Treasure maps

Agriculture.com Staff 05/27/2010 @ 4:19pm

You'll find four topics in the next few pages. Low-Cost Layers shows how farmers are using soil-color maps, topographical data, yield maps, and experience to fine-tune nitrogen rates.

View Fields In 3-D introduces a new aerial imaging service that lets you see fields in a new dimension. pH Mapping tells how on-the-go sampling improves the accuracy of field mapping and variable-rate lime application. Sharp Maps describes LiDAR, a new technology that creates topographical maps with elevation data accurate within 8 inches.

Gathering the data to map out management zones doesn't have to cost a bundle.

In the early days of precision agriculture, many users believed fields would have to be sampled in 1-acre grids to capture the tremendous variability they often found. That budget-busting approach has been modified to rely on more affordable data gathered in 2 1/2-acre grids and smart samples taken in even broader management zones. Now, the focus is on inexpensive ways of gathering the data needed to build those management zones.

Colorado State University precision ag specialist Raj Khosla is finding that effective zone management maps to control nitrogen application can be built with data many precision farmers already have on hand or that they can easily obtain.

"We're building effective nitrogen management zones for irrigated fields by using soil color, topography, and the farmer's experience," says Khosla.

Khosla's approach begins by creating soil color maps using aerial images available at most local FSA offices. "Most offices have digitized the information and can provide hard copies at no charge. Meanwhile, digitized scenes are available from FSA's Aerial Photography Field Office (APFO) in Salt Lake City, Utah, for $13. And if no-till residue now blocks the bare-soil images that are needed, you can go into the local archives to scan old maps, also usually at no charge."

Khosla points out that any farmer using an RTK-quality guidance system is already recording the topographical data needed for mapping. "A decade ago, we had to use expensive survey-grade equipment. But now many farmers are using RTK guidance, and this provides excellent elevation data."

The final piece of Khosla's mapping puzzle comes from on-farm experience or from multiple years of stabilized yield maps. "We need to know which areas of the field typically yield high, low, or in-between. A farmer can do that by drawing on experience and creating polygons on a digital tablet. Or we can use the multiple years of yield monitor data that most growers now have on file," he says.

However, Khosla cautions against putting too much faith in the yield maps, unless the results from multiple years are normalized. "Yield is a function of many parameters. So allowing it to be a major  driving factor in designing management zones could be a mistake," he says.

"We recommend dividing fields into three management zones with high, medium, and low yield potential," Khosla continues. "Corresponding yield goals and nitrogen rates are set for each of these zones. In studies on several farms in northeast Colorado, we've found this results in a $13- to $27-per-acre increase in net returns compared to a uniform yield goal and nitrogen application."

Eaton, Colorado, farmer Rod Weimer thinks the management zone approach is netting him an additional $35 to $40 per acre. "We use the topography map from our guidance system and multiple years of yield maps to make our management zones. We tried using soil color as another data layer, but there wasn't much variation in our fields," he says.

Weimer says his fields typically have two or three management zones, and he varies the application rate of nitrogen, phosphorus, and potash between them.  "We use georeferenced soil sampling sites located in representative areas of each zone and take samples annually to monitor the soil fertility," he says.

Khosla points out that while his approach helps to develop low-cost maps to change application rates between management zones, new on-the-go nitrogen sensors allow rate changes within the zones. "This capability, along with the ability to automatically control individual boom sections on the sprayer, can really fine-tune nutrient applications," he says.

North Dakota State University agronomist Dave Franzen agrees. "We started out thinking we needed 1-acre grid sampling but have also learned that topography, satellite imagery, and yield mapping data provide similar accuracy at a much lower cost. It allows growers to create management zone maps that guide fertilizer, seeding rate, and other management practices that enable them to do the right thing, in the right place, and at the right time," he says.

You'll find four topics in the next few pages. Low-Cost Layers shows how farmers are using soil-color maps, topographical data, yield maps, and experience to fine-tune nitrogen rates.

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