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Soil sampling techniques yield a variety of views, practices

Aug. 25, 2014 | 0 comments

TOWN OF SCOTT

What's the best strategy for taking soil samples? How does the chosen strategy affect the results? What's the value of a particular soil sampling strategy?

Those questions were considered by the private sector speakers at the Sheboygan County Forage Council's program on soil sampling, field zone management and variable rate fertilizer applications that was held on the Mark and James Ramel families' Windy Hill Farms.

From their varying perspectives, the presenters gave somewhat different answers on what they consider to be ideal, practical or affordable. They also addressed the potential differences between what is shown by soil sample analysis data and what the landowner or operator knows about the cropping history at various sites.

Sharing their views and experiences were Nick Guilette of AgSource at Bonduel, Steve Hoffman of InDepth Agronomy in Manitowoc, Scott Fleming of Rock River Labs in Watertown, Roger Lucky of the Adell Co-op and Karl Harpstead of the Kettle Lakes Cooperative. The program was moderated by Sheboygan County Extension Service crops and soils agent Mike Ballweg.

Solving a puzzle

Guilette likened the process of coordinating soil sampling and fertilizer applications to the assembling of a puzzle. He listed such pieces as soil and plant tissue tests; seeding rates; matching plant populations with the topography; product use and management; yield monitors; and the compatibility of information that is accumulated.

Taking soil samples to represent 2.5-acre portions of fields continues to be the most popular practice for submittals to AgSource, Guilette said. The cost of $10 per sample, when viewed in terms of the three or four-year intervals for sampling the same land, leads to the question of "Can you afford not to do it?" he said.

He agreed that reducing the size of the area represented by a soil sample would point out the differences in soil pH that could be significant within a field. The remedy for low pH is lime, which is needed most often in the western parts of Wisconsin.

In addition to the known nutrient inputs for crops, drought affects the availability of some of those nutrients, Guilette said. He recommended such practices as tissue testing, the use of aerial imagery, special attention to parts of fields where manure application restrictions apply and the comparison of soil types with yield monitor data as elements of "precision agriculture."

Looking beyond soil samples

Given the other factors that can easily affect crop yields, Hoffman said he has doubts about the reliability and value of 2.5-acre soil sampling grids. In many cases, he finds there isn't a very good correlation between soil sample testing data and crop yields.

That's often due to the significant influence such variables as drainage and weed control can have on yields, Hoffman explained. He noted that the application of lime "helps a lot where it is needed" but added that "manure is not always a good fit."

The existing soil type maps are not at the scale nor do they have the accuracy for use in "precision agriculture" practices, Hoffman said. He cited the instances of shallow soils, suggested that imagery is better than soil maps in many cases and heralded the value of farm operator knowledge on field patterns and production history.

To illustrate his point, Hoffman displayed various sets of data from the same 66-acre field. He described how a straight line arrangement for taking soil samples would not capture the existing soil variability within that field.

A call to think differently

"We need to think differently" on how to cope with variability within fields, Hoffman said. He emphasized it is not possible to eliminate variability, acknowledged it is possible to boost yields in some spots and indicated some portions of fields are likely to produce high yields at all times.

He also noted variable rate input technology has been available for about 25 years but is not widely used.

Yield maps have good value when they are paired with zone management practices within fields, Hoffman said, and he predicted more use of that approach within the next five years.

Instead of heavy reliance on soil samples drawn from standard field grids, Hoffman opts for zone management as a better way to identify variability within fields. With use of that data, along with attention to basic agronomy practices such as weed control, variable planting and nitrogen application practices can follow and be beneficial.

Availability of data

Rock River Labs, which works only with retailers, also subscribes to 2.5-acre grids for taking soil samples, Fleming said. The grid sampling technique is good for repeatability, accuracy and tracking changes.

Fleming emphasized that the information is easily available to all parties involved in crop production. The business focuses on working with partners, establishing trust, helping growers set goals and calculating a cost versus benefit strategy.

A centered method (10 cores from a 10-foot circle) within a grid is a better way to identify variations better than the collection of samples in a wiggly or W pattern because the latter could leave gaps of up to 60 feet, Fleming said.

For integration with some other data systems, parcels of 4.4 acres would be ideal for soil sampling, he added. For his model of potential variability, he displayed the pH data and nutrient residuals for a 137-acre field. He likes the use of overlays of soil and yield maps.

Tweaking of perfection

"Perfection is the enemy of getting it done," according to Karl Harpstead of the Kettle Lakes Co-op. He spoke from the experience of having used soil sample data for applying variable rates of fertilizer on "thousands of acres" in recent years.

Harpstead cited the relatively low cost of $1 to $2.50 per acre, averaged over four years, for sampling soil in 5-acre grids. He noted that sampling in grids of 2.5 acres is also a choice. Beyond soil sample data, Harpstead suggested the aerial imagery if the chore is to apply varying rates of nitrogen on corn.

There is a good potential for high variability on pH, the potassium needs for some crops are quite high and overall nutrient needs can vary greatly within a field, he said. On the other hand, it is worthwhile to recognize frequently drowned out spots where there could easily be a high residual of nutrients.

In practice, those factors call for a variable rate applications rather than a straight rate, Harpstead said. He noted that all of the Kettle Lakes terragators are equipped for variable rate applications and that one of them has a twin bin for applying two different nutrients.

"Don't waste, and don't apply too little," Harpstead said. He acknowledged that there isn't solid proof yet of better yields but variable rates "have that potential."

Limitations on variable rates

Agreeing with the latter assessment was Adell Co-op agronomist Roger Lucky. He, too, noted that he doesn't have any yield data to calculate a payback timetable. For that reasons, he advises farmers to employ a variable rate strategy on a few fields in order to test the practice for themselves.

Lucky doesn't promise farmers they will save on the total application of fertilizer but that the differences would be on where it is applied within a field. He finds that the best situation for variable rate application is on combined fields where the parts have a different cropping history.

One drawback with the equipment is that a shut-off option is not available on any portion of the 70-foot wide air flow application units geared for handling one or two products, Lucky pointed out. Changes of rate are typically accomplished within a travel distance of 20 feet.

Soil sampling in grids of 2.5 acres is the standard practice for Adell Co-op, Lucky continued. He also cited a sampling cost of $2 to $2.50 per acre averaged over four years but added that there's an extra cost for variable rate applications.

Lucky said Adell Co-op generally follows the Extension Service recommendations, which are based on the combination of soil type, the crop being grown and cropping rotation history, but it will also honor the requests of the farm operator. For the latter, it is also important to keep an eye on the nutrient management plan for the farmland, he said.

Considering the points made by the presenters, Ballweg suggested that farmers "take some baby steps" toward where practices seem to be headed in the long term. He noted farm suppliers now have the equipment for providing variable rate application services.

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