pH nutrient availability
Before I get into the specifics of each nutrient, I MUST stress that when using liquid salt or chemical fertilizers, it is extremely important that you monitor the pH of the soil, at leats until you have a ‘system’. Failure to do will eventually give you a headache.
Organics are much more stable and consistent, so you can usually ignore pH and just let nature take its course…but using salts/chemical ferts is much more unpredictable. pH will swing more often and many elements will be out of the necessary pH range for uptake. (see the graph) They then build up in the soil, locking up other nutes while your plant starves if it doesnt burn first.
In addition to affecting nutrient absorption, the pH of organic media influences the activity of microorganisms. Bacteria are more prevalent at pH greater than 5.5, while fungi are most active at pH less than 5.5. The soil pH also influences the process of nitrification, which occurs most readily at a slightly below neutral pH (5.5-6.5). I’ll explain the nitrification process a bit more when I get to the specific nutrients.
For most plants, cannabis included, nutrients are more readily taken by roots in slightly acidic soil. Rule of thumb…stick around 6-7 pH and you will be fine. Try to set up your soil in a neutral pH manner right away and you’ll avoid the complications of balancing as you fertilize…there lies the advantage of organics! In the times that you DO need to alter pH, go by the following advice.
If you need to RAISE the pH, DOLOMITIC LIMESTONE (25%Ca, 10-20%Mg) also supplies calcium and magnesium. Some sprinkle it in with the soil when first planting, but it leeches easily, so you shouldn’t really bother when growing indoors in containers… Oh, and because it’s so much easier to find in many areas, it’s tempting to use Hydrated Lime which supplies Ca alone, but I find that it REALLY plays with pH and it also leaves most fertilizer mixes too high in Ca and too LOW in Mg…Mg is the element I find to be consistently deficient in fert mixes. Using the Dolomitic Limestone rather than Hydrated Lime adds extra Mg while reducing the volatility of limestone’s pH raising power.
I mix in at watering at about .25-.50 grams per liter. be careful…if you have been using an ammonium-nitrogen fertilizer, introduction of limestone will produce ammonia gas…in other words, it steals N ions from the soil medium…stealing them from your plant!! (If you aren’t sure what form of N you have, just read the section on Basic Elements) Therefore, if you DO use lime, do not add any ammonium nitrogen fert for at least one week. (Hopefully, you wont BE using ammonium nitrogen at all after reading this thread!) If you use dolomitic limestone, remember it has a healthy dose of Mg, so keep this in mind if you use epsom salts.
When you buy limestone, remember that a finely ground source will react quickly compared to a coarsely ground form. Look at the label…limestone is graded according to the percent that passes through a 100-mesh screen; look for one that has at least an 80% grade.
If you want to LOWER the pH, try PHOSPHORIC ACID or IRON SULPHATE. If you use iron-sulphate remember that it will drastically change pH and it leeches from soil very easily. Do NOT overdo these…when in doubt go light.
I bought a wand pH tester for $15. They also sell test kits with tablets, but you only get a few uses. Here are a few home-made test kits for the truly thrifty (cheap)!
- Take a spoonful of soil and add a few drops of vinegar. If it fizzes up, the pH is above 7.5. Take another spoonful and moisten with distilled water. Add a sprinkle of baking soda…if fizzles, the pH is below 5.0. Not exact, but at least you know if you are safe or at an extreme.
- Mix some soil in distilled water and use litmus paper. Again, not exact, but peace of mind range!
SALTS & LEECHING
With the use of chemical or salt ferts, its important to understand the properties of leeching. For a more detailed perspective, do some reading on ion exchange between nutrients and the soil medium. Helps to understand salinity too. Borders, Barnes & Noble has some great books.
I’ll get right to the point… some soil components have a high “CATION EXCHANGE CAPACITY” which allows higher levels of nutrients to adhere to soil particles until dissolved in water for plant uptake. A medium with high Cation Exchange Capacity will act much like long term storage for nutrients. This is great for slow release or low concentration organics…or if you water every couple days but only fert every couple weeks. But…
In the context of Salt/Chemical ferts, I will focus on underlying strategy of high dosage-rapid release liquid fertilizers. With these ferts, if you aren’t careful, the salt content can get very high, and quickly. Even more so if you have low humidity and high evaporation rate. (This isn’t the best fert system for an HPS closet grow…tried it, stressed me, learned a lesson)
So it would be much safer to use soil components with LOW cation exchange capacity. That simply means that extra nutrients wouldn’t be stored as well by the soil…they would just leech out…this would prevent salt buildup (salt buildup limits water uptake) and toxicity which is easy to do with concentrated liquid ferts.
ThE salt index is a rough barometer of a fertilizer’s potential to prevent water absorption from the soil by plant roots…this is in comparison to the potential of an equal weight of Sodium Nitrate, which is therefore given a value of 100. Obviously, a high index has greater chance of freaking your plants! Sodium Nitrate is pretty risky itself, so if a fert has a rating OVER 100…ya better be darn sure you know what you’re doing 
Excessive salt content doesn’t just inhibit water uptake…it actually creates a kind of reverse osmosis (i think that’s the right term) where water is drawn OUT of plant by the roots) so your plants shows symptoms of dehydration or nutrient deficiency when another fert and/or watering is actually the LAST thing it needs!
In general, if you have a fert with a index over 25, be sure to dilute with water…DO NOT apply such ferts right into soil. Here are a few common ones…I’ll see if I can come up with a better list and edit later.
Potassium Chloride-116
Ammonium Nitrate -105
Sodium Nitrate -100
Ureas-75
Potassium Nitrate-74
Ammonium Sulfate 69
Calcium Nitrate-53
Potassium Sulphate-46
I hope I explained that ok… So what’s the point here?
Many growers add Perlite or vermiculite to soil for greater drainage and aeration. USE PERLITE! It has a LOW CEC around 2.0, while vermiculite has anywhere between 85-150!!! In other words, Perlite doesn’t ‘hold on’ to ionic elements…while vermiculite ‘clings’ to them!
With Perlite, if you only water once a week or use light doses…you are less likely to accumulate salts leading to toxicity…many get washed down and away with each new watering!
The other main difference is that perlite also allows for greater evaporation of water due to the millions of microscopic ‘craters’ or ‘pores’ which result in a huge surface area…vermiculite on the other hand, retains moisture. Often, use of vermiculite can lead to a dense, compact medium which doesn’t drain as well and can suffocate the roots. Just be careful of one thing. Because perlite doesnt bond with ionic salts, you cannot let your soil become bone dry…doing so leaves your roots exposed to these salts burnin the heck out of them and your roots will start sucking water BACK from the plant to establish an equilibrium. The plant wont win that battle. Not good.
Perlite’s characteristics also make it great for flushing. If you dont fert for the last 2 weeks or so, the plant will use up the majority of nutes in tissues and root area. That way, you have a plant free of excess chemicals when harvested. This is especially important in the case of Nitrogen as it creates a poisonous gas when oxidated…AND it forces the plant to burn at a higher temp which destroys much of the THC before inhaled. Much smoother, cleaner, better tasting final product when flushed.
With perlite, you can actually flush the container with warm water and get the nutes out of the soil, so the plant will use up the nutes in its tissues. Vermiculite holds on to the ionic elements much better, so the plant has to use up elements in the soil AND in its tissues. Either way can be successful…just know what you’re dealing with start the flush stage appropriately.
Sticking with the strict 2 week flush is also a good way to monitor your fertilization…for example, if after 2 weeks, your fan leaves are still primarily green, it has an excess of N in the soil to feed from. Therefore, you know to lower the concentration of N in your next grow…your hacking cough will remind you!
Perlite is also great if you want to reuse soil between grows without burning the heck out of seedlings. Usually after a harvest, the soil is LOADED with salts and isn’t appropriate for introduction of a newly germinated seedling. Perlite allows a greater capacity for leeching the soil so that it is mild enough to take seedlings.