1) Somewhere I heard that EC meters don't work well for soil grows to measure your nutrient solution because it doesn't conduct or somethin... true / false?
These are used to measure dissolved content in water. EC meters in particular are measuring the electric conductivity of the water (I believe this is potential, but I could be wrong). I have never seen a TDS or EC meter for soil. Though I have seen devices to measure the fertility of soil (NPK levels), just have no idea how they work.
2) Will an EC meter take the guess-work out of feeding? I want to give my plants as much as they can eat without burning them, because usually my plants grow really slow and I never know if I'm underfeeding them or if I'm gonna burn them.
Yes, but this is not an endeavor of formulas and clear cut rules. This plant has wildly different genetics from strain to strain, and can exhibit a wide range of recessive traits that will make many situations unique. Meters will tell you exactly what your concentration of nutrients in solution is, no more no less. They will not tell you what your plant needs.
4) If PPM and EC can be derived from each other, why is EC better?
I'm trying to decide whether to buy a $30 EC meter or a BlueLab Truncheon.
(why does a meter need to readout in EC and PPM? what's the point of that?)
I'm not sure one is any better than another, though I can tell you that in water filtration (and from what I gather about hydroponics) , TDS is more common. They are measuring things differently though, and I don't really know what a TDS meter uses to determine PPM. EC's are used for other things as well. For instance, when measuring salinity in an impure sample, a TDS meter isn't very useful (it will measure calcium, and all sorts of things other than salt), however you can use an EC, because salt in the water has a direct correlation to conductivity, and therefore is measurable using an EC. At the same time you could also use a specific gravity meter, or even a refractometer which looks at how the salt changes the spectrum of light traveling through the solution. All are simply different ways of measuring the same thing, and some could have different applications (specific gravity is used to measure a lot of things). For your application, I really don't think there is a big difference. I would just make sure that whatever you get is high quality. I can also tell you EC's have been around longer, and are used more often in commercial green houses.
5) What does 100N-100P-200K-60MG mean? are these ppm numbers?
Wouldn't that just mean an N-P-K of 100-100-200, and a mag of 60, the way it's written makes me think tsp/gallon again.
If I follow a feeding schedule suggested by a company, most likely the only way I'll know when I'm giving them too much is when I burn them, and burning your plants is not fun. It's also a bitch to flush out multiple 3-5 gallon pots.
Using half the suggested feeding amount I don't prefer either, because then you won't ever know if you're under feeding your plants or if you're even giving them the right amount of NPK.
I understand your desire to do everything you can, but this is simply something you have to learn by experience, and keep faith. This plant has been cultivated and spread throughout the world for longer than we have records. It has evolved over millennial to almost every habitable environment on the planet. The genetic diversity is incredible, and recently a lot of very specialized breeding has been done, mixing up these genetics even more. Feeding schedules are guidelines, not the formula for perfect plants. Oftentimes they don't even tell you what type of plants their instructions are for, and needs can vary dramatically from species to species.
The more you get experience with this species, and in particular, the strains you are dealing with, you will get better and better at reading the plant and knowing what it needs. If you are diligent you can spot deficiencies quickly and correct them before they have had any effect on the plant, the more you do it, the better you get at it. It is ALWAYS better to slightly under fert and respond to the first sign of a deficiency than it is to nute burn a plant. Nute burn seems to be more common than deficiencies (not couting PH related deficiencies which seem to be very common).
I also am not convinced that under fertilizing will significantly effect growth without any signs of a deficiency. Personally my plants seem to do well in a range of N-P-K concentrations, but once overdone, they burn fast, and can get really damaged.
Last, but not least, plants need to get used to ferts, so starting at 1/2 strength and moving to full strength slowly might work perfectly, when starting at full strength might have burned your plants.
