Considerations for Proper Soil Sampling

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

It’s the time of year to start planning soil sampling, but remember all soil samples aren’t the same. Cropping system, soil type and past management contribute to how you should sample.

Consistently sampling the correct depth is key to good fertility recommendations. A depth of 6-8 inches is the typical recommendation for fields under tillage, since that much of the soil is often mixed together. To get a good idea of nutrient or lime needs for pastures, samples should be taken no deeper than 4 inches. The reason for this is stratification of nutrients and acidity. In pastures only rainfall moves nutrients and lime into the soil profile, versus mixing that upper layer with tillage. Deeper samples may cause you to overestimate nutrient or lime needs for pastures. Michigan State University observed 228% more P2O5 and 180% more K2O at 4 versus 7 inch samples. The deeper samples may cause you to add potash to pastures that don’t yet need it.

No-till fields can be similar to pastures when considering nutrient stratification, but a 6-8 inch fertility sample is still recommended. Sampling in no-till systems should be based more on nutrient placement. If you band fertilizer, be sure to avoid sampling those locations. Don’t avoid entirely, it still represents soil nutrient levels, but bands can skew your soil tests and cause you to under apply. One recommendation for dealing with bands, is for every one sample taken within a band, take another 8 samples x the distance (in feet) between the bands (http://www.cropnutrition.com/efu-soil-sampling).

Acidification from nitrogen fertilizers also creates sampling issues. Broadcast or surface applied N often only causes acidity in the upper few inches, so a deeper sample may not capture the lime needs at planting. For surface applied N in no-till fields, a sample from 0-2 inches to determine pH is best. Knifed or injected N will cause acidity deeper in the profile, typically about 2-3 inches above the knife depth. For those soils, samples should be taken deeper in the profile, and the typical 6-8 inch fertility sample may work fine for determining soil pH.

Proper sampling can reduce error on your end. If pH or nutrient levels vary between years, consider whether you:

  • Sampled a consistent depth
  • Have a tillage, continuous no-till, or pasture system
  • Where and how you applied your nitrogen (banded, broadcast, knifed, etc.)

Errors in sampling can easily cause under/over application of fertilizer or lime. If you have any questions about where you should sample in your system, email me at Jarrod@udel.edu.

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