How deep should you plow?

Saturday, November 3, 2012

Opinions differed at this year's Canada's Outdoor Farm Show. Most thought the plowing was too deep while some felt the plowing was too shallow. Plowing is an art

by PAT LYNCH

There was a lot to learn again at this year's Canada's Outdoor Farm Show equipment demonstrations. This year, it was plowing – not the art of plowing but demonstrating plows.

But it is hard to separate the plow from the plower and hard to separate plowing from plows.

I asked various equipment manufacturers why they were plowing so deep and they said they felt that was what farmers wanted. I said no, they want to plow shallow. They asked me how shallow do they want to plow and I said, "Six inches maximum and four inches would be better."

Then one or more said they were only plowing six inches deep, to which I said, "No you are plowing deeper than that." So we got out the tape measure. We couldn't even agree on how deep the measurement was. I put one foot in the furrow, one foot on unplowed ground and measured the difference. When I did that, I was getting their plow depth to be about two inches deeper than they felt they were plowing.

I had an interesting exchange with one of the demonstrators, who had hired someone to run their plow. They said they wanted it shallower, but the driver said no, this ground must be plowed that deep. I argued with this driver for a bit but gave up. He was in control of the plow depth.

Then we had the discussion about bringing up subsoil. I said, "You should never bring up subsoil."

He said that is how you make more topsoil, mixing in some subsoil. I assured him that was wrong.

Subsoil has much poorer soil structure and soil texture than topsoil. When you dilute topsoil with subsoil, you generally end up with soil that is much poorer than what you started with.

It is better to have only four inches of good topsoil than six inches of poor topsoil. I think that some yield increases producers saw when they went from plowing to no-till was because they were not constantly bringing up subsoil.

Bringing up subsoil also dilutes soil nutrients. The high calcium in the subsoil ties up phosphorus. If you take soil samples from the top six inches and compare that to soil samples from the six to 12-inch depth, you will see how much higher the nutrient level is in the top six inches.

When I talked to the manufacturers, they all said their plows could plow more shallowly than how they were set for that demonstration. That was the good news.

In defense of the plow manufacturers, I had made the decision not to chop the corn stalks. Looking back it would have been a better demonstration had the stalks been chopped.

Most of the plows left little residue on top. This goes back to the old idea from plowing matches of "clean plowing." Part of this is to be able to get a suitable seedbed with minimum secondary tillage in the spring. But from what I see with the advances in secondary tillage, you can get a suitable seedbed with one or two passes in the spring if you leave corn stalks showing.

And this is the ideal. You want to plow to mix the organic matter, crop residue and nutrients in the top six inches. You want to do this so you can plant with minimal passes in the spring. And you want to have 30 per cent trash cover next spring after planting.

Moreover, you don't have to plow every year. I believe in rotation – rotation of crops, pesticide families, genetics and tillage. You can rotate through minimum tillage, no-till and plowing. And you do not have to own all this equipment. Rent it. You rent land and other pieces of equipment, so there's no reason not to rent tillage equipment.

Interesting listening to the attendees. Most thought the plowing was too deep. There were some who felt the plowing was too shallow. Plowing is an art. BF

Consulting agronomist Pat Lynch, CCA (ON) formerly worked with the Ontario agriculture ministry and with Cargill.