You really don't need to worry about the microbes growing outdoors in the ground. They'll be there and thriving whether you help out or not. Unless you have really heavily used soil, like an old old garden or a former tobacco field the nutrients your plant needs are for the most part already there and available. When the plant has the room to really spread her roots out she'll likely find what she needs. All that aside mixing a bag of black earth, big bucket of compost, or a wheelbarrow full of composted manure into your soil when the growing season is over is never a bad idea. Unless you have perennials or bulbs you want to keep. Then you'd just top dress it.