All the chemical elements plants need to live and flourish are naturally present in water, soil, and air. It is natural to want to add extra nutrients, with increasingly better mixes and formulas to take the most out of every single plant. People are often ready to spend lots of money on the most advanced nutrients, yet this is no guarantee of great results - sometimes it can make things worse. What kind of nutrients do cannabis plants really need, and how much?

Cannabis plants need nitrogen, phosphorous and potassium to grow and thrive (N, P, K are the symbols for these elements). These elements are not standalone in the growing mix, and are usually combined in more complex molecules that plants are able to absorb. Organic fertilizers contain N, P, K under more complex and natural molecular forms than the equivalent mineral ones.

Growing in soil is the simplest way to cultivate cannabis, as it can forgive feeding mistakes thanks to its buffering activity between chemicals and the roots system. Some growers like to pump as many nutrients into their plants as possible, and more. Others prefer to keep fertilizing to a minimum. While ensuring optimal nutrition will certainly help your cannabis grow as strong as it can be, getting too involved in the nutrition game can cause distraction in less experienced growers. The result is other environment parameters getting out of control, potentially putting plants’ health at risk.

THREE STAGES, THREE DIETS

Just like with animals, each stage of a plant’s life requires slightly different nutrients. When growing in a good and rich soil, no additional nutrients are needed during the seedling stage of plant’s life. Just be sure pots are large enough to provide enough soil’s humidity and room for plant’s roots to grow freely.

When growing in artificial mediums that have no natural nutrients, root boosters and seedling nutrition can be used. Roots boosters contain enzymes, bacteria, and other compounds that encourage healthy root growth. Seedling nutrients contain a mix of the three main nutrients, usually in the correct percentage for sprouts. A similar nutritional effect would be obtained by feeding young plants with feed used during the vegetative stage, at one-quarter of the normal vegetative stage nutrient dose. Remember, unless it is a fertilizer specifically produced for cannabis, there is a good chance it will be too strong, and need reducing in dose.

During the vegetative growing phase, plants need high nitrogen and potassium levels, and a medium intake of phosphorous. As a general rule, the amount of phosphorous should be around half of nitrogen, while potassium levels can vary from one-third to half of nitrogen. In the flowering stage, nitrogen level must be drastically lowered, while increasing phosphorous and maintaining the same level of potassium used in vegetative stage. All commercial cannabis fertilizers are differentiated for grow and bloom phases according to these basic principles.

Switching to the proper nutrient mix when plants start flowering just means feeding them with a different NPK balance. Nitrogen, phosphorous, and potassium percentages are indicated on product labels with three numbers, along the lines of NPK 5-18-5 – with each number indicating the ratio of N, P and K respectively. The correct proportions of the three main elements are calculated by the manufacturer. Therefore, the grower just has to add water and stick to the right dosage.

In addition to the NPK variable mix, cannabis actually needs small quantities of secondary and micronutrients. Some of these are more important than others for plant’s health. Most are already present in a good soil mix, while others are often included in cannabis feed in addition to NPK. Specialist additives can also be bought, but this is only advisable for more experienced growers, as overfeeding can be detrimental.

UNDER AND OVER FEEDING

When growing cannabis, it is always better to be cautious than overzealous. A plant growing with just a little added nutrition will develop better yields than a plant growing in over-fertilized soil. Add nutrients to water only every other watering, and if you are in doubt about the doses, just cut them in half. These are the most simple and valid rules for feeding a cannabis plant.

Nutrient deficiencies and overdoses can be detected by a change in the plant’s appearance and colour. Many overfeeding symptoms have similar aspects to those of nutrient deficiency. The most evident signs of a nutrient unbalance are drooping and yellowing leaves, irregular leave shape or dimensions, brown spots, burnt edges. Yellowing leaves only signal a problem in the vegetative and early flowering stages of the cannabis plant life - it is normal when happening towards the end of the flowering stage. Of you want more info on nutrient problems and what they look like, check out our blog showcasing signs of deficiency.

If you find that you are not capable of fixing a problem through a correction in nutrition, flush the soil with pure water at a neutral pH for a few days, then start again with the nutrient mix at half dose. Remove the top few centimetres of soil which are likely full of excessive nutrients. This will give you a “clean slate”, but is a drastic measure.

Remember: if you seriously underfed, or overfed your cannabis, chances are they have been irreparably damaged in some way. They will take time to recover, and even then, your harvest will be reduced. If you work with fast-growing autoflowering strains, nutrient mistakes can seriously compromise your yield. 

ADDITIONAL CONSIDERATIONS

pH is critical for a correct nutrients uptake by your cannabis. All fertilizers modify water and grow medium pH to some degree (some more than others), usually lowering it. A wrong pH level locks out nutrients from plants, thus causing an underfeeding situation even if you are adding the correct nutrient mix to water. As such, always keep a close eye on the pH of your grow, and adjust it accordingly with solutions relevant to your grow (such as pH Up or Down, or natural alternatives in an organic grow).

Nutrients are often stored within the cannabis plant itself after uptake. This can result in in a bad chemical taste in the final product. Most of growers stop fertilizing days or even weeks before harvest, and flush their soil to force the plants to use up these excess nutrients, resulting in a smoother smoke. You can learn more about this final flush, here.