Fall: A Perfect Time to Soil Sample

Posted November 7, 2012 at 8:00 am


Ruth Beck

Agronomy Crops Field Specialist

The weather this fall is providing a perfect chance to catch up on soil sampling on your farm. Fall can be the best time to sample soil to determine fertilizer needs for the next crop for a couple of reasons. Soil testing in the fall spreads the work load for producers, the soil is often drier making it (usually) easier to sample and results from a fall soil test allow time for decisions to be made for the next crop.

The level of many soil nutrients does not change over winter months. Phosphorus, potassium, pH, soluble salt content and many micro and secondary nutrients are not affected by sampling time, and measured levels should be similar in the fall as to test results from spring samples. Soil biological activity does affect nitrogen and sulfur soil test levels especially when soil temperatures are above 50 deg F. If the soil temperature is below 50 deg F. and/or soil remains dry, soil biological activity will be limited reducing changes in measured nitrogen and sulphur.

Recommendations for nitrogen are to sample every year since nitrogen levels can fluctuate a lot depending on conditions. Sulphur is also a mobile nutrient and should also be sampled annually. For accurate soil testing for nitrogen and sulphur, sampling should be done at the 0-6 inch depth and at the 6-24 inch depth. Samples from the different depths should be submitted separately.

A zone sampling program based on yield zones within a field could offer some large nitrogen fertilizer savings by putting fertilizer back to areas where it is needed next year and by not oversupplying other areas in a field.

Uniform zones within a field or uniform fields should be sampled in a random pattern across the area by collecting approximately 15-20 equal size cores per 80 acre parcel. Cores should be air dried or frozen prior to shipping. Whenever you are soil sampling be sure to allow for time for the soil samples to air dry, ship to lab, and for generation of an analysis at the lab.

Dr. Ron Gelderman, SDSU Soils Extension Specialist, said that in dry years such as 2012, there can be a higher than average carryover of nitrate-N in soils. This difference can mean some savings for producers next year. However guessing at carry-over levels could also possibly lead to yield losses as situations can also occur where the carryover is not there. It is possible for phosphorus and potassium levels to have increased in soils because of less being removed with lower yields. However, soil tests after a drought have also shown, at times, that potassium is actually lower than in a normal year because lower rainfall has not moved the potassium from the plant residue to the soil.

Soil testing is really the best way to determine soil nutrient levels. Sampling cost is inconsequential compared to potential fertilizer savings or yield gain.

More information on soil sampling is available at; http://www.sdstate.edu/ps/extension/soil-fert/upload/SamplingSoils_SDFS50.pdf or check out this short video http://igrow.org/livestock/pork/soil-sampling-factsheet-and-short-video-available/

A list of nearby laboratories that perform soil sampling is also available;


Mark your Calendars

*Ag Horizons Conference: Nov. 27-28 in Pierre

*Soil Health Info Day; December 11, in Mitchell, SD