20 autoflower guerilla grow

Hello all. This season I plan on growing 21 auto flowers. The strains consist of 7 auto white widow, 7 auto super hash, and 7 auto anesthesia. These were purchased from Herbie's and pyramid seeds are the breeder. Here is the rundown, and I have some questions aswell:
The plants will be divided into two plots. One will have 2 different strains, containing 14 plants in total. The other plot will have the other 7 plants.
I will be growing in 3 gallon pots and directly in the ground. Medium is MG moisture control and perlite.
Questions: I am using soil from last year's grow which still looks nutrient rich. Should I fertilize it anyway before planting or fertilize a few weeks after planting when the plant is more mature?
How liberal/conservative should I be with fertilizer, if using any at all?
(Pic is plot one. It can only fit 8 in the ground now. Goal is to fit 12 in ground along with 2 pots to equal 14.)

Sounds good. Autos should t need much in terms of nutes early on. I always started low and increased. In 3 gallon pots you'll have to water often depending on your climate. I'm doing 5 gallon pots indoors and need water every 36 hours at 70% humidity. I also have big ass autos that suck up 1500ppm every 36 hours too. So the nute thing is just off of the plants signs.