KTIC Radio Crop Tech Cafe Corner: Soil sampling depth

soil sampling depth

Soil probe marked with v-shaped notches at 6 and 8 inches. Easy way to manually mark (I used a small triangular metal file) a soil probe for visibility during sampling to help push to a consistent depth. V-shaped notches reflect light back up to your eyes while sampling.  No need to use marker, tape, dowel, etc.!

Listen to this week’s Crop Tech Cafe Radio Corner on KTIC: Soil sampling depth


This is Dr. Nathan Mueller, your local agronomist with Nebraska Extension for Dodge and Washington counties. Whether you take soil sample yourself or hire someone to do it for you, there are some important guidelines to follow when soil sampling.

It’s extremely important to control the depth at which soil samples, especially for managing soil pH and phosphorus. So how deep should soil samples be taken?  The answer is it depends! It does depend on the nutrient or nutrients you want to manage and the research done behind the soil test interpretations.

The soil fertility research done at land grant universities has really determined the best soil sampling depth. And here is why soil sampling depth matters… We know that most soil nutrients, quality, and pH vary or change with depth. Typically soil organic matter and phosphorus concentrations decrease as we sample deeper. However soil pH, calcium, and magnesium concentrations can increase with sampling depth. Yet some nutrients are mobile in the soil such as chloride, sulfate, and nitrate and sampling deeper is helpful, even down to 2 for 3 feet.

Let’s focus on phosphorus this morning. I recommend sampling the top 6 or 8 inches. You notice I didn’t say 6 to 8 inches. Calibration and correlation research work done at UNL is based on a 0 to 8 inch sample and Iowa State University is based on a 0 to 6 inch sample (KTIC radio covers eastern NE, western IA, and little of southeast SD). It may not seem like a big deal, but it is. The interpretations of the soil test results and need to add fertilizer to meet the crop’s need could change if you sample 0-6 inches versus 0-8 inches deep and use the wrong interpretations (UNL versus Iowa State). In short, if your soil is sampled to 6 inches deep, then use the Iowa State soil test interpretation. If you sampled deeper to 8 inches, follow UNL interpretations. If you sampled 8 to 10 inches deep your soil test phosphorus value will likely decrease compared to shallower sampling for the same field, and you are more likely to over fertilize your crop.

So pick a depth, mark you soil probe (see picture and caption above), and push the probe to a consistent depth each time when making your composite soil sample. And if you hired someone to soil sample for you, you might want to ask them how they took your samples!

For more information on best management practices while soil sampling, contact me at 727-2775 or visit our locally-focused website at croptechcafe.org. Know your crop, know your tech, know your bottom line. This is Dr. Nathan Mueller, your local agronomist for Nebraska Extension on KTIC radio.

KTIC Radio Crop Tech Cafe Corner: Soil sampling depth
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