Intensive soil sampling makes dollars, and sense

By David Scheiderer, Integrated Ag Services

Traditional soil sampling is done on a 2.5-acre grid or more, and its effectiveness relies heavily on whether or not the person doing the sampling does a good job. This reliance on human precision leaves room for error. Such errors are unacceptable for many farmers trying to manage record high fertilizer prices against unimaginable volatility in the commodity market. There is the opportunity to gross $1,000 or more per cropland acre, and a farmer wants to capture as much of that as possible by applying costly fertilizer and lime at peak efficiency.

How a soil sample is taken can greatly impact the results. If the soil is wet, soil will compact inside a traditional soil probe, making it difficult to clean out between samples. As the person sampling travels through the field taking core samples over a period of hours, the effort it takes to keep  the probe unplugged can become frustrating. A partially plugged probe does not collect a uniform sample, adding tremendous variability to the test results.

Problems also can arise if the soil is too dry. In really dry conditions, it’s almost impossible to get the soil probe into the soil at the right depth. Many times what happens is the person taking samples will only get core samples at depths of 4 to 6 inches, instead of the more desired 6 to 8 inches. This can dramatically affect the results.

Farmers also may see varying results from their soil data if the sampling is done at different times of the year. So, depending on the time of year and the soil conditions at the time of sampling, the obtained results can be greatly impacted. Beyond the actual sampling technique, soil test results also can vary based on quality control checks at the laboratory. This is the least likely source of variability when it comes to soil sampling, because quality soil labs likely have controls in place to minimize lab error. However, University of Kentucky research shows that instrument variability between labs can cause pH results to vary by +/-2%, P results to vary by +/-12% and K results to vary by +/-8%.

To minimize lab and sample collection variability, farmers should:

• Use a reputable lab;

• Don’t switch from lab to lab;

• Sample fields at the exact same depth;

• Sample the same time of year (fall, spring) each sampling cycle, if possible;

• Use repeatable sampling techniques that collect samples exactly the same way each time. Repeatability reduces the noise in the numbers, providing more reliable results.


Benefits of intensive soil sampling

To reduce undue soil test variability and increase the reliability of their soil test data, farmers should consider more intensive soil sampling. Years of experience and testing done by Integrated Ag Services (IAS) show that the reliability of sample information increases dramatically going from 2.5-acre grids to 0.5-acre grids. To test how much in-field nutrient variability there really was, IAS intensively sampled a 140-acre farm every 100 square feet and another 165-acre farm every 150 square feet. A total of 860 soil samples were collected. Sampling at 2.5-acre transects yielded results for pH, P, K and organic matter at an average reliability of 26.7. In comparison, the reliability of these same soil results when tested at 1-acre intervals jumps to 57.1, and at half-acre intervals is further refined to 80.1 reliability.

While farmers may think soil sampling at these more intensive levels is impractical, that is not the case:

•       With today’s expensive input costs, more intensive sampling intervals can be justified.

•       The same can be said for high grain prices; intensive sampling intervals can be justified with very small incremental increases in yields.

•       The technology to automate soil sampling is what makes 0.5-acre and 1-acre sampling an economical solution.

•       Other information gleaned from accurate GIS soil analysis is very helpful in understanding more complex crop production issues.

To get a more accurate picture of the benefits of intensively collected soil data, here are some economics of soil sampling. Traditional 2.5-acre grid sampling costs about $8 per acre, and when amortized over the recommended four-year sampling window equates to $2 per acre per year. In comparison, 1-acre sampling costs $15 per acre per year, or $3.75 per acre per year over four years. Sampling at 0.5-acre intervals costs $20.50 per acre, or $5.12 per acre per year over four years. However, because the reliability of half-acre sampling is much higher, sampling only needs to be done every six years. So, amortizing the cost over six years equates to $3.41 per acre per year.

This increased expense, $1.65 per acre per year, of 0.5-acre sampling compared to 2.5-acre sampling can be easily justified. The added reliability of half-acre sample data and resulting increased efficiency of fertilizer and lime applications can translate into savings on input costs. A small decrease in usage of any one nutrient will more than offset the additional annual cost ($1.65) of 0.5 acre sampling

In terms of yield, by applying fertilizer and lime more reliably where needed, and at the rates needed, only small fractional increases in yield are needed to offset the cost of sampling. For corn, 2.5-acre grids require an increase in yield of 0.3 bushels per acre over four years to pay the cost of sampling, 1-acre grids require an increase in yield of 0.65 bushels per acre over four years, and 0.5-acre grids require an increase in yield of 0.60 bushels per acre over six years.


An automated system makes the difference

For the past four years, IAS has been working on a system to take intensive samples more accurately and efficiently. The result is the new patent-pending IAS Precision Soil Sampling System.

Using this system, IAS takes a sample every 150 feet, for a 0.5-acre grid or every 200 feet, for a 1-acre grid. Existing GPS technology is used to mark the location of these samples to develop application maps for farmers and fertilizer applicators. Human error is completely removed from the equation, because the samples are taken with the fully automated IAS Precision Soil Sampling System, which is mounted to the back of a tractor.

For example, when collecting half-acre sampling, every 150 feet, a knife cuts vertically into the soil at a 7-inch depth for approximately 30 feet. This means the sample is not just taken from one small spot, but in a vertical slice that is 7 inches deep and 30 feet long. And, this sampling technique is done exactly the same way every single time.

The automated sampler, combined with auto-steer technology, allows the operator to watch the system to be sure it is working correctly at all times. Strategically placed cameras are incorporated into the system, allowing the operator to watch every aspect of the operation. GPS and a bar-code reading system ensure each sample is positively identified with the GPS location it was taken from. This sampling technique, combined with a good quality lab, enables IAS to give farmer-growers the best information possible.


Three-tiered benefits

The benefits of intensive soil sampling are three-fold. With today’s high input costs, it’s not uncommon for a farmer to spend as much as $150 per acre on fertilizer, or more. As described earlier, 0.5-acre grid sampling provides farmers the most cost-effective and efficient information to ensure that fertilizer investment is spent as wisely as possible.

Second, also as described earlier, intensive soil sampling enables farmers to maximize yields in the best means possible.

Finally, this intensive sampling method ensures inputs are placed where they are needed, and at the rates they are needed, which is good environmental stewardship. No farmer wants to spend any more money than he has to for fertilizer, and no farmer wants to put his farmland at environmental risk. Intensive soil sampling allows farmers to capitalize on the economics while being good environmental stewards at the same time.

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One comment

  1. Amazing article! Thanks for spreading the word on correct soil sampling procedures!

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