Can I use the same soil mix from last year?

Reusing soil us the way to go. It will need to be amended with nutrients before it's useable. You can purchase mist if the things you need at garden store ur at hardware stores. I would add kelp meal, alfalfa meal, perlite, worm castings, peat moss, rock dust, neem meal, composted chicken manure, bone meal, blood meal and maybe some seabird and bat guanos. There are a few other things you could add if you wanted too. Fish meal is also good. The more diverse the soil the better. Some people say to not use bone and blood meals if planting gorilla due to animals attracted to scent. You'll have to decide based in location. To mix soil it's easiest to use a cement mixer or you can mix by hand. If you build a good soil it will continue to improve year after year. It's the microbes in the soil that make it so great and font destroy them with chemical nutrients.

 

yes, re use it but add 30% perlite and 10-20% of slow release nutes or other nutes

 

If you can add the entire list the plants will be better off and produce better and larger quality buds. If money is an issue you could get away with perlite, kelp, alfalfa, bone and blood meals. It will grow bud but will not be as great if you add more nutrient sources the better the plants will be.

 

the main thing is sun hours hitting the plant directly (morning sun is important against mold)
If you add anything it would be perlite for airing so the plants roots get 02.
Imo, food is food, and cannabis ain't that special, a good organic slow release fert with some worm castings in the upper part of the mix will be perfect. I do add high P bat guano and Kelp (K) but personally don't go beyond that. With enough sun and air, they'll be beasts and yield heavily with a great taste. Again choose the sunniest spot possible and nature will do the rest. 
I'e never tried it but I know some guerilla growers who only use native soil (black humus is rich) as food, some perlite for airing, some sand (for elements), ta da!

 

Also water crystals; although siad not to giving the best taste, they will greatly help to keep them alive in between long periods of draught or no watering. Taste can be affected but not the high or the plant's resistance. Some add gypsum, silica, rock dust, miccorhyze etc...; but they are not necessary to get good buds! (and expensive) You only need sun, an airy soil mix, water and food (slow release is fine), good outdoors genetics, good timing, a secluded spot, a dry fall. Also growing, autos, normals and later plants is good (sometimes, the later ones do better as late season can be dryer than beg october when the normal ones come along). I always have a few end october plants who aren't big enough in beg. october to get moldy from rains. I just use femmed seeds to not bother with any male culling that can seed a whole area.

 

Also for guerilla grows, 10-15 gallon holes/pots is enough. You'll get 1/2 pound or more if given enough sun (8 hours minimum). No need to overkill with 50 or more gallons. 

 

Mix them all together. I usually add a handful or two per 10 gallons. The bags should list application rates. To be honest it's hard to mix to much amendments. With organic the plants take the nutrients as they break down so over feeding is usually not an issue unless you also feed teas. Even then the plants should be fine as long as you watch them and only feed when needed. It's not unheard of to only feed water once planted and still produce quality bud. Feeding do help inverse quality and yield. I look at it as the more money spent building as soil the more I make on the end product. Trust me it pays off to spend the money. Growing marijuana is like growing $20 bills. Every bud turn into money or savings of money not spent buying it.

 

What part of Cali are you?

 

There is always a chance of bacteria in the soil as well so putting the soil in a clear plastic bag and sitting it in the sun a few days would kill any bad bacteria. It may be fine but many people recommend the clear bag method to cook anything bad that may be in it.
That's a beautiful area Ox up north. I am in the SF Bay area.

 

I wouldn't I would use fresh soil so nothing is contaminated and has good nutes in it to start with and not depleted ones

 

What's up triangle neighbor...just southwest of you.  Great advice above...add perlite and worm for certain and as many viable nutrients as possible.  If your soil was just happy frog with water run thru it then I would feel comfortable adding at least 2 cups of your amendment mix per cubic foot(7.5gal)...I would not put your soil in a plastic bag that is sealed just mix in the perlite worm and amendments moisten and let sit.
 
You should be able to find everything you need locally in hayfork or weaverville...if it's not too much of a pain to get into town