In Gardening: An Ecological Approach, I offer five reasons (of hundreds) to grow a garden. My post from April 12, 2013, stated Reason #1 (to grow food for health).
Here is Reason #2.
To Enhance Soil Fertility
"Nature cycles nutrients and other materials in functioning ecological communities. Where life exists on Earth, the general long-term trend has been for soils to increase in depth and fertility under the influence of the plants and animals that the soils support. Where industrial agriculture prevails, however, the tendency has been for soils to erode or to be degraded and for fertility to diminish. The Earth should not lose its natural productivity because humans have lived there." (from preface, Gardening: An Ecological Approach)
Here is our opportunity to offset a little of the detrimental impact of industrial agriculture in our own backyards or community gardens. Composting garden and kitchen "wastes" and actively seeking out the "clean" organic wastes that others would otherwise send to a landfill with the trash are two ways to increase the biological capital (fertility) on our small plots of ground. Some coffee shops may donate their coffee grounds, some grocery stores may donate their past-prime organic produce, some juice bars may donate their organic waste, and your neighbors who don't spray and don't garden (yet) may give you their grass clippings and autumn leaves.