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Soil charges variable rate application

In 1992, Louisiana farmer George LaCour was doing variable rate nitrogen application of a sort. He started by putting a piece of gray tape on the front axle of his rig. He then told the men driving the rigs, “When the cotton gets below the height of the tape, turn the fertilizer rig on. When it gets above the tape, turn it off.”

LaCour, who farms near Morganza, La., knew he had a problem getting nitrogen in the right spot. He'd put 25 units on a field and end up with one end that wouldn't close the row while the other end would be 6 feet tall.

Since going with true variable rate fertilizing, things have changed for LaCour. This year, that same former-problem field had cotton that was even from one end to the other. And LaCour's Pix rate was substantially less.

Consultant know-how

Harold Lambert, a Louisiana consultant, stays atop the new agricultural technologies. He knows his business and says there's a prerequisite for getting involved in precision agriculture.

“If a producer doesn't have a firm grasp on some of the fundamentals of his farm business, he needs to work on those before venturing into precision agriculture. Forget the maps and prescriptions until the business side is fully understood.”

That said, Lambert is using soil electrical conductivity (EC) to geo-reference soil texture changes across a field. The reason he's doing that is simply convenience: soil EC is a good surrogate parameter for texture.

Lambert is able to precisely map clients' fields at a resolution fine enough to be acceptable for varying nitrogen and seeding rates in cotton.

“While we do subjectively delineate zones within in a field, we want to get better at how we establish those zones. We also want to get better at zone prescriptions. The system we use isn't perfect, but it certainly beats the way we used to do it,” says Lambert, who spoke at the Precision Agriculture Conference in the Monroe (La.) Civic Center on Jan. 18.

Lambert is using variable rate application work with only two growers — LaCour and his farming neighbor, Marty Frey. A third Lambert client does a lot of yield mapping but not variable rate applications.

To check the soil EC, Lambert uses a conductivity rig — a specialty instrument — purchased from the Veris Company. The Veris EC implement is pulled behind Lambert's pickup truck. The implement has metal coulters that are in contact with the ground.

Two of the coulters discharge an extremely low electrical current. That current is received back by the other four coulters.

Onboard instruments measure the drop that occurs during that process and matches that with a latitude and longitude reading from an integrated GPS system.

Once that data is displayed with GIS software, Lambert can view the data in map form. From that, he can process and analyze it and, in time, produce a prescription for the land.

LaCour and Frey own the remaining hardware needed for variable rate work.

“There must be some way to read the prescription and control the application as a planter or fertilizer applicator travels through the field. In the case of both LaCour and Frey — each owns an Agleader yield mapping system — they have the instruments to read the prescription and send a message to a hydraulically driven Rawson controller.

The Rawson controller must be integrated with a GPS system so the system knows its location relative to the data in the prescription,” says Lambert.

The two farmers also employ John Blue fertilizer pumps that are common on many applicator rigs. The pumps aren't pieces of precision agriculture equipment and aren't essential for variable rate fertilizer application, says Lambert.

“You could make use of some type of centrifugal pump or something else. But we wanted to use the same equipment to vary seeding rates as well. The John Blue pump was the most versatile to use. And the John Blue company has come out with a new flow divider. It's said to be much more accurate at low volumes of liquid fertilizer.”

Farmers' perspectives

“We determined we needed variable rates because our crop yields often change with the soil type in the field. We farm along many bayous.

“Often there are very sandy soils on one end and 500 yards away at the other end of the field you can make a big clay ball. That's what soil variability is for us,” says LaCour.

“I asked Harold to take samples inside each of the different soil types and textures. He took three samples of each. He gave me a map that showed everything from no nitrogen needed to massive amounts needed.”

LaCour cautions other farmers thinking about getting into precision agriculture to take things slow. “Don't look for a lot out of this at one time. If you take that approach, you're liable to be disappointed. This is a building process and there's a lot of trial and error. Just start small and work your way up.”

Making a profit is still the driving force, he says. “We got caught in a situation this year that delayed harvest. We didn't get a yield monitor in a field of uniform cotton. Precision agriculture just wasn't worth as much money to me as getting the crop out of the field.”

In preparing for variable seeding rates, LaCour and Frey use a small attachment that can be put on or taken off in about an hour. That way, if something goes wrong it doesn't shut planting down for a long time. “We take it off and keep going,” says LaCour.

In getting his feet wet in the precision ag pond, Frey's first inclination — as was LaCour's — was to go with a yield monitor. He obtained some useful data. But when it came down to applying variable rates, it wasn't enough. So Frey had to look at something else.

That something was an Agleader product — a PF 3000. Frey harvested his 2000 rice crop with it. In 2001 he put it on the planter.

“We planted 1,120 acres of variable-rate cotton. We followed that (using the same tractor) with variable-rate fertilizer across that entire acreage. Seeding rates varied anywhere from 27,000 to 48,000 in the same field. Fertilizer went from 10 pounds to 115 pounds. Once we did that, without question we saw some major effects — especially in some of our heavier ground where we'd had trouble with short cotton. We made a good crop,” says Frey.

Getting set up was pretty simple, says Frey. “They sent us the systems and we mounted them ourselves. We made mounting brackets and put them on in a way that would allow us to attach and detach the systems easily. That way, if we have problems with the systems, we can just disconnect them and go back to planting conventional.

“One of the main things is to be careful in hooking these things up. Take your time and make sure your cables are routed properly. It only took us a day or so to hook the system up,” says Frey.

Frey does have radar, although it isn't essential. While it's true that GPS offers ground speed readings, radar offers more accuracy when it comes to seeding. It probably isn't as important with fertilizer. But with seeding, it's definitely useful, says Frey.

“Without question, I have no intention of going back to the conventional way of doing things.

“From a cost standpoint, it's difficult to put a specific number on what the systems are doing. There are many ways to compute it. But figuring in seed technology fees and other things, it's pretty safe to say the thing is saving us $15 to $40 per acre,” says Frey.


When you're setting up prescriptions in April, it's difficult to figure in 8 inches of rain in June and 14 inches of rain in August and early September, says Lambert.

“Don't think this will be easy and perfect. If you consider that things now mundane in farming were once more difficult, this falls into line. Anything new can be frustrating.”

Lambert says it would be most helpful to the adoption of precision agriculture if public-based research could be oriented in certain directions. Private business people can't do some things on their own.

“I believe there needs to be a way to monitor by various soil, crop, and environment scenarios what the subsoil residual nitrate trends are. Perhaps this could be monitored in various places across the state on a monthly basis. It would be helpful to know that subsoil residual nitrate levels are unusually high or low. That would allow us to set our nitrogen levels more objectively, and so this is important agronomically and environmentally,” says Lambert.

Keep it simple

LaCour says he wants simplicity and, in using precision agriculture, is looking for ways to make money.

“Agleader came out with a cotton yield monitor. We bought one. I have three pickers and two combines we use. But I only have the one cotton yield monitor. Why? Because I'm still learning and I don't think putting a cotton yield monitor on all my equipment is justified yet. By the time I get to that point, the precision ag equipment I now have may be useless — about like the last three computers I've had. But it's a learning curve that we all go through.”


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