How soil samples are treated after they leave the field can have dramatic effects on soil test accuracy, according to studies in Oklahoma and Kansas. Inaccurate tests can lead to both under and over fertilizing crops, particularly if sampling takes place when conditions are higher than 80 degrees F.

Recent Kansas State University (K-State) research by soil fertility specialist Dorivar Ruiz-Diaz showed significant changes in soil test nitrogen (N) credits occurred between refrigerated and unrefrigerated samples over a 14-day storage period.

“Soils are home to a diverse population of microorganisms, many of which help decompose crop residue and cycle nutrients in the soil,” Ruiz-Diaz says. “This nutrient cycling is crucial for crop production but can skew soil test results if it continues in soil samples after they have been collected.”

The N cycle in the soil is particularly complex and is strongly influenced by microbial activity and, therefore, temperature and soil moisture conditions. During the process, N contained in the organic matter undergoes several transformations, ultimately converting it to plant-available ammonia, also known as mineralization.

Ruiz-Diaz says microbial activity conditions continue as long as temperatures and moisture levels are conducive to their activity. As a result, soil tests for plant-available N have the potential to change substantially if samples are not handled correctly. This is important because such tests are used to determine the profile-N credit and, ultimately, adjust N fertilizer recommendations.

Temperature Tests

The K-State scientists demonstrated the changes which can happen if soil sample submission is delayed by collecting similar samples at the university’s agronomy farm, mixing and sieving them, and then placing them in sample bags randomly assigned to different combinations of storage temperature and duration. One set was refrigerated, while the other set was stored inside a cargo box in a truck bed. To monitor the changes in soil test levels over time, 3 sample bags were removed from the refrigerator and box every 48 hours and tested in the lab.

Over the 14-day test, mineralization and nitrification led to more than triple the increase in soil test nitrate (NO3) in the undried and unrefrigerated truck cargo box treatment samples. Meanwhile, refrigerated soil test nitrogen levels showed insignificant changes.

“Nitrogen credits were nearly 100 pounds higher for unrefrigerated samples…”

“Profile N credits calculated from the soil tests were nearly 100 pounds of N per acre higher for unrefrigerated samples,” Ruiz-Diaz says.

K-State recommends soil samples be submitted to the lab as soon as possible, ideally on the same day they were collected. If same-day submission is not possible, samples should be air-dried or placed in a refrigerator set at 40 degrees F or less.

Next Step

In August, Oklahoma State University (OSU) extension precision nutrient management specialist Brian Arnall and his associates took the tests a step further and tested the impact of sample bag types. They collected samples, ground and homogenized them, and placed them in 3 types of bags: Ziploc, a closed commercial, resin-lined paper bag and a breathable material bag known as SWFAL. Samples in all 3 types of bags were placed in a minivan in the field and also brought to an OSU laboratory.

“With both sets of samples, we had temperature loggers collecting hourly data,” Arnall says. “Every 3 days, we collected 4 samples from each treatment, determined the moisture content and ran a full spectrum of analysis including CO2 burst. The soil for both treatments initially tested 1.1% organic matter and had a pH of 6.1.”

Average daily temperatures were recorded in both the sitting vehicle and the laboratory. On days 2-5, the average temperature of the van was over 100 degrees F, but the temperatures dropped after that. The readings from the van reflect an average of wide-ranging highs and lows between morning and midday, while the office temperature remained stable throughout the day.

“The nitrate levels changed the most in the temperature test…”

“The nitrate levels changed the most in the test,” Arnall says. “This is the form of N measured in most soil tests. With a starting point of 3.4 pounds of N per acre based on a 6-inch soil sample depth, the field samples had jumped to 15.6 pounds by day 6. The office samples had much slower increase in NO3 with it taking 9 days to reach 10 pounds.”

Arnall says the position of the samples in the van also showed a significant effect on soil test results.

“We set each replication of samples in a different spot in the van — the front, middle and rear seat — and the thermometers showed slightly different temperatures with each replication, which in turn affected the samples,” he says.

While the majority of nutrients were relatively unaffected by the bag type or storage, the bag and where it’s stored will affect nutrients that are more heavily influenced by organic matter and biological activity. Arnall reminds growers that sealing a soil sample in Ziploc-type bag creates a greenhouse effect where moisture cannot escape and provides a perfect environment for microbes to thrive. This significantly speeds up the mineralization of NH4 N to NO3 N. In the OSU test, the use of SWFAL soil bags provided data showing no significant impact of storage or time.

Arnall says the results show the importance of proper storage and proper storage bags for those who use laboratories that run the Haney test, which includes measures of NO3, NH4, organic N, CO2 respiration and water-extracted organic carbon.

“It is critical that the samples be brought into climate-controlled facilities as soon as possible when using these types of bags,” he says. “It’s also good to remember the soil we used had organic matter of 1.1%, initial NO3 of 3.4 pounds and NH4 of 14 pounds. A soil with higher organic matter percentage could have even greater change. Another variable is if the soil sample depth is greater than 6 inches. If your sample depth is 8 inches, then the value is 33% higher.”