Best Soil Nutrients for Cannabis?

Published Aug 23, 2020

by Nebula Haze

Soil is so simple. So close to nature. Add seeds, remember to water your plants, and wait for an awesome harvest, right? Unfortunately, many growers run into problems with cannabis plants in soil. The culprit is almost always either your environment, watering habits, or nutrients/supplements. We’ve covered environment and watering habits already, so this cannabis soil nutrient tutorial will focus on which soil nutrients and supplements you need to get the results you want.

Recommended Soil Nutrients

Recommended Mineral-Based or Synthetic Soil Nutrients – This isn’t a comprehensive list of great cannabis nutrients, but these are the nutrient system I have experience with and recommend for growing cannabis.

Simple Duos – Use one bottle for the vegetative stage, and the other bottle for the flowering stage.

  • Dyna-Gro Grow + Bloom – Originally designed for orchids, Dyna-Gro nutrients have proven to work exceptionally well for cannabis plants in soil. If you want professional results without spending a ton of money or having to mix multiple bottles together, these two bottles give your plants everything they need from seed to harvest. The best simple duo in my opinion.
  • General Hydroponics FloraNova Grow + Bloom – Cheap and easy with excellent results. The biggest issue I have with these is the liquid is so thick it can be annoying to measure out.
  • Botanicare Pure Blend duo (or buy individually Grow + Bloom) – Consistent results every time. The Bloom formula is especially great.

Dyna-Gro Grow + Bloom is my favorite simple nutrient duo

Multiple bottles at a time – Typically there are 3 bottles for the base nutrients but sometimes only 2. This class of nutrients needs all bottles at all times, which get mixed in different ratios depending on the stage of life. More flexibility to alter ratios throughout the plant’s life.

  • Fox Farm trio for soil (mostly organic with some synthetic ingredients) – This is the first nutrient set I ever used for growing cannabis and I was thrilled with my results. They also offer a starter kit that includes their most popular supplements so you can try the entire nutrient line without having to invest in big bottles of everything.
  • General Hydroponics Flora trio – One of the most popular cannabis nutrient systems. Not just for coco and hydro, it also works great in soil! Follow the instructions on the side of the bottles at half-strength for cannabis plants in soil. This is my go-to nutrient system. Great growth rates, yields, and bud quality.
  • Botanicare KIND trio (it’s often cheaper to buy smaller versions of the bottles individually: Grow, Bloom & Base) – The 3 bottles provide everything a cannabis plant needs with more flexibility than the Botanicare Pure Blend duo.

You can’t go wrong with the Fox Farm trio for soil. Just follow the instructions on the label at half-strength for cannabis plants.

If using filtered or RO water (which contains few micronutrients) it’s a good idea to also add a Calcium/Magnesium supplement by the same brand that makes your base nutrients. Cannabis plants go through a lot of Calcium and Magnesium!

  • Dyna-Gro Nutrients – get Mag-ProNote: Don’t use regular Cal-Mag supplements with Dyna-Gro base nutrients as it can result in unwanted interactions.
  • Fox Farm nutrients – get Bush Doctor Cal-MagNote: they’re currently re-branding this supplement from “Gringo Rasta Cal-Mag” to “Bush Master Cal-Mag” and it can be difficult to find right now. If you can’t find the Cal-Mag version by Fox Farm, use Cal-Mag Plus. It won’t cause unwanted interactions with their base nutrients.
  • General Hydroponics nutrients – get CaliMagic
  • Botanicare nutrients – get Cal-Mag Plus

How to Adjust PH Using Synthetic Nutrients

Learn more about testing pH

Test and adjust the pH of your water to get faster growth and prevent nutrient deficiencies in soil

Cannabis vegetative plants growing in Super Soil / Coco Loco potting mix

Organic Soil Nutrients

There used to be a ton of organic soil nutrients in bottles (like the now-discontinued GO Box) but many options have disappeared as growers switch to using amended compost for their organic nutrient needs. Why? The organic nutrients in bottles get similar results to mineral nutrients. The biggest difference in organic growing seems to come from using actual compost and/or amended soil.

Best supplement ingredients for organic growing

  • Sea kelp – a source of K, helps plants resist heat and stress
  • Leonardite – full of humic acids for happy roots, resist stress
  • Protein hydrolysate (contained in many supplements) – helps plants produce big roots and be more resistant towards water stress
  • Potassium Sulfate (also called “Sulfate of Potash”) – the sulfur and potassium promote smells and bud development in the flowering stage

My favorite organic supplement contains all: Floralicious Plus

How to Adjust pH in an Organic Soil Grow (regular PH Up and PH Down can harm the natural balance of the soil)

Learn more about testing pH with organic soil

Organic cannabis growing is rewarding!

 

Summary of a Marijuana’s Nutrient Needs in Soil

Now that you’ve seen some suggested marijuana soil nutrients, let’s go through a quick overview of a cannabis plant’s life in soil as it relates to nutrient needs.

Seedlings Have Plenty of Nutrients in Any Soil

Just about any soil potting mix contains enough nutrients for seedlings

Soils Starts Running Out of Nutrients

  • Vegetative Stage – Fast-growing cannabis plants uptake way more nutrients than your average houseplant. Depending on how “hot” your soil was at the beginning (how full of nutrients), plants may start running out of nutrients during the vegetative stage. If you started with a regular soil potting mix, begin adding nutrients when plants are 3-4 weeks old to keep them growing fast and healthy. Plants want “all-purpose” or “grow” nutrients in the vegetative stage, which are high in all three macronutrients (high NPK). If you started with a big soil container or a hot soil mix like Ocean Forest soil, you may not need to add extra nutrients quite yet. Keep in mind than anytime you get runoff water out of the bottom of the pot, you’re washing away some of the nutrients. Therefore, it’s a good idea to avoid getting runoff unless you plan on adding nutrients in the water.

With many soil mixes, plants run out of nutrients and start growing slowly after a few weeks. When a plant looks lime green all over (especially if bottom leaves are turning yellow and falling off), it needs more nutrients overall. The left plant received plain water and started turning yellow after the soil ran out of nutrients. The right plant has been receiving nutrients in the water and is a nice healthy green color.

Major Deficiencies May Appear if You Don’t Add Nutrients

  • Flowering Stage – Cannabis plants devour nutrients when making buds, especially Phosphorus and Potassium (“PK”). As a grower, you must prevent nutrient deficiencies in the flowering stage or your bud quality and final yields will suffer. Many soil plants run into major nutrient deficiencies without extra nutrients in the flowering stage. At this point, you need to do one of three things: add nutrients to the water, apply a top dressing/amendment, or transplant plants to fresh soil. There’s one exception: If your plant has a “reservoir” of nutrients at the bottom of the pot (for example a bottom layer of heavily amended, composted soil), roots will dip in and take nutrients as needed all the way until harvest. This “super soil” strategy is the best way to grow cannabis plants if you only want to add water.

This flowering cannabis plant has several weeks before harvest, but it’s already losing bottom leaves and turning pale all over. This means the plant wants higher levels of nutrients overall. Without enough nutrients, more leaves will turn yellow and buds will stop getting bigger.

I gave the plant extra “bloom” nutrients (using the GH Flora trio). This reversed the yellowing and buds continued fattening. Here is that plant at harvest.

Plain Water

  • Harvest – If you’ve been adding nutrients and supplements to the water, it’s often recommended to give plain water for a week or two before harvest to “flush” the plants/buds, though some growers give nutrients until just before harvest. If you’ve been giving plain water from the beginning, nothing for you to do here; just wait until buds look ready and harvest!

Although you want to avoid discolored leaves for most of the flowering stage, it’s normal for leaves to start turning yellow the week before harvest.

Major Takeaways

  • Cannabis plants need a lot of nutrients for a good harvest
  • Nutrients come from either the soil or the water
  • If a plant uses up all the nutrients in the soil, you must add more
  • “Hot” (nutrient-heavy) soil mixes last longer
  • Bigger pots have more soil and therefore last longer
  • Runoff water washes away nutrients in the soil, so avoid runoff if not adding nutrients

How to Add Nutrients:

  • Transplant to fresh soil – a chance to start fresh, but can stress plants
  • Apply top-dressing or an amendment – nutrients flow to the roots when you water the plants
  • Add nutrients in the water – covered in detail above!

Alternative: Create a “reservoir” of nutrients under the soil (“just add water” setup)

Example of great cannabis soil

Example of great cannabis soil

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