Cheers guys. There are two things I think you two kind of correctly indicated but are also lacking a bit of a better understanding to it (The measurement in the scale is a pretty good idea though).
- Soil composition: In order to know what could be going wrong with your plant you definitely will have to go into the soil, but with avocados this is a really hard thing to do because their root system is a very delicate one. Any stress will dry your plant brown. With avocados there isn't such a thing as too much watering, the thing that could be going wrong is the soil's density and the way you have your draining system setup. This means that if your soil is tightening too much it could be harmful to the root system. This is something easy to figure because you stated that there are also other plants now showing these same patterns from lack or nutrients, and maybe just putting a little more oxygen and nitrogen to the equation would better some things. For a correct evaluation I would require to see how well is the soil doing in every pot (or spot) where you are growing them. But it's an easy thing to learn, you just remove a bit of the soil and dig (as far as possible from the roots) and you will notice if the soil is too tightened up or if it is already a bigh block of dirt, either way you will have to move the entire block (only when it's a tightened block of soil, never take out an avocado plant with its roots out!) into a new pot or place it with a new and fresh kind of soil, taking into account that you need this soil to breathe well and to drain water enough so it doesn't stagnate, that would be the issue, not "watering too much". Avocados would grow even more if there were 24/7 rains over them, almost every vegetable does so, it's all about the draining!
- Nutrients: This is what I think could be your second and even first problem with your avocado plants. There is a correct formula for them but we have to take care of the soil density first, so always keep in mind that the soil has to be the perfect media for the root system, the nutrients can be easily solved with a solution or straight vermicompost. What your leaves are showing is a lack of capacity to fully develop, and the yellow and falling leaves are just an indicative that winter is there. What can you do about it? Two things (also lol):
1.- The vermicompost or solid nutrients always have to go over the soil and using a little portion. Even an avocado plant can have a problem growing if the nutrients are too much or if it has the incorrect ph. Little by little you can test if a root hormone product, or one for a straight growth for vegetables will do the trick, but remember to not over put nutrients on them.
2.- This is my first trick, and I will try to explain it the best that I can: You can cut or take off almost every leave of an avocado plant (going full throttle could also make our plant to dry), and I would recommend to chop the top of the plant as well, the upper part will grow stronger and you will have a quicker development on the plant. Remember to learn about your soil, how it drains and how it's composed, because that's where you will find what is going on with your plants.
I believe that I can help you, maybe I don't have enough photos right now but I can assure you that I can be of help. I currently have more than 40 avocado trees and I have an opposite problem; the heat is too strong and the humidity isn't enough. But I still make it work and my older trees are going to turn 4-5 years old this 2018. So, I will help you, at least to the point where you can understand exactly what is going on. That is the first step to really solve the problem!