Hey-hey Skunkers! Soil amendments and pH, when growing cannabis in containers especially, is uber important to at least have some kind of a grip on. Once a grower said to me: “Almost every issue is a pH issue at its core.” I whole heartily agree with that, however, there are important things all around that statement, some of which I will get to today.
Controlling your pH with things like pH-Up or pH-Down in a living soil is sacrilegious and does way more harm than good. The “good” is fleeting much like using any kind of liquid amendments when leveraging living soil growing in containers. pH-Down (phosphoric acid) dives the pH hard and kills a great number of bacteria. pH-Up (potassium carbonate or potassium hydroxide) is so chalked full of mineral salts (potassium/K) that they accumulate in the root zones effecting pH in those zones negatively, sometimes a month later. Not to mention a straight up P or K overdose is ugly.
I’m going to enlighten those of you unfamiliar with pH effects brought about by using certain (or not using certain) additions to your soil. From messages I have gotten from many of you there is a lack of basic pH understanding of additions, like neem seed meal, as one example of many. Let’s boogie…
Soil Amendments and pH: The Rhizosphere
The Rhizosphere Illustrated Above
- A: Amoeba. These guys eat bacteria and supply the roots with NH4—AKA, highly available nitrogen.
- BL & BU: Different species of bacteria that feed on sloughed off root cells etc.
- RC: Root derived carbon.
- SR: Sloughed off root cells.
- F: Fungal Hyphae
- N: Nematode
The above illustration of the plant’s rhizosphere makes it a tad easier to wrap your head around the real zone that pH influences effect the most. Regardless of your soil’s pH, the pH in the rhizosphere (mostly manipulated by the plant) is what really matters and is where additions of too many minerals/salts accumulate and matter the most.
On the flip side of the coin, bombing your container with organic acid concentrations tends to kill off a lot of the good bacteria. This allows the fungi an “open door” situation in the rhizosphere with no resistance from bacteria. Plus, the lower pH is highly favored by the fungi. A fungus dominated rhizosphere equals a low pH rhizosphere below 6.2 normally, which does not make cannabis happy at all and often kills plants, ugly.
Soil Amendments and pH Effects: Peat Moss
Soils that contain high ratios of peat moss are tough to handle when using the living soil in containers. However, outdoors in the ground or in raised beds, this type of peat moss heavy soil often works fantastic when mixed with the native soil, especially clay heavy soils. Peat moss is jam-packed with fulvic acid. As an acid it tends to lower the pH of the soil. This is especially problematic in containers growing cannabis.
In containers, your soil is already insanely high in organic matter, compared to mineral matter. Decomposing organic matter tends to lower pH in the soil. So, if you double down, using rich organic soil that is also high in ratios of peat moss, you get pretty epic pH diving going on in those containers. This kills off much of the good bacteria that need a pH above 6.8ish. Cannabis favors a bacteria dominant soil, so that’s bad-bad out of the gate.
If you are thinking about recycling your soil like in the True Living Organics method, this high ratio peat moss soil continues to tend to lower the overall pH and tends to get fungal dominant easier; both are bad for your cannabis plants. Coir makes a better soil cut than peat moss, pH-wise, but coir must be thoroughly rinsed off (flushed) with plain water first, or toxic salts/minerals (like potassium) will kill your plants ugly. Coir should be used lightly, if at all; in living soil mixes.
Lime Usage as a Soil Amendment and pH Raising
The kind of lime you use in your soil matters quite a bit. Hydrated lime for example, can take your container soil up to 11.0+ pH quickly. If you don’t know, trust me here when I say that is very-very bad. Dolomite lime is your best bet here when growing cannabis in containers, and this is because of two big reasons: 1-Dolomite lime brings calcium (Ca) and magnesium (Mg) to the plant, and cannabis has full pull need for both of these nutrient elements. 2-Dolomite lime is a true buffer. What I mean by that is, dolomite lime takes the pH towards 7.0 pH, no matter if it starts higher or lower. Most lime or calcium soil amendments tend to take pH upwards to various degrees of intensity.
You don’t have to use proper lime for pH considerations in a custom soil mix. You could use ground up oyster shells, DE (diatomaceous earth, food or agricultural grades only), shellfish meal, or wood ashes. Something like wood ashes you would use very lightly like a teaspoon’s worth per gallon of soil mix. Something like crushed oyster shells would be more like 1 tablespoon per gallon of soil mix.
By keeping your soil pH at or above 6.8 is important for the bacteria to get established in the containers’ soil. Once established they can do things via their excretions to keep the pH up higher where they like it, much like fungi can do in your soil keeping it at a lower pH. Cannabis prefers soil pH where the bacteria like it.
Almost all amendments, including lime, need to be composted, or fast composted (cooked) first, before coming into contact with living roots.
The Soil Amendments and pH Effects Bullet List
Here’s a list of some common soil amendments and pH effects they tend to have on your soil. Anything calcium heavy will always take pH upwards, so will most potassium (K) heavy amendments. Container soil is always very high in ratios of organic to mineral matter, and organic decomposition without any kind of pH buffering/controlling tends to lower the soil’s the pH rapidly.
Soil Amendments that Tend to Raise the pH
- Coconut coir fiber (this must always be rinsed/flushed thoroughly first before use)
- Shredded bark, twigs, dried brown leaves
- Wood ashes
- Langbeinite (use only with extreme caution as OD is very easy, and a K overdose is a death sentence to your plants)
- Oyster shells
- DE (diatomaceous earth)
- Ground gourd or squash seeds (ground seeds in general)
- Shellfish meal
- Feather meal
- Bone meal
- Kelp meal (minimal but noticeable)
Soil Amendments that Tend to Lower the pH
- Blood meal
- Bat and bird guanos
- Humic Acid ore
- Coffee/tea grounds and all green plant materials
- Raw fish and fish meal
- Raw barnyard manures, also rabbit poop
- Cottonseed and Neem meal
- Alfalfa and soybean meal
- Gypsum (this will initially lower pH then switch it up and slightly raise pH which is why it is awesome when constructing a soil mix before composting “cooking” it)
- Soft rock phosphate
- Peat moss
One thing that is really a nice countermeasure here if it’s too late, and your containers’ soil pH is diving, you can buffer your water using dolomite lime. To make dolomite lime water: bubble 1/8th teaspoon of dolomite lime in a gallon of low PPM chlorine free water for 24 hours. You will end up with water that is just about 7.0 pH and around 60 PPM which will buffer all but the most insidious pH dives, like dives driven by anaerobic activity in your containers’ soil. Which also loves uber low pH.
Speaking of anaerobes… Using coir as a growing medium, or even in high ratios in your soil mix can turn deadly fast. Initially the coir is actually an aerating type amendment but as it decomposes it becomes aggregated hunks of water holding non-aerated biomatter.
Remember that low PPM water has no buffering abilities, so if your soil mix has no buffering either it will tend to dive that soil hard into low (deadly) pH zones.
Hope you enjoyed today’s article and that it helped clear you up on your basics when understanding soil amendments, and the pH effects that will likely result. Grab yourselves some killer healthy genetics over at Kingdom Organic Seeds and start your all-natural cannabis garden right. Check out this article: Using Lime in Growing Containers – Skunk Magazine, by yours truly, for more insights on lime usage when growing container cannabis. Byeeeeee…
- REv 😊
I'm The Rev, and I have been with SKUNK for about a decade now. I hail from Southern California, spent mucho time in Northern California, and now reside in Southern Oregon; always coastal. I am an all natural style cannabis grower and I have written a couple books on the subject - check out True Living Organics 2nd Edition on Amazon - I have been growing for over 45 years, and I have been breeding cannabis for over 30 years. Check out kingdomorganicseeds.com to see some exotic selections. Growing connoisseur cannabis is what I teach mostly, growing it in living soil without using liquid organic nutrients to feed the plant. I am also a highly skilled synthetics grower, hydroponics, aeroponics, DWC/SWC/NFT, Ebb and Flow, and soilless, but I cringe when smoking synthetic grown herbs, so for the last 15 years or so I preach the artisan style of all natural growing, specializing in container growing. Cheers and welcome aboard.