Field Notes: Dial In Nitrogen Use

September 22, 2009 07:00 PM

by Mike Starkey

In 2001, we were applying 200 lb., and sometimes 225 lb., of nitrogen (N) per acre because that's what the universities recommended—1.1 lb. of N per bushel of anticipated corn yield. But we found out it doesn't take that much. Now we apply about 0.75 lb. per bushel, and we expect to reduce that over time.

Spoon-feeding (applying all our N with the planter or by sidedressing), on-farm test plots and continuous no-till have made us more efficient.

In our former N program, we applied 15 lb. of N as we planted and sidedressed about 200 lb. more. Today, we apply about 80 lb. of N per acre as we plant. That includes 25 gal. per acre of 28% N solution, along with 5 gal. per acre of Thiosul, placed about 3½" from the seed. On the seed, we apply 3 lb. of N in a low-salt 9-18-3 pop-up fertilizer.
(Thiosul is ammonium thiosulfate. Our plots showed that we gain 5 bu. to 8 bu. per acre from applying sulfur. We don't get it from the air anymore because of reduced vehicle emissions.)

Annual strip tests. As we sidedress, we vary the rate of N based on management zones. The zones, which we tweak every year, are based on soil types, yield data and information gleaned from N strip tests.

In the strip tests, we apply from 0 lb. to 100 lb. of N per acre, as we sidedress, in a field containing soils representative of our farm. We run the strips all the way across the field, on each side, and record the locations. In the fall, we check the yields with our combine yield monitor.

The tests tell us the point at which additional N no longer pays for itself, based on the cost of N and the price of corn—sort of like the law of diminishing returns. The most efficient rate for our soils right now is near 140 lb. per acre (total applied at planting and by sidedressing), varying somewhat by management zones.

We repeat the strip tests every year because our soils are changing. Studies have shown that our no-till soils are supplying 40 lb. to 50 lb. more N per acre than tilled soil because of increased biological activity after seven years of continuous no-till.

We first looked at no-till in the early 1990s to reduce our equipment cost and cover more ground without hiring additional labor. No-tilling soybeans was easy, but we gave up on no-till corn. At that time, the corn hybrids could not handle the stress of cold soils, and our equipment was not properly set up for no-till.

In 2001, we discovered the Nu-Till system, which is an easy way to set up your corn planter for no-till. It enabled us to get consistent seed depth and has worked well.

Other keys to making no-till work include good drainage and applications of gypsum where needed to improve soil structure. We apply gypsum where soil is high in magnesium but low in calcium and the pH is in the 6.5 to 6.8 range. (If the pH is low, we build it up with high-calcium lime before applying gypsum.) On high-magnesium soils with good pH, gypsum makes a big improvement in water infiltration.

Putting on a sufficient amount of N at planting to tide the plants over for 30 days gives us time to accomplish sidedressing. We also note which hybrids need N early and which prefer it later in the season and plan our application schedule accordingly.

Other efficiency efforts. Next year, sidedressing will go faster because we plan to switch from anhydrous ammonia to 28% N solution for safety and convenience reasons. We'll be able to travel faster because we won't have to seal the slot. And the narrower knife used for liquid application throws less soil, so we won't worry about covering up corn plants.

We're trying to become even more efficient with our N by planting annual ryegrass as a cover crop. We hope the ryegrass will scavenge N from the soil as it decomposes, releasing the N for the following crop.

To see how much N the annual ryegrass takes up, we seeded ryegrass on one side of a creek, but not on the other. Scientists from a local university will monitor the nitrate content of tile water running out of both sides.

One more factor in our effort to reduce N is that today's corn hybrids use N more efficiently. That's another incentive to do some testing and see how much N we really need. At today's fertilizer prices, we don't want to waste it.

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