Soil Sampling

Fall and winter is a good time of year to test soils due to a reduced workload and more time to use test results to develop a nutrient management plan for the following crop.

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Figure 1

Figure 1. Sampling areas based on variation across the field as indicated by differences in soil type and an old fence line.

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  • Soil tests are recommended every 3 to 4 years or when there are specific fertility concerns.
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  • Key items to consider when soil sampling are timing, spatial resolution, depth, and tillage system.
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  • Fall and winter sampling should happen after harvest and before the ground freezes, and prior to any tillage.
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  • Samples collected from the top 6 to 8 inches of the soil can be used to determine soil pH, lime needs, and the amount of organic matter, phosphorus, potassium, sulfur, and zinc present.
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  • To assess nitrate-nitrogen in the root zone, samples should be collected to a depth of 24 to 36 inches.
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  • In no-till or reduced-tillage systems, nutrients can become stratified. Consider having a separate analysis run on the upper 2 inches of the soil cores.
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  • For greater accuracy in year-to-year comparisons, samples from the same field should always be taken at the same depth and time of year.
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  • For general soil fertility measurements, take 10 to 15 soil cores for each composite sample.
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  • Samples can be grouped according to different management areas of the field such as soil type, slope, and yield potential (Figure 1).
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  • Check with your local soil testing laboratory for specific instructions on collection of soil cores, preparing, and shipping samples.
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Once you have an accurate and comprehensive field assessment from soil sampling, soil testing can be repeated at the same locations to track fertility and pH changes over time. Subsequent sampling can be redefined in order to analyze only representative areas of each field. Another option may be to reduce future sampling to only problem areas.

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By examining the pattern of variability, a grower can learn more about his field. The patterns of high and low soil property values may be tied to soil type or slope, or they may help explain yield variability. Interpreting the variability in soil test values can take a bit of detective work. It may require looking at past farm records or recalling farming in the past.

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In summary, the quality of the sampling methods greatly influences the accuracy of soil test results. An investment in soil testing is an opportunity to make more accurate soil management decisions.

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