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ONE of our key assumptions about how seeds germinate turns out to be wrong. They don’t absorb water directly from the soil: instead, most of it comes from water vapour in the surrounding air.

The discovery, by Stewart Wuest, a soil scientist with the US Department of Agriculture in Pendleton, Oregon, will be a wake-up call to those gardeners and farmers who believe that seeds germinate and grow best when they are in direct contact with the soil, and that packing down soil on top of seeds is the way to a blooming flower bed or bumper crop. It also explains why the first shoots in a newly sown plot all tend to emerge around the same time, even though the amount of contact between each seed and the surrounding soil is far from uniform.

Soil particles have irregular, angular shapes, so soil is full of millimetre-sized air spaces. As a result only a small area of each seed is in direct contact with the water films that cling to soil particles. So they get most of their water from the moisture in the air spaces.

Wuest made his discovery through a series of simple experiments. He glued wheat seeds to the edges of a number of 3-millimetre-wide plastic dowels and inserted them into holes of varying diameters cut into a block of moist soil. Seeds forced into 2-millimetre-wide holes were in close contact with the surrounding soil, whereas those in holes 11 millimetres wide had no soil contact.

But after 24 hours, the seeds in the largest holes had absorbed almost as much water as those in the smallest, although they took a little longer to germinate. Wuest calculates that at least 85 per cent of the water absorbed by the seeds could be attributed to vapour alone (Society of Soil Science Journal, vol 66, p 1760).

He thinks that gardeners’ habit of packing down soil does more harm than good. All seeds need is enough soil coverage to prevent the wind from drying them out.

The finding could also mean that agricultural engineers need to redesign the seed drills farmers use. Most of these machines use an elaborate array of discs and rollers that compact soil on top of the seeds.