Firm packing | Generally solid contact in wet soil has been the standard, but is it the right move for producers to make?
The assumption behind nearly all drill designs is that better packing and seed to soil contact result in a better rate of germination.
Stewart Wuest of the U.S. Department of Agriculture in Pendleton, Oregon, has been studying the question for nearly two decades.
Not only is the model of seed soil contact mythical but water itself may have little to do with seed germination, says Wuest.
“The current concept of how seeds absorb water from soil was based upon the idea that seeds are in contact with minute water films on the soil particles,” he says.
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“Recent research demonstrated that seeds are capable of germinating without soil contact and that 85 percent or more of the water absorbed by the seed can be directly attributed to vapour.
“Seeds can imbibe water in the form of vapour without physical contact between seed and soil.”
Wuest says in a soil that’s near 100 percent water holding capacity, only about 10 percent of the seed surface touches the water film on the soil particles.
That means 90 percent of the seed surface is exposed only to the atmosphere in the soil.
Heavy packing of a seed trench compacts the soil and crushes the open-air pores, reducing the space for soil atmosphere and soil vapour.
Wuest came across another startling conclusion.
Regardless of how dry or wet a soil may seem, relative humidity in the soil often remains close to 100 percent.
“This explains why such a wide variety of drills, with and without packers, in tilled and untilled soils, all are capable of producing good stands.
“Some farmers seed with drills they know are producing poor seed soil contact, then follow with a light harrow to cover seed with a loose layer of soil.
“Large uniform fields of emerging wheat would be a rare sight if germination were highly dependent on good seed soil contact.”
He concludes that knowledge of how vapour is sufficient to germinate seed should help guide future improvements in seeding equipment.
Pressing seed into firm soil is only necessary if it helps maintain high relative humidity near the seed.
“Seed should germinate just as quickly in loose moist soil as in firm moist soil.”
LeRoy Richard, who developed the Ag Systems Air Drill concept, says that putting only enough pressure on the soil and seed to seal the soil, about 3.5 pounds in his machine’s case, is enough.
