Salts, E.C. and Leeching
With the use of chemical or salt ferts, its important to understand the properties of leeching. For a detailed perspective, do some reading on ion exchange between nutrients and the soil medium. Helps to understand salinity too. Borders, Barnes and Noble have good books.
Basically, 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. Its kinda 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.
In this thread, I will focus on underlying strategy OG high dosage-rapid release liquid fertilizers. With these ferts, if you arent careful, the salt content can get very high. Even more so if you have low humidity and high evaporation rate. So it would be much safer to use soil components with LOW cation exchange capacity. That simply means that extra nutrients wouldnât be stored as well by the soilâŚthey would just leech outâŚthis would prevent salt buildup (which also limits water uptake) and toxicity which is easy to do with concentrated liquid ferts.
This salt index is a rough barometer of a fertilizerâs potential to prevent water absorption from the soil by plant rootsâŚthis 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, so if a fert has a rating OVER 100âŚya better be darn sure you know what youâre doing 
These ferts donât just inhibit water uptakeâŚthey actually create a kind of reverse osmosis (i think thatâs the right term) where water is drawn OUT of the roots!
In general, if you have a fert with a index over 25, be sure to dilute with waterâŚDO NOT apply such ferts right into soil.
Here are a few common onesâŚIâll 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.
Whatâs the point? 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 doesnât âhold onâ to ionic elementsâŚvermiculite âclingsâ to them! With Perlite, if you only water once a week or use light dosesâŚyou are less likely to accumulate salts leading to toxicityâŚ
This is also key 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 isnât 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.
This is also why you should also leech a plant for the last 2 weeksâŚno nutes. 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. Much smoother, cleaner, better tasting final product. This is especially important in the case of Nitrogen as it creates a poisonous gas when oxidatedâŚAND it forces the plant to burn at a higher temp which destroys much of the THC before inhaled.
The E.C. value measures the salinity of soil by measuring its conductivity. You want an E.C. under 2.0âŚanything around 4.0 needs immediate leeching. There are devices for measuringâŚhonestly, I havent bought one because I am pretty comfortable with thisâŚbut if you experiment may be worth a look!
Despite my natural aversion for ppm measurement, it would be a good idea when mixing ferts as a beginner. Check your nursery or Home Depot. Iâve F-ed up a plant or two in my day, always because I THOUGHT I was more accurate with eye measurement than I really was!