Keys to Optimal Nutrient Levels in Soiless and Wet Hydroponic Grows

By Silas Sativarius

The cannabis industry is without question on the absolute cutting edge of plant nutrient science. While the perfect nutrient combination may have a greater effect on Cannabis yield than other crops, there is no concrete evidence of that. But the real reason nutrient research and products are booming in the Cannabis cultivation industry is simply the vast amount of money Cannabis growers are willing to spend on nutrients compared to ANY other commercial crop. The difference is literally ten fold higher than any other crop bar none.

And with this willingness to spend for the “best” has come a plethora of exaggerated claims to support this huge market. I will not try to evaluate any specific products or claims in this article, but instead I will attempt to help you understand the basics of how to best use your expensive and high tech nutrients.

Nutrient Concentrations and Feeding
Let’s start with the fact that overfeeding or excessive nutrient ppm levels is THE most common mistake inexperienced or ambitious growers make. One misconception that can lead to this mistake is the idea that the maximum level of ppms a plant can handle continues to go up proportionally as the plants growth rate increases. In reality, the maximum PPM level a plant can tolerate, or better, that is optimum (they aren’t the same) is based on a very complex relationship of factors in the root zone far too complicated to discuss here. But the bottom line is, the optimum is relatively static within a narrow range and does NOT increase proportionally with plant growth. So how does the plant get more nutrients as it grows faster? Water transpiration.

Water Transpiration
Water transpiration is the amount of water that literally flows through the plant from the roots to the leaves and out into the atmosphere. There are strict limitations on the concentration of minerals in the water flowing through the Xylem (the plants vascular system) So the more water transpiring, the more nutrients are transported. This is the most important factor in nutrient uptake. And it depends on several factors.

1) Temperature
The higher the temperature, the faster the plant will transpire water to cool the plant. CO2 intake also increases. See 3 Keys for Maximizing.. for more details on Co2 and Temp. Higher temperatures also increase evaporation rates.

2) Humidity
The lower the relative humidity (RH) i.e. the dryer the air, the less resistance to evaporation and the easier it is for the plant to evaporate water from the vacuoles in the leaves. The more water taken IN, the more must go OUT, so decreasing humidity helps facilitate water transpiration and prevents the potential for water condensation on the leaves which further restricts transpiration and makes the plant vulnerable to fungi like mildew.

3) Root capacity
The more root capacity the plant has, which literally comes down to the total surface area of the entire root system, the faster water/nutrients can be absorbed. Root surface area can be dramatically increased in two ways, optimizing air space in the soil, and Mycorrhiza.
• Air space – in general, the optimum ratio of air to water in the root zone is 50/50. Soil porosity or the amount of air space in the root zone throughout the life of the plant is CRITICAL to root function, and lot’s of air space creates “feathery” roots with high surface area.
Mycorrhiza – Mycorrhiza are a variety of fungi that grow on the root surface creating a webbing network that not only dramatically increase the functional surface area of the root, but also helps break down and make more available certain nutrient compounds. More on Mycorrhiza in the upcoming Root Zone article.

4) Osmotic pressure
Osmotic pressure is created by the ratios of solutes (minerals) between the exterior and interior of the root. It is here that nutrient PPM really comes into play because lower PPM’s decrease Osmotic pressure, but too high will stop transport completely. So for this reason, unless you suspect you have excessive nutrient levels in the root zone or salt buildup, plants should never be watered with pure Reverse Osmosis (RO) water, for it can negatively impact transpiration.

So you can see that keeping the root zone conditions optimal is ESSENTIAL to both nutrient and water uptake.

Evaporation in this context is the root zone moisture that is evaporated away from the roots without being used by the plant. When this moisture evaporates, it leaves the dissolved nutrients behind, initially just increasing the nutrient ppm level in the root zone, but ultimately, if the root zone gets too dry, usually towards the top, the nutes will dry completely and oxidize into salts, which are difficult to redissolve and create unhealthy conditions in the root zone.

There are 2 environmental factors that effect evaporation. Air and/or root-zone temperature, and air relative humidity (RH). Both an increase in temp and /or a decrease in RH will increase evaporation. Also, the “airier” or more porous the media and/or container, the faster the evaporation.

To compensate for this loss of unused water in the root zone, nutrient levels have to be reduced to effectively keep the existing root zone nutrient ppms consistent. This can be accomplished by either lowering feed water ppms on-going, or by periodically watering with 1/4 to 1/3 strength normal nutrient mix. The amount of reduction depends on the level of evaporation, but a good way to test this is to water the plants starting with a 10-20% reduction and measure the ppms of the nutrient run off coming out of the media. Adjust the feeding nutrient ppm until you get roughly the same coming out (or within 100ppm or so) as going in, each feeding. And remember. The harder you push the plant, i.e. the closer you run it to that maximum threshold, the less headroom you have for environmental fluctuations.

As mentioned in Tips and Tricks for LED’s,  LEDs produce no Infra Red (IR) and consequently require higher ambient air temperatures to achieve similar plant metabolism rates to that achieved with HPS, all other factors being equal. With this increase in air temp comes an increase in evaporation, so it is extra important when using LED’s to adjust your ppm schedule to accommodate these changes.

There is another factor that has a big impact on both the ability of the plant to absorb nutrients, and consequently your optimum levels of nutrient ppm. It is Chelation.

Chelates are compounds that react with the more stable nutrient molecules found in fertilizers (so they can stay unchanged in the package) and make them less stable, but far more absorbable by the plant. Chelates can also break down harmful salts, turning them back into absorbable nutrients. EDTA, Humic Acid, and Fulvic acid are the most common Chelates used in fertilizers. EDTA and other Chelates are also the active ingredient in most “Final Flush” type additives, (such as Advance’s Final Phase) advertised for use in the last two weeks of flower because they break down salt residue preventing nute lock up in the crucial final weeks.

Chelates are maybe the one nutrient ingredient that really can have a significant impact on growth rate because they make the nutrients more easily absorbed. There really isn’t any toxic threshold for these compounds, but the more you use of them, the less actual nutrients you can add to maintain a safe PPM level, so it is a balancing act.

TIP FROM SILAS – I personally use Liquid Karma (and no they don’t pay me to say this, I just love the stuff) as my organic Humic / Folic acid chelation agent (as well as providing some vitamins) through all phases of growth and I have found that while I may adjust the ratios of the various nutrients –nitrogen, potassium, phosphorus etc—for the different stages, i.e. veg, pre-flower, flower, and ripening, I slowly increase the ratio of Karma to Nutrients from 1 tsp/ gal Karma in Veg, to as high as 4tsp/gallon Karma at peak flower. Do it and WATCH what happens! I will give more details of this and all topics discussed on this site in my “Secrets to Massive Cannabis Yields” book coming out soon. There are many other sources besides Karma for organic Humic /Folic acid chelation additives so take your pick, but Karma is the only nute I have used religiously for almost a decade. I also add a little EDTA additive (1/4 label) anytime I use chemical nutes or flower boosters.

Because chelates make the dissolved nutrients (and salts) suddenly more absorbable to the plant, they can take a perfectly safe ppm level and make it toxic. Thus, whenever you use Chelation additives, you should reduce the nutrient ppm levels by 20% or so and then increase them slowly over time. And never use an EDTA flushing agent at full strength immediately after feeding with nutrients at full strength. Always flush with clear water at least once prior to using these additives at label strength. However, they are actually more effective if used throughout the grow cycle at roughly ¼ label strength, but nutrient PPMS must be adjusted accordingly. As with any changes in nutrient PPM, start low, say 6-700ppm and gradually increase and watch carefully. The fastest way to detect nutrient toxicity is a rapid browning of the normally white stamens of the flower, (Literally over-night) anytime before the last week of flower. This will happen before the leaves may show any symptoms. Spraying of flowers can have the same effect however. Also, browning of the leaf tips can be a sign of longer term excessive nutrient levels.

Root cleaning
As roots grow, they slough off their outer skin like a snake. This skin, is a carbohydrate material and can accumulate and reduce the water /nutrient uptake of the root. In organic humus based soil, there are naturally occurring bacteria such as Bacillus Subtilis and Bacillus Cereus, that eat (breakdown) the organic material in the soil as well as the carbohydrates shed by the roots. These are the same bacteria that break down plant matter in a compost pile.

However, in wet or soiless hydro, healthy root bacteria (flora) do not occur naturally. You have to add the bacteria and then provide them food ongoing, because most hydroponic nutrients do not have significant organic components that could provide food for these organisms, and they will quickly reproduce beyond the amount of food the roots are producing and then begin to die off. And without them, the root system will not be able to uptake water or nutrients efficiently, or breakdown any extra organic material from organic based nutrients that can accumulate and dense up soiless medias, reducing the oxygen present.

So it is important for proper nutrient uptake, that from the first week in veg, you supply periodic doses of healthy bacteria through supplements or compost teas to keep the roots and media clean and healthy, and it is strongly advised that you include a small amount of simple sugar (Glucose or Fructose) to feed the existing bacteria when the root detritus runs low (i.e your roots are clean). ¼ the label recommendation of most sugar products is plenty, but don’t worry, if the sugars are simple forms like glucose or fructose, they can be absorbed by the roots (and leaves) and thus can replace some of the sugars the plant normally has to produce through photosynthesis. Feeding sugar will also replace a small part of the nitrogen requirements. But DO NOT use excessive sugars in the nutrients because it can cause unhealthy conditions in the root zone. Less is more, and 50 (normal) to 100ppm (late flower) is a safe upper limit for sugar supplementation. People go WAY overboard with sugar, let me be clear, sugar DOES NOT translate directly into fragrance or resin. Phosphorus drives resin production. I will discuss this more in future articles.


Normally when plants grow in organic soil with all that good humus being acted on by countless bacteria and fungi, the soil will naturally stabilize around a PH of 6.0-6.5. This is adequate for proper absorption of organic forms of nutrients which are more stable and slightly less bio-available, and watering with simple PH7 water like rain does not alter the PH of this system virtually at all.

But when you are growing in a Hydroponic environment, both wet, aeroponic, or soil-less (peat, rockwool, etc..) the media the plant is growing in has no organic material except what you add in the nutrients. For this reason, it is very difficult to establish prolific enough flora (bacteria etc..) to really buffer the PH conditions. Thus YOU have to adjust your PH on every feeding to the correct level.

Different minerals (nutrients) favor different PH ranges, but when you distill it all down with the dozen or so minerals really essential to an optimally healthy plant, the sweet spot for all hydroponics (not Soil) turns out to be PH 5.7- 6.0. Phosphorus is the most sensitive nute to PH above 6.0 and it is ESSENTIAL to high quality Cannabis. And if you want optimum growth, and you are not using exclusively organic nutrients, you had better keep your PH between 5.7 and 6.0 ALL THE TIME. I’m not going to try to explain it here, but look at the chart above and suffice to say, you need to keep your PH in that range EVERY TIME you feed. Not just when you mix the tank. Every time you feed, RECHECK IT. Be diligent, and you will notice the difference.

And buy a GOOD PH meter and keep a back up meter as a reference, because when they fail, they don’t tell you. They just give you incorrect readings and you won’t know till it’s too late and your wondering why everything is SH*T. And it can only take a couple feedings that are PH 6.2 or higher, especially during the CRUCIAL first 2-3 weeks of flower, and especially if your using chemical nutes, to ruin an entire crop from Phosphorus deficiency. In those first weeks, the plants aren’t sucking up the nutes as fast, so it sits there longer, and often there is a bunch of fresh clean media (Peat, etc..) from a recent re-pot to a larger media, so there’s just not much resident organic residue and bacteria to help buffer the PH, so it has lots of time to mess with your plants. And it will RUIN an entire crop. Oh the plants will still grow, but they will never be optimal, and there is no fixing it.

You can protect yourself from PH issues somewhat by always using an organic Base nutrient. Also, bacterial treatments help as well. Organic nutes are less PH sensitive and over time leave behind material that will set this natural PH buffer mentioned previously. But this material also slowly clogs up the media, so if you use organics with soil-less media, increase the aeration in your starting media with additional perlite etc.. But ask any pro grower, organics nutes are safer and easier, and make for a little better taste and smell etc.. but plants grow much faster and bigger with high quality chelated chemical nutes. So it is a tradeoff. For Wet Hydro, organics will simply clog your jets and kill things, so you have to be very careful in avoiding nutes with organic particulates, which is in almost all of them. I will get into more detail in future publications.

But spend the money, spend the time, and watch your PH like a HAWK. 5.8-6.0. 6.0 for the first 2-3 weeks to favor Nitrogen and keep them good and green, then you can slowly begin to slide it down towards 5.8 during ripening to favor Phosphorus and Potassium absorption for flower production. Doing so will make the sun leaves begin to yellow, but don’t worry, that’s ok, they are less important at this stage than high phosphorus absorption.

TIP FROM SILAS:  Ammonium based nutrients will slowly decrease the media PH,  most other forms of nutrients will slowly increase media PH.  So ammonium based nutes will be less sensitive to excess (>6.0) PH levels because the absorption at the root naturally creates free hydrogen that drops the PH. If your media or soil PH is too low (exp. general leaf yellowing from nitrogen starvation), use non-ammonium based nutrients to correct.

I hope I have not sacrificed too much literal accuracy (my disclaimer for all you PHD plant biologists out there) in the interest of simplification here, but I think you can see that optimizing nutrient uptake is a very complex system that requires careful and gradual adjustments to find the optimal levels for your plant and environment. And the most important thing to remember is — of the core factors in plant metabolism (growth rate), namely Light, CO2, Temperature, Water, and Nutrients — Nutrients are the LEAST important factor in optimizing yield, and the most problematic. Ppm levels DO NOT need to be raised significantly as plant metabolism goes up, because the plant naturally increases the amount of water and nutrients taken up into the plant proportionally with the metabolism. You are better served by optimizing the other elements that facilitate water transpiration, namely temp, humidity, and root health. Increasing nutrients should be the last consideration in trying to optimize yield, and you should always be conservative. There is little downside to running conservative PPM’s and LOTS of potential downside to running too much.

To those who grow, we salute you!


2 thoughts on “Keys to Optimal Nutrient Levels in Soiless and Wet Hydroponic Grows”

  1. I mix dolophril ( which is dolomite and magnesium and calcim) and worm castings in with my promix soilless mix. When I mix my food to feed the plants that our in the soilless mix should I ph the food at 5.8 to 6 or higher? I use jacks 20/20/20 powder nutes cal/mag supplement and diamond nector for nutes…..


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s