Passage II

As earthworms move through the soil, they leave behind open burrows. These burrows may enhance the growth of plant roots by providing paths that the roots can easily follow as they grow.

A study was done to see if more plant roots and/or deeper plant roots are produced in soil containing 1 of 2 types of earthworms compared to soil without earthworms. Type A earthworms create horizontal burrows down to a maximum depth of 15 cm that collapse within minutes. The burrows of Type A earthworms typically do not reach the surface. Type B earthworms create vertical burrows from the surface down to a maximum depth of 30 cm that remain open for several days.

Nine boxes without bottoms and having sides 0.5 m high were prepared, and each was placed in a hole in the ground at an outdoor site so that the tops of the boxes were even with the ground surface. Each box was then filled with identical fertilized soil. Forty Type A earthworms were added to each of 3 boxes, and 40 Type B earthworms were added to each of 3 other boxes. The remaining 3 boxes contained no earthworms. Five days after the earthworms were added, seeds of barley, wheat, or oats were planted in the boxes.

Equal numbers of barley seeds were planted in 1 box containing Type A earthworms, 1 box containing Type B earthworms, and 1 box containing no earthworms. The same procedure was followed for wheat and oat seeds. All 9 boxes received equal amounts of sunlight and water. After 12 weeks, the dry mass of the roots in each box was measured for each 5 cm depth of soil in which roots were present (see Figure 1).

Figure 1 adapted from S. D. Logsdon and D. R. Linden, "Interactions of Earthworms with Soil Conditions Influencing Plant Growth." ©1992 by Williams and Wilkins.