Abstract: A growth medium moisture replacement system particularly useful in bioassays for Heterodera glycines—Glycine female development includes a first container of growth medium for soybean cultivars, an enclosed second container for water, and a wick which extends upwardly from the water in the second container into the growth medium in the first container to replace moisture in the growth medium after establishment of a hydraulic gradient. The wick includes a rope member having an impermeable sheath around a middle portion of the rope member, leaving its ends uncovered, and sealed to an opening in the bottom of the first container.