If you have always thought of earthworms as slimy little pests, think again. Earthworms perform a valuable service in soil maintenance and renewal. Although earthworms do not directly affect plant growth, they create ideal soil conditions through their castings (excrement). As earthworms consume and excrete decaying organic matter, what they leave behind is rich in nitrogen, calcium, magnesium and phosphorus. Those are elements that plants need to grow healthy and strong. Unlike chemical fertilizers, earthworms' castings do not burn seedlings or young plants.
Earthworms' eating habits do more than just produce high quality fertilizer. They also help to mix different layers of soil. Dying plant matter is pulled below ground to be eaten, and rich deep layers of soil are brought to the surface and mixed with their castings. Earthworms can be divided into three categories: those that live on the surface, those that live in the upper soil and those that burrow deep into the soil. All three help mix and fortify dirt.
Earthworms create burrows and nests in the dirt. The little tunnels they leave behind aerate the soil, which serves two purposes. First, water is retained and can penetrate deep into the soil. Soil that is overly compacted cannot absorb or hold water, causing flooding and killing off plant life. Worm excrement is porous, and the soil they live in has many channels for the water to run into. In addition to creating ideal irrigation conditions, these channels allow plants to establish a strong root stock. Growing roots can easily expand downward and out, resulting in a healthier overall growth.
Although earthworms have few natural enemies, heavy use of chemical pesticides and fungicides kills them off quickly. Deep tilling also harms them; it deprives them of decaying organic surface material and destroys their burrows. If you want to promote healthy plant growth through an abundant earthworm population, reduce your use of chemicals and learn conservation tillage methods that will not harm earthworms that already live below the surface.
You could have your very own ecosystem in your garden once you have a healthy earthworm population. Your plants will have nutrient-rich, well-aerated soil to grow in. Plants and earthworms form a symbiotic relationship. As plants grow, mature and die off, earthworms act as natural composters. In turn, the soil they inhabit becomes richer, promoting strong plant growth.