well, for starters, 300 plants is probably to big a number for you to handle. Especially if you cannot afford to properly grow them. It will be hundreds if not thousands of dollars at the very minimum to grow them to a good potential. Its a lot easier to check on 20 monster pound yielding plants, as opposed to 300 small quarter ounce plants. But if your intentions are to grow 300 plants, and you have no money to grow them, your best bet is to buy bagged slow-release all purpose fertilizer. 20-20-20, a 20 dollar bag will go a long ways. Keep in mind, that will only help give the plants some nutrients, that does not include any soil additives that help improve the soil. My advice, spend less money on seeds, and more on proper growing ingredients. I can see 300 plants not happening, you will overwhelm yourself, wasted a bunch of time and money, instead dedicate your time and money to fewer plants, and grow them up to be monsters, why not aim for 20 plants and try to yield a pound off each one?
P.S, you would have to spread out 300 plants like crazy, helicopters would spot that many plants in no time at all.