nutrients to function properly.
Optimum rates will also depend on the environment in which plants are grown. Plants driven in a more intense environment that drives more photosynthesis or transpiration with brighter lights or greater vapor pressure deficit will have greater nutritional needs than those grown in a gentler environment.
Nutrient use and needs are affected by plant growth stage and intensity of growing systems, so fertilizer rates must change accordingly. Therefore, there’s a large market for cannabis-targeted fertilizer lines, but their recipes often over-apply nutrients. Growers can also create their own custom nutrient formula, but that requires advanced expertise and multiple components. When it comes to supplying your plants with the water and nutrients they need, It’s important to consider your irrigation, fertigation, and integrated water systems.
Want to dive deeper into nutrient formulations and view the recommended ranges for cannabis fertilizers at various growth stages?