Vapor--Not Liquid Water--Spurs Seeds' Sprouting

A new finding by Agricultural Research Service soil scientist Stewart B. Wuest has stunned many plant scientists. Until recently, it was generally believed that seeds must be in direct contact with soil to obtain liquid water needed for germination. Then Wuest discovered the importance of water vapor.

From his research on wheat seeds at ARS' Columbia Plateau Conservation Research Center in Pendleton, Ore., as well as from studying previously published data, Wuest concluded that water vapor in the soil is actually what makes seeds germinate. With a relative humidity of close to 99 percent in soil, the seeds didn't need to be tightly compacted in the soil to grow. In fact, seeds that were separated from the soil by crop residue still germinated, because the vapor was able to reach them.

Wuest also found that, thanks to water vapor, seeds separated from soil by a layer of fiberglass cloth germinated just as well as those touching the soil. He was even able to germinate seeds suspended in air above water, using just the vapor rising from it.

Water vapor is all around us, measured as humidity. That's what makes a dry cracker left out in a room with high humidity turn soggy from absorption of water from the air. Similarly, seeds are able to absorb their needed water from vapor in the soil. In fact, liquid water is not nearly as important as previously thought and may only account for 15 percent of water taken up by germinating seeds.

In light of this discovery, approaches to water absorption models and measurement techniques may need to be changed. The design of some seeding equipment may also change, since actual seed-soil contact is not as important as earlier believed. Emphasis is likely to shift to tactics for retaining water vapor near the seed.

Findings from Wuest's research were published in a recent issue of the Soil Science Society of America Journal.

ARS is the U.S. Department of Agriculture's chief scientific research agency.