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Soil Testing: When, Why and How

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Soil Testing: When, Why and How


Although soil testing continues to be a best soil management practice, the way to go about it has changed. With the global positioning system (GPS), farmers can be smarter about how they conduct soil tests.

“One of the many advantages that this technology brought was related to soil sampling,” says Curt Woolfolk, senior agronomist for Western North America at The Mosaic Company. “Retailers and growers can now dissect fields based on GPS coordinates and soil sample at a much smaller resolution than ever before. The soil test values from those small management areas within the field can be used to drive variable-rate fertilizer applications. They also allow harvesting equipment to track low- and high-production areas within a field.”

Sampling your soil once a year is still the ideal recommendation for tracking your soil nutrient budget, but if once a year isn’t feasible, still consider increasing your frequency.

“If you are testing every four years, try every two instead,” Woolfolk says. “You don’t need to do intensive sampling each time, but closely track crop removal or do a more basic soil test every year, or as often as possible.”

Frequent soil testing helps farmers decide whether their current management is robbing future productivity and profits. Combined with local calibration data from university research, soil testing serves as the best guide available for determining nutrient needs for growing crops. Soil testing to provide a balanced fertility program is a vital component of sustainable farming programs that are profitable, efficient and environmentally responsible.

The right time to take soil samples is in rhythm with the crop rotation. Normally, it’s best to sample following back-to-back plantings of the same crop, which creates a consistent basis for comparing fields and picking out trends over time. Most samples are taken in late summer and fall to allow ample time for planning a crop nutrition program based on the 4Rs of Nutrient Stewardship: applying the right nutrient source at the right rate, time and place.

In the past, soil samples were taken almost at random across a field, mixed together, and sent to the lab. Now, with GPS capabilities, grid sampling has become the favored technique.

“We do grid sampling in 2.5-acre grids so that the field is split into areas,” says Glen Franzluebbers, director of professional ag services for Central Valley Ag (CVA) Cooperative, headquartered in York, Nebraska. “That way, we know what is needed for fertility in every part of the field.”

More retailers are adding precision agriculture to their offerings, and each year, more farmers are interested in these management tools in order to become more efficient. The first step in this process is soil testing, which is the foundation for understanding the variation in yield potential across a field.

“We do a lot of yield mapping, so instead of setting a blanket yield goal across the field — instead of fertilizing the whole field for 200 bushels an acre — we can start looking at those yield maps and historic yields, and almost create a yield potential map,” says Franzluebbers. “Then we can variably apply the nutrients, according to what the real potential is in each area of the field. If there’s a poor yielding area, where it might only make 150 bushels, then we should fertilize for 150 bushels.”