This document printed from the University
of Illinois Extension From the Fields at http://www.extension.uiuc.edu/rockfordcenter/
Time to Soil Sample
October 5, 2009
Jim Morrison
Extension Educator, Crop Systems
Rockford Center 1601 Parkview Avenue
Rockford, IL 61107-1822
Phone: 815-395-5710
FAX: 815-395-5726 morrison@illinois.edu
Even though it is not perfect, soil testing is the most important guide to profitable application of phosphorus, potassium, and limestone. An accurate soil test provides a framework for determining the fertility status of a field.
Buy using a soil probe, collect at least five cores or subsamples within a ten-foot radius around a sampling point. When mixed together, these subsamples comprise a soil sample.
The number of samples taken from a field is a compromise between what should be done (information) and what can be done (cost). There are various sampling strategies depending upon cost, equipment to be used, past fertilization practices, and the potential response to fertilizer application. These strategies are explained in the new Illinois Agronomy Handbook, 24th Edition. One such strategy is one sample per 2.5 acres.
A seven-inch sampling depth is commonly recommended. If limestone or nitrogen has been applied to the soil surface and not incorporated, it is important to monitor surface soil pH by collecting samples to a depth of two inches from at least three areas in a 40-acre field. These areas should represent the low, middle, and high ground of the field.
Sampling every four years is encouraged when soils are at an optimum level of fertility. When maintenance levels (crop removal) are not being applied in cropping systems that remove large quantities of nutrients, such as hay or corn silage, soil testing should be done every other year. To improve consistency of results, collect the samples at the same time of the year, and from the same locations within the field.
Late summer and fall are the best seasons for collecting soil samples, because potassium test results are more reliable then. Potassium test results, unlike phosphorus and pH, tend to be cyclic with low levels in late summer and early fall.
For commercial soil testing services in your area, see this web site www.soiltesting.org , or contact your local University of Illinois Extension office, or fertilizer dealer.
For further information on soil testing and soil fertility consult the newly revised Illinois Agronomy Handbook, 24th Edition, available at the University of Illinois Publications Plus web site https://pubsplus.uiuc.edu/C1394.html , or contact your local Extension office.
