Editor’s Note: This is the 1st part in a 4-part series on the 4Rs and nutrient management.

Soil sampling and testing is not an exact science, but it is one of the many tools farmers have to improve profitability and maximize yield potential. Soil testing combined with plant tissue testing, yield results and other field data, help growers to make sound nutrient management decisions all season long. I’ve outlined some best practices for successful soil sampling on your farm in this, part-1 in a 2-part series, covering “Nutrient Management and the 4Rs.”

Sample Number
A soil probe is the best tool for sample collection. Typically, five or more core samples are collected and mixed to make one composite sample.

  • This sample should represent a 2.5-acre area for fields in row crop or hay production.
  • Samples should be representative of the area within that 2.5-acre sampling unit.
  • Avoid highly eroded areas and waterways/toe slopes – soil test results can be skewed in those areas.
  • Avoid straight line sampling, it can be influenced by fertilizer, manure or starter fertilizer applications.

Sample Depth
The Illinois Agronomy Handbook (IAH) recommends a 6 2/3 inch soil sampling depth for all no-till and tillage systems because their limestone and fertilizer recommendation tables are calibrated for this sampling depth.

Sampling Pattern
There are a variety of soil sampling patterns that can be employed. With the 2.5-acre Grid Sampling, one composite sample consisting of five or more cores is collected from a representative area within the grid. While this approach is more intensive and expensive, it is the best option on fields that have never been sampled or sampled infrequently, to get a good picture of the soil test variability within the field.

Zone Sampling has the same number of cores per composite, but fewer samples per land unit area. The zone pattern can be based upon soil type, crop management or yield zones. Only a few composite samples are collected from each zone of like soil properties. This fits the larger, more uniform fields of southern Illinois, and especially central Illinois, where there are larger areas of a single soil type or relatively uniform crop yield levels

Sample Timing
Fall is the best time for soil sampling and nutrient management planning. Soils are at maximum drawdown on nutrients after harvest and this timing allows limestone to be applied prior to any fall tillage operations. However, it is possible to pull soil samples throughout the winter months and early spring, as soil conditions permit.

  • But remember, once you start a soil sampling program it is always best to resample as close to the same time of year and soil conditions as the previous sampling.
  • There can be a certain degree of variability in soil test results when collected in the fall versus the spring.
    • Depending on your location (north versus south), these changes can be lesser the farther north you go, provided the soils freeze over winter and stop microbial activity.
    • In the southern-most reaches of Illinois high degrees of variability can be seen due to the non-stop decomposition of crop residues all winter long.

Sampling Conditions
Soils are best sampled when crumbly in the hand, yet with a touch of moisture. Avoid sampling fields that have been excessively dry or wet for extended periods of time. Excessively dry or wet soils can result in false potassium (K) values. In the case of the dry soils it can take anywhere from 9 to 24 months before the soil K balances back out. Soil moisture conditions do not have an appreciable influence on soil pH or soil test phosphorus (P) values. Also, don’t sample frozen soils. Results will be highly variable and a uniform soil core may be hard to collect, if you can collect a sample at all.

Submitting Samples
Good soil test results begin in the field with sample collection.

  • Thoroughly mixed composite samples should be placed in a clean paper or lined paper bag with information clearly marked on the outside of the bag.
  • The bags have perforations in the lining to allow air and some moisture to escape.
    • Many commercial laboratories provide these bags to their customers at no charge.
  • Once you have all samples collected they need to be shipped to the testing laboratory as quickly as possible.
    • Samples straight from the field with adequate moisture should not be held for extended periods of time, microbes in the soil continue to grow and decompose the organic residues.
    • This decomposition can throw off the soil test results as compared to the day they were collected.

A typical laboratory soil test report package will include, but is not limited to, the following: water pH, buffer pH, P, K, Ca, Mg, percent organic matter (OM), CEC and percent base saturation. The P, K, Ca and Mg will be reported as “available” in the soil. The other nutrients like S, B, Cu, Fe, Mn, Zn, Na, available NH4+ and available NO3- are available upon request or in other packages from most laboratories.

If you are just starting out with a soil sampling program, fix the soil pH, P and K first. Then follow up with the S and micronutrients. Initially, start with the big stuff and don’t sweat the small stuff. After the soil pH, P and K issues are addressed then chip away at the other nutrients.

For a list of soil testing laboratories that service Illinois go to the following website: www.soiltesting.org.

Remember the Units
You will need a basic understanding of the reporting units on your soil test reports. Some laboratories report the results in pounds per acre (lbs/acre) and others report them in parts per million (ppm). There is a Rule of Thumb that states ppm x 2 = lbs/acre. This assumption only holds true if the soil sample was taken to a depth of 6 2/3 inches. However, as a general estimate this rule is often followed because of ease of calculation.

If you are unclear on the data presented on the report forms, consult the personnel at the laboratory or your certified crop adviser (CCA) – they will be able to assist you with making these conversions and understanding your report sheets.

Note: Coming up in Part #2 of this discussion: Nutrient Management, the Illinois Loss Reduction Strategy, and the 4Rs of Environmental Stewardship.

Terry Wyciskalla is an independent crop and soils consultant based out of Nashville, Illinois. He specializes in soil sampling, fertility recommendations, precision ag services and crop problem diagnoses. He serves a 12-county area throughout Illinois. He earned his 4R Nutrient Stewardship certification in 2015 and is happy to be a part of the 2016 Soy Envoy team.

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About the Author: Terry Wyciskalla

Terry Wyciskalla is a Certified Professional Agronomist, a Certified Crop Adviser, and a 4R Nutrient Management Specialist. He has a Master of Science (MS) in Plant and Soil Science and has spent 25 years as a soil fertility agronomist/precision agriculture consultant in a 10-county region in southern Illinois while also spending 16 years as a researcher in soil fertility and an instructor at Southern Illinois University Carbondale.