For a productive garden, the first thing you need is healthy soil. But keeping garden soil fertile over time can be a challenge, because as you grow and harvest crops, you’re also removing vital soil nutrients. Gardeners use many different strategies to keep the soil productive, including applying commercial organic fertilizers, but when buying fertilizers, it’s not always true that you get what you pay for. Commercial fertilizer prices can be high, and in most cases, they're simply not necessary. Two of the best organic fertilizers are free from your own backyard: grass clippings and compost. Here’s how to figure out how much fertilizer your garden needs and guidance for choosing the best free or low-cost options.