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  • Professor Sprout

Cannabis Grow Guide PART 4: What Causes Weed to Have Black Ash?

Updated: Apr 24, 2020

It’s time for the final instalment of our great black ash experiment.  To summarize up to this point, we’ve discussed many of the questions about what can cause black ash in cannabis; from moisture content and curing procedures to organic versus synthetic nutrient programs and more.

We’ve been able to dispel some of the most common myths and do away with a lot of the bro-science that abounds in the cannabis forums and discussion groups online.

We tested 8 different soil mixes, 3 different concentrations of synthetic nutrients, and even threw in a test with bottled organic nutrients.  Two plants were grown for each soil mix/feeding program for a total of 24 plants.  All plants were grown in the same room under identical growing conditions and plants were shuffled around periodically into new positions under the grow lights to ensure all received equal light distribution.

Once harvested and cured, the best buds from the healthiest looking of the two plants for each group were selected.  2 grams from each were ground up and rolled into joints with a handheld rolling machine.  The joints were then sealed into a glass container containing a Boveda humidity pack and left for one week in order to stabilize the humidity at 60-62%.

Each joint was then carefully lit by baking the tip with a lighter flame for 10 seconds to ensure the most even combustion, before fully igniting and then drawing in three small puffs to get a good ember burning.  The joint was then carefully placed on a wooden block and a stopwatch timed how long the joint remained lit.  The ash was then photographed and the burn time recorded.

It was not always evident to the eye which ash looked whiter when comparing two samples, so burn time was a helpful indicator as to which sample had the superior quality.  One of the reasons why black ash in cannabis is so undesirable is that it almost always burns poorly and refuses to stay lit long enough to pass it to a friend.  These tests certainly reinforce that observation. So enough with the chit-chat, here are the results:

General Organics Bottled Nutrients Burn Time:  124 seconds

Advanced Nutrients Synthetic Low Dose Burn Time:  96 seconds

Advanced Nutrients Synthetic Medium Dose Burn Time:  104 seconds

Advanced Nutrients Synthetic Medium Dose + Overdrive Burn Time:  87 seconds

Advanced Nutrients Synthetic Label Dose* Burn Time:  48 seconds

*This test was done posthumously to assess what effect feeding at the manufacturer’s suggested dosing would have on burn quality.  Although this plant was grown out after the original crop was harvested, the environmental conditions were the same.  It should be noted that this plant suffered severe nutrient burn, diminished yield, and terrible burn quality.  Aroma was greatly reduced and flavour was very unpleasant.  The smoke was harsh and burned the back of the throat.  I would strongly advise against following the manufacturer’s suggested dosing schedule if you decide to use Advanced Nutrients unless you are growing a strain that has an exceptionally high nutrient tolerance.

Soil Mix A - Bagged Sea Soil - No Amendments Burn Time:  175 seconds

Soil Mix B - Bagged Sea Soil Plus Pre-Mixed Dry Organic Nutrients (Gaia Green) Burn Time:  113 seconds

Soil Mix C - Soilless Peat-Based Planting Medium (Promix) Plus Pre-Mixed Dry Organic Nutrients (Gaia Green) Low Dose Burn Time:  160 seconds

Soil Mix D - Soilless Peat-Based Planting Medium (Promix) Plus Pre-Mixed Dry Organic Nutrients (Gaia Green) at Label Dose Burn Time:  125 seconds

Soil Mix E - Professor Sprout’s Mix at Full Dose Burn Time:  181 seconds

Soil Mix F - Professor Sprout’s Mix at Half Dose Burn Time:  156 seconds

Soil Mix G - Subcool’s Super Soil  Burn Time:  112 seconds

Soil Mix H - Subcool’s Super Soil Plus KMag  Burn Time:  140 seconds 

Additional Tests

If you do any research online regarding what causes black ash in marijuana you will most often read that it is due to three main issues:  A: moisture content too high, B: lack of flushing, and C: lack of curing.

In order to put these rumours to bed, I photographed three additional tests that I think are quite revealing.


I think the photo above illustrates quite clearly that you can’t ‘cure-out’ black ash in weed.  If you fucked up something in the growing process and you’ve got black ash, I’m sorry to say it’s going to stay that way.  The top sample was grown outdoors in the ground in organic soil that was severely deficient in both calcium and phosphorus (as revealed by a soil test).  The bottom sample is the same clone grown outdoors two years later in soil that was heavily amended with fishbone meal, oyster shell flour, and well-rotted compost.  As you can see, even after two years the cannabis grown in deficient soil still burns very black, whereas the improved soil produced a plant that burned to a nice white ash with only a 2 month cure.


It still astounds me how often people blame black ash on moisture content.  I mean, it’s stupid simple to test this out for yourself.  If you get black ash and you think it’s because the plant material is too wet, then let it dry and see for yourself.  Why is this so hard to figure out?   

Anyway, to show definitively that moisture content is not related to black ash in any way, I rolled two joints from the same bud.  One was placed in a cigar humidor at around 75% humidity (for the record, I would not recommend storing your weed at anything above 62% humidity) and the other was left at an ambient room humidity of 55%.  Three days later they were tested.  

While the top sample was certainly harder to draw a puff from and didn’t stay lit as well due to the very high moisture content, you can clearly see there is almost no difference in the brightness of the ash.  There is a little colour difference, but it is negligible in my opinion.  

Myth busted.


This one needs a little explaining.  Flushing or leaching is the process of watering your plants with nothing but plain water during the final two weeks (or more) of ripening.  The goal is to cut off the plant’s nutrient supply so that it begins to feast on it’s own internal reserves.  The commonly held belief is that this leads to a cleaner and whiter-burning end product as many of the mineral deposits inside the plant tissue have been metabolized.  While there is little to no scientific information on this process, it is nevertheless an industry standard practice.  

It should be pointed out that this method only works with synthetic nutrients as organic soil cannot be flushed.  Many of the nutrient stores within organic soil are held in reserve by microbial life and are released based on the plant’s demands.  Many of the nutrients in organic soil are also inert until broken down by these same microbes, therefore even if the medium if flushed heavily with plain water, as long as the microbial life is still alive, nutrients will be constantly released to the plant.

Synthetic nutrients are a different story.  These are salts that use the process of osmosis at the root level to ‘push’ food to the plant.  As such it is easy to overfeed the plant and force them to take up higher levels of nutrients than would otherwise be done by a natural living soil base.  

And therein lies the rub.  I have found that if you overfeed a plant, especially during the final weeks of ripening as growth begins to slow to a crawl, no amount of flushing will force the plant to use up it’s internal food stores.  Once the plant’s metabolism slows down enough, those minerals are going to be locked up inside the plant tissue permanently.  Curing will do no good since these minerals will not volatilize and evaporate.

A prime example is the photo below.  These two joints were rolled from buds taken from the same plant which was synthetically fed at higher than optimal levels during the last two weeks of growth.  A sample was cut at 8 weeks and the rest of the plant harvested at 10 weeks after a 14 day period of plain water flushing. 

As this was the first time I had grown this particular strain, it was unknown exactly how long it would take to finish.  In this case, the breeder has listed the strain as taking 10 weeks to ripen, however this particular phenotype finished much earlier than expected at just past 8 weeks.

As you can see, the two weeks of flushing did little to whiten the ash.  There was some improvement, but overall the ash was still quite black.  In this case, feeding should have been ceased at 6 weeks while the plant’s metabolism was still active and consuming nutrients.  Had this been done, I am confident the flush would have been effective in producing a much whiter ash. 

The Losers

I was quite surprised to see that the vast majority of the testing plants produced a very respectable, white, light grey, or salt and pepper ash that smoked very smoothly and burned evenly without constantly extinguishing itself.

However, there were some definite losers.  In no particular order the worst of the bunch were:

Bagged Sea Soil(Soil Mix A).  You might be surprised to see this one make the loser list since it had one of the whitest ashes in the group and the second longest burn time at 175 seconds (just a few seconds shy of Professor Sprout’s soil mix at full dose).  However, it burned very quickly and extremely hot, and was quite harsh on the throat.  Aroma and resin production underwhelmed, and yield and bud size was quite poor.  This plant was severely nutrient deficient from about week 3 of flowering onward and it was quite obvious that this straight-out-of-the bag soil mix was simply not up to par and didn’t have nearly enough food to support the plant’s demands beyond a few weeks.

Subcool’s Super Soil(Soil Mix G).  I was quite disappointed with this one since I had read such great things about it.  I know there are a lot of people getting great results with Subcool’s recipe and the man himself always puts up top notch material with this mix, but for some reason it just didn’t produce for me.  These plants had massive calcium, magnesium, and potassium deficiencies almost immediately, and became so sickly looking that I didn’t think they were going to make it to harvest.  Yield was terrible and the smoke was almost completely unpalatable.  It had a horrible acrid taste of burning timber and was so harsh on the back of my throat that I spent the next two hours coughing my lungs out after sampling the buds from this group.  It was so bad that I ended up tossing it all in the compost.  Note: Soil Mix H (the same Super Soil mix with the one simple addition of Kmag) resulted in a much more pleasant taste, healthier looking plants and overall better smoking experience.

Advanced Nutrients Medium Dose Synthetic Nutrients + Overdrive.  This was the first clear example of that horrid black ash we are talking about.  This one burned out quickly, took longer to light, and was very black.  The smoke left a thick, nasty taste in my mouth that was very unpleasant as well as a lingering heaviness in my lungs that was unsettling.  A definite loser, and a significant clue into one of the major causes of black ash.  Even though the overall concentration of the nutrients were the same between this group and the other synthetic nutrients fed plants at 1000ppm, the addition of Overdrive (a potassium and phosphorous booster) was a game changer - for the worst.  Further investigation into this has led me to believe that excess phosphorus in the final weeks of ripening can wreak havoc on your buds.  And while the yield and density of the buds were superior with the addition of Overdrive, the quality of the smoke was certainly not.  And how many growers would choose to sacrifice quality for a better yield?  Unfortunately many, as evident by the absolute proliferation of phosphorus heavy bud boosters and hardeners on the grow shop shelves.  

Advanced Nutrients Label Dose(Super High Dose) Synthetic Nutrients.  This plant was grown at exactly the feeding rate the manufacturer specified, and it resulted in the absolute worst weed in the entire bunch.  These plants had signs of severe nutrient burn, yield was dismal, and resin production and aroma were both noticeably reduced.  When smoked, the joint actually sparked and crackled like a sparkler.  It burned the back of my throat, left my lungs feeling congested and phlegmy, and was an absolute bitch to smoke.  Not to mention it had the absolute blackest ash out of the entire group.  This one went in the compost as well.  Perhaps an extremely nutrient tolerant strain would have fared better with this feeding program but I would certainly think twice before hitting your plants at these dosages.

The Winners

1. Professor Sprout’s MixFull Dose(Soil Mix E).  I’m not the type of guy who likes to toot my own horn (insert sophomoric sexual reference here), but the fact remains that this soil mix grew the healthiest looking plants with the strongest aroma, best resin production and even the top yield (about 10% higher than the next best in line).  Not only that, but the ash was bright white and burn time was best in show.  Interestingly, the potency of these plants were also noticeably higher than the others. 

I chalk the success of this soil mix up to the research I did into cigar tobacco production as well as nutrient ratios.  With this information I was able to design a recipe that optimized phosphorus, calcium, and magnesium uptake; three of the major nutritional factors in producing a white ash in tobacco production.  A lot of folks think you can just toss anything into an organic soil and let the microbes do all the work, but it is crucial to balance your amendments.  Too much of one element will cause reduced absorption of another leading to deficiencies even when the soil contains adequate amounts of all macro and micro nutrients.  This is especially true with calcium and magnesium.  If magnesium levels are too high, it will reduce calcium absorption which can spell disaster for cannabis.  Both of these elements are essential to a white ash so it is very important to get this right.  Dolomite lime, one of the most commonly soil amendments used in marijuana growing, is too high in magnesium.  Thus by adding oyster shell to into my mix (which is almost entirely calcium) I am able to provide a much more balanced ratio leading to enhanced absorption of both minerals. 

Also of importance was providing my soil mix with both rapidly available and sustained release nutrients coupled with diverse bacterial and fungal inoculations that ensured the plant was adequately fed at all stages of growth and never wanted for anything.

I have since used this soil mix with at least a dozen other strains in our catalogue with great success.  Most importantly it produces beautiful white ash and mouth watering flavour every single time with very satisfying yields. 

2. General OrganicsBottled ‘Veganic’ Nutrients.  While this pair of plants took a little more TLC due to the fact that the nutrients needed to be constantly bubbled and agitated to keep from spoiling once mixed, (I also found bubbling these nutrients for a minimum of 24 hours helped to stabilize their shockingly low ph) the extra effort was worth it.  The plants looked very robust and healthy all through their life cycle and yielded very well (2nd highest yield, just under Professor Sprout’s Mix).  Their aroma was strong and very pleasant, resin production was impressive, and the smoke was smooth and flavourful with a nice white ash.  I was pleasantly surprised by this line of nutrients.  Note:  I don’t receive any product or reimbursement from General Hydroponics for writing this - in fact I doubt anyone at General Hydroponics even knows who the fuck I am.

3. Promix Plus Gaia Green Dry Organic Nutrients Low Dose (Soil Mix C) .  Again, I get nothing from endorsing Promix or Gaia Green, these just happen to be the products that I have available at my local garden supply.  I have included this one in the winner category because it not only produced a very pleasant smoke with a respectable yield and clean ash, but it was also superior in flavor and aroma to many of the other test plants.  However, the best part of this soil mix is it’s simplicity.  You could basically make a pile of this soil mix with four ingredients:  A bale of Promix, a bag of earth worm castings, a pail of Power Bloom, and a pail of All-Purpose.  Mix, moisten, let cook, Bob’s your uncle.  No spending hours on the interwebs to track down a bunch of obscure ingredients that cost a fortune to ship, and no running all over town to gather supplies.  

So that concludes the great black ash experiment; my investigation into the factors that lead to black ash in cannabis.  Hopefully this has been helpful and has dispelled some of the misinformed myths and rumours that are so common in ganja growing circles.  I hope the information in these posts helps you grow potent, flavourful, and clean burning herb with a gleaming white ash.

Happy Gardening,

- Professor Sprout

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1 Comment

Farty Booty
Farty Booty
Jul 06, 2021

Some of the links for your ash pics seem to be broken. Great read though.

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