BLOOMINGTON, Ill. — Soil sampling, testing and interpretation can be confusing. To help navigate this  complex issue soil sampling was one of  the topics of a recent soil management meeting held at the Illinois Soybean Association (ISA) office. 

According to John Jones, Assistant Professor of Agronomy and Soil Fertility Extension Specialist at the University of Illinois, recent research in collaboration with Precision Planting and Cropsmith, Inc. will serve to update the Illinois Agronomy Handbook in regards to crop soil testing recommendations. Jones, who spoke to farmers during the ISA’s “Talk Dirt to Me” seminar on Thursday, December 11, said on-farm and clinical research funded by the Illinois Nutrient Research and Education Council (NREC) using improved technology is reshaping current recommendations for agricultural soil testing. 

“The exciting thing to think about is which new technologies and sciences we can apply to get to a more accurate and precise prediction of fertilizer response,” said Jones. U of I’s ongoing NREC-funded research, which is serving to update soil test phosphorus (P) and potassium (K) interpretations, specifically, is the largest and most expansive P and K calibration effort in the U.S., Jones added. 

“We’re (testing) corn and soybean every year, conventional, strip till and no till comparison, a lot of rate and timing comparisons and with the amount of research sites we have across the state (around two dozen) I think we can represent the state soil regions very well,” he said. 

Jones advised farmers to avoid wet chemical methods of soil testing, which, he said, should be field-calibrated by an independent source if utilized. He also eschews recommending soil testing for micronutrients when testing soil nitrate levels. This should only be done when there is reason to suspect a soil deficiency. 

“The biggest consensus is that soil testing for micronutrients has generally failed us in terms of predictability,” said Jones, who grew up on a farm in south-central Wisconsin located on Saybrook and Plano soils. “I would say soil testing for zinc has been most consistent in having a relationship with response to yield, but boron, manganese, iron…we just haven’t had that clear relationship between soil test levels and yield response.”

Corey Lacey, environmental policy manager for the ISA, was in attendance at the meeting. As President of Agricultural Laboratory Testing Association, Lacey knows that farmers’ decisions improve with help from soil data provided by high-functioning soil-testing laboratories.

“Soil testing represents one of the most cost-effective investments on a farm, typically amounting to only $1 to $2 per acre in a standard testing cycle,” Lacey said in an ISA news release. “Despite its low cost, it yields one of the highest returns on investment. This is because it enables farmers to use precision agriculture technology (grid sampling, variable rate technology maps, etc.) to make informed fertilizer and lime applications, which can amount to hundreds of dollars per acre in input costs.”

The University of Illinois Extension offers recommendations for soil testing methods. Their most recent recommendations, co-authored by Jones, include the following guidelines: 

A soil probe is the best implement for obtaining soil samples, although an auger or a spade may be used if care is taken to collect an exact depth with a consistent slice thickness. 

Soil samples should be a composite of 10 to 15 cores (whether grid or zone sampling) from the intended management area within a field, depending on field characteristics such as soil types, land or fertilizer uses, and terrain. Collecting 15-20 cores per area can improve representation. Soil sample bags can be obtained from soil testing labs or input dealers and should be filled to the indicated line, around 1 quart.

The fertilizer recommendation system in Illinois is based on crop response to fertility levels in the top 7 inches of the soil, which is the proper sampling depth for pH, phosphorus, and potassium. 

Samples that are too shallow will overestimate actual soil fertility, leading to the underapplication of lime or fertilizers. Samples taken too deep or where part of the top portion falls off during sampling will underestimate current soil fertility, causing over-application of lime or fertilizer.

Soil samples should be collected to the depth and at the same time which the soil test methods were calibrated. In order to provide a true measurement of fertility, soil samples should be collected from a depth of 7 inches, the same points of the field each time, and at the proper time as well as collected with the appropriate sampling distribution. 

Soil testing is strongly suggested every 2-4 years if soils are at an optimum level of fertility, based on producers’ goals, and base applications have been calculated. Samples should be collected at the same time of year. 

Sampling should be avoided within a few months after a lime or fertilizer treatment due to higher nutrient values. After harvest in the fall is an ideal season to sample because soil moisture conditions are generally more stable, which can affect measured values of soil pH and potassium. Sampling in the spring is a viable option if soil sampling conditions are favorable, however, many times these results are not timely enough to guide P, K, or lime application until the following fall.

Sample locations can be identified using a global positioning system (GPS) unit or by accurately measuring the sample points with a device such as a measuring wheel. 

Depending on the farm goals, the recommended tests include pH, phosphorus, potassium, calcium, magnesium, and sulfur. 

Agricultural producers should contact an Extension office or fertilizer dealer to find a recommended soil lab near their operation. More information on U of I Extension crop soil sampling recommendations is available at: https://extension.illinois.edu/crops/soil-sampling. The Illinois Agronomy Handbook can be accessed at https://extension.illinois.edu/global/agronomy-handbook. 

Contact Jones with soil sampling questions at jones86@illinois.edu.