You were asking how deep soybean roots go, i found the following in the this link from corn and soybean digest.
http://cornandsoybeandigest.com:80/soybeans/drought-beater-soybeans

Drought stricken soybeans that pushed yield records had several advantages:

Perfectly timed August rains.

Reproductive resilience. While corn has only one chance to develop an ear and tassel, soybeans can abort many flower flushes and still produce more. Soybeans can compensate if environmental conditions change.

Normal August day and night temperatures.

Deep roots; some reported at 5, 6, 7 and even 9-ft. depths to find moist soil.

Drought suppressed pests and disease.