Dial In Down Pressure

March 9, 2012 04:56 PM
Dial In Down Pressure

Correct down pressure is critical to perfect stands

The genetic potential of a corn seed is essentially unlimited. Once you place the seed in the ground, whatever happens next doesn’t add to yield potential—it subtracts from it. That’s why Farm Journal Field Agronomist Ken Ferrie is quick to remind farmers that a corn plant should never have a bad day.

Getting corn off to a positive start requires placing seeds at uniform depth, closing the furrow and firming the soil, taking out air pockets so moisture cannot escape. At Farm Journal Corn College, Ferrie draws upon lessons from Farm Journal Test Plots field studies to explain how to set down pressure and select a closing system to help plants jump out of the ground with their yield potential intact.

Think of planting not merely as dropping seeds in soil but as creating a microenvironment for growth around every seed. The result of too much down pressure is similar to that of planting in wet soil, Ferrie says. The soil around the seed will be overcompacted, preventing uniform emergence and slowing early growth. Those late plants will lag behind all season long.

It also smears the soil along the sides of the trench. "Sidewall compaction is often thought of as a no-till problem," Ferrie says. "But you can get sidewall compaction in conventional tillage too, if you apply too much down pressure."

Excess down pressure causes scoring on the top of the furrow, which causes the furrow to open up, letting moisture escape, Ferrie adds.

p54 Dial In Down Pressure   chart

Too little down pressure causes some seeds to be planted shallower than others, resulting in uneven emergence and lost yield potential.

Hidden yield robber. If your entire field looks the same (because plants in every row are showing symptoms), too much down pressure can become an invisible yield robber. The effects were evident when Ferrie compared 100 psi, 250 psi and 400 psi of down pressure in adjacent rows. The study included moldboard-plowed, vertical-tilled, strip-tilled and no-tilled soil.

The effects of excessive down pressure included delayed crown root formation after the soil cracked and separated. "Crown roots can’t grow in air," Ferrie says. "The roots stop growing and wait for you to throw some soil over them, or for rain to fall to wash soil into the slot and cover the roots.

"Plants with poor root systems want to tip over and lean in the wind. Eventually, there are big differences in plant height," he adds.

In the study, conventionally tilled ground needed the least down pressure and no-till needed the most. Finding the ideal down pressure for your soil conditions is more complicated.

No one intentionally applies too little or too much down pressure, Ferrie notes. But, in the stress of planting season, it’s easy to forget to consider the effects of tillage, planting speed and planter attachments and to make the necessary adjustments.

Speed kills. Speed might have the biggest impact on seed placement. "A farmer bumps up the throttle, then discovers seed depth is all over the place," Ferrie says. "That’s because his planter is bouncing up and down.

Instead of slowing down, he cranks up the down pressure. Then he gets sidewall compaction—it’s a chain reaction."

Poor seedbed preparation can also cause a frustrated operator to pour on more pressure. "Below the surface, soil density must be uniform," Ferrie says. "Tillage must be uniform, so you have 3" to 4" of uniform soil to plant in. If it is uneven under the surface, it’s like planting over speed bumps. Operators find uneven seed placement, then apply more down pressure to get through the rough spots."

Horizontal tillage tools must be properly set, and the wings must stay in the ground. "Run correctly, moldboard plows, field cultivators and soil finishers create a pretty nice seedbed," Ferrie says, "although they leave a horizontal layer that may impede water management later in the season.

"Vertical tillage tools, such as chisel plows, inline rippers, disk rippers and vertical harrows, also can create a good seedbed without the horizontal layer."

Setting down pressure. New technology that tracks downforce and lets you adjust it on the go makes finding and setting the right amount of down pressure fairly simple. If you don’t have that technology, Ferrie recommends that you go to the part of the field that is toughest to plant. Stop the planter, leaving the planting units in the ground. Grab the depth-gauge wheels and see if you can turn them. You should be able to slip them with little effort. If you can’t move them, you have too much down pressure. If they spin easily, you have too little.

Perform this test each time you enter a new field or switch from one tillage system to another, Ferrie suggests. When you make a down pressure change on a tough spot, ground-truth it elsewhere in the field to make sure you’re not smearing the sidewall where the conditions are better.

"If you’re in doubt, err on the side of too much down pressure rather than too little," Ferrie adds. "Too little down pressure leads to depth control problems, which is a bigger sin."

Remember that anything you add to your planter row unit will affect down-pressure requirements—especially starter attachments and nitrogen applicators. Contrary to what Ferrie expected, his study showed that attachments mounted on the front of the planting unit require significantly more down pressure than the rear-mounted attachments.

"Angled fertilizer openers take quite a bit more down pressure than straight cutters," Ferrie adds. "Fixed row cleaners require more pressure than floating row cleaners."


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