The accuracy of a fertilizer recommendation depends on the quality of the soil sample collected and analyzed to produce a soil test value. Taking the time to collect a quality soil sample is the first and perhaps most important step in developing a sound nutrient management plan. A quality soil sample should represent no more than 25 acres and be from a composite of no less than 10 soil cores. Sampling smaller areas (2.5 acres) with more soil cores (~15) provides more reliable information with greater confidence.
There are many resources on soil testing including:
- Ohio State University Soil Fertility Resources: agcrops.osu.edu/fertilityresources
- Purdue Extension Soil Sampling Guidelines: extension.purdue.edu/extmedia/AY/AY-368-w.pdf
- Michigan State University Soil Fertility and Plant Nutrition Resources: soil.msu.edu and canr.msu.edu/spnl
Soil Sampling Strategies
Four factors are generally considered when taking soil samples:
- Spatial variability of soil within a field
- Depth of sampling
- Time of year when samples are taken
- How often an area is sampled
Spatial Variability
The degree of spatial variability determines how many soil samples are needed for a field. All fields have some degree of natural horizontal and vertical soil variability, so the density of soil sampling should increase as field variability increases. Deciding on a soil sample strategy is always a trade-off between the collection and analysis cost and the level of detail of the information gathered. Soil samples should represent no more than 25 acres and be from a composite of no less than 10 soil cores. A common practice is a 2.5-acre grids (1 soil sample to represent 2.5 acres), and many growers are moving to higher density samplings such as 1-acre grids. These higher density approaches require a larger investment in soil testing but can provide more precise information to manage nutrients profitably.
Sampling Depth
Soil samples used for nutrient recommendations should be taken to the same depth each sampling period to ensure changes in soil test levels can be reliably tracked over time. The recommendations here are based on a 0 to 8-inch soil sample, following the original tri-state recommendations. However, many agricultural practitioners sample at different depths than 0–8 inches (e.g., 0–4″, 0–5″, 0–6″). Soil samples based on shallower sampling depths typically do not align perfectly with the recommendations presented here, as shallower soil samples often return higher soil test values relative to 0–8″ samples due to stratification of pH and nutrients.
Time of Year to Sample
Soil sampling after harvest in the fall or before planting in the spring is recommended. Fall sampling is preferred if lime applications are anticipated. Soil samples taken in the spring can produce different results than fall samples due to the effect of moisture on soil pH and nutrient levels, particularly K.
Intervals Between Sampling
Most fields should be sampled every three to four years. Phosphorus, potassium, and pH are highly buffered in this region, so changes in soil test values from one year to the next are typically modest. Shorter sampling intervals (one to two years) are recommended on low cation exchange capacity soils (CEC <5 meq/ 100 g) where rapid changes in fertility can occur. Although not essential, consider sampling at the same time within a crop rotation.
Adaptive Nutrient Management
Soil testing provides the foundation of an adaptive nutrient management strategy when sample depth, time of year samples are collected, and intervals between sampling are all kept consistent. Maintaining consistency over many years enables a grower to monitor soil test trends and evaluate how management practices and nutrient management regimes are performing. This can be critical information to further refine a fertility program, control input costs, maximize farm profitability, and meet management goals.
All soil nutrient test data in this publication are reported as parts per million (ppm) rather than pounds per acre (lb/acre). Preference is given to ppm since it represents what is actually measured in the laboratory. Soil test values are an index of availability and not the total amount of available nutrients in soil (a common misconception when reporting in lb/acre). To convert soil test data from lb/acre to ppm, divide the lb/acre value by 2.
Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa
Executive Summary | Soil Sampling, Handling, and Testing | Soil pH and Lime Recommendations | Nitrogen | Phosphorus and Potassium | Calcium, Magnesium, and Sulfur | Micronutrients | Additional Resources | Authors and Acknowledgements
This website provides a summarized version of the Tri-State Fertilizer Recommendations, reporting only the main points of the document, but lacking comprehensive detail. For complete information, please see the full version which is available from The Ohio State University Extension Publications Store.