Honestly there are things you can add to your soil but I'm guessing as long as your native soil is the bulk of your growing mixture it will just compact right up with any weather etc. I would start with large bags of peatmoss/pro-mix which is peat moss vermiculite and perlite. That is a soilless medium which means it has almost no nutritional value. Though it drains amazingly and is a very good medium in my opinion.
You can dig holes and fill with promix or something like that or you can use pots or grow bags. For nutrients you can take 2 paths. One would be a weekly feeding of a high nitrogen liquid fertilizer during growth and a high phosphourous and potassium liquid fertilizer during budding. The other route you can take is adding long term nutrients to the medium which will break down over time. I would suggest vics super soil which means mixing many things with pro-mix such as blood meal, bone meal, lime, worm castings kelp meal and epsom salts. But the amount of mixing and then bringing the expanded soil out to your grow would not be an easy feat.
What I suggest for a scenario such as yours would be looking into a product called Heavy Harvest by Advanced nutrients. It's a 3 part time release fertilizer designed specifically for the production of cannabis. You put around 1/4 cup of dry fertilizer around the base of each plant and as long as it has adequate water it should be taken care of. Mind you there are 3 different types of this you need to purchase for different stages of growth. But it would work very well with the pro-mix as not only does it provide the basic 3 nutrients N-P-K but it has all the micro nutrients and minerals which you would find in the soil and is ph buffered.
I found out about this product when watching the browndirtwarrior video series which I advise you look up as it would probably help you a great deal. His grow method was the same as I explained to you. 1. Dig hole 2. Put in bag 3. Fill with compressed soil of 3.8 cuft2 compressed soilless medium 4. Make hole and transplant sexed female 5. Apply fertilizer as directed 6. Water in.
As long as your plants are watered you need to return 2 more times to apply fertilizer during summer and beginning of fall. This in my opinion is a lot easier than having to return weekly to liquid feed your 50 plants nutrients lol. During the last week or so I would scrape the top layer of fertilizer off the soil and give each plant a heavy dose of water so as to flush them of any odd taste the fertilizer may leave. I hope you have some help on this grow because chopping, trimming and curing 50 properly grown plants is gonna be one pain in the ass but the mountain of weed in the end may very well be worth it.
