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Leandro Vieira (Leo), Olson, Gregory, Gauthier, Ted
How can the grower efficiently balance nutrient management for productive and resilient plants? For this to be possible, there is no shortcut. The collection of a soil sample to have it tested is a valuable step needed to begin the process of understanding your crops and their potential. Soil testing can be considered the greatest return on investment when planning for fertilization. Without a soil test, it is possible that the nutrient that is limiting yield and plant performance is not among the nutrients that the grower is regularly applying during fertilization. As the Sprengel-Liebig’s Law of Minimum infers: “Plant growth is limited by the scarcest resource, not the total resources available” (Figure 1).
Moreover, if soil pH is not within the ideal range for the target plant, time and resources can be spent adjusting soil pH instead of additional fertilizer applications that will not help amend the soil pH. Soil pH plays a major role in the availability of nutrients (Figure 2). A pH below the recommended range will limit the availability of all macronutrients (nitrogen, phosphorus, potassium, sulfur, calcium and magnesium) and a micronutrient (molybdenum) in the soil. Thus, testing your soil is essential to ensure long-term sustainability by preventing plant growth decline and promoting nutrient use improvement while directing grower available resources towards supplementing what the soil is missing.
Figure 1. Graphic representation of the Sprengel-Liebig’s Law of Minimum using the barrel stave concept: “Plant growth is limited by the scarcest resource, not the total resources available.” Illustration provided by Leandro Vieira
Figure 2. Soil pH affects the availability of elements in the soil. Illustration provided by Leandro Vieira
Selecting a soil testing lab is important; however, properly collecting a soil sample is even more important. The soil test report you receive will only be as good as the sample collected. Furthermore, many soil testing labs have personnel that have been extensively trained and have proper facilities to conduct all appropriate soil analyses. Thus, properly collecting your soil sample is crucial to ensure that your fertilization program will be as effective as possible. The axiom “a chain is only as strong as its weakest link” applies to the soil testing line of sample collection and analysis.
Collection > Submission > Analysis > Reporting > Interpretation > Application > Crop Yield
If the sample collection process has any issues, the rest of the steps along the way can magnify those issues.
When selecting a soil testing lab, the chosen laboratory must use the Mehlich-3 extraction process to quantify the amount of plant-available nutrients in the soil (Mehlich, 1983). Different extractants remove different amounts of nutrients from the soil. Because of this, different extraction processes are usually not comparable. Basically, different extractants are developed for different regions and/or different soil characteristics. Mehlich-3 methodology is the one that better estimates the amount of plant-available nutrients in the native soils of Louisiana and is the primary method used for fertilizer recommendation studies.
In this publication series, additional fact sheets will focus on the specifics of soil sampling for different scenarios (e.g., homeowners, field crops, pastures and hayfields). The foundation of collecting a representative soil sample is based on six pillars (Figure 3). Following these instructions will ensure that the soil test report will reflect as much as possible the fertility of your soil. Additionally, never use an oven to dry your soil samples at home. This will change the chemical properties of your soil (especially for temperatures above 140 F). These chemical changes will affect the reliability of your soil test report.
Step 1. Divide field areas into areas (less than 5-20 acres each) for sampling based on differences of slope, crop to be grown and soil type.
Step 4. mix the 15-20 cores thoroughly in a clean container.
Step 2. Take soil samples at the depth of 6 inches.
Step 5. Fill the soil box (or soil bag) with 0.5 pounds of soil and label it with field identification and customer information.
Step 3. Collect 15-20 cores in each area of interest to obtain a composite soil sample.
Step 6. Fully fill out the lab submission form and place it with your samples in the shipping box.
Figure 3. Follow these general guidelines for proper soil sampling. Illustrations provided by Leandro Vieira
The LSU AgCenter Soil Testing and Plant Analysis Laboratory (STPAL), located at 128 M.B. Sturgis Hall on the main campus in Baton Rouge, provides soil analyses and interpretation for a small fee. STPAL also provides plant and soilless media analyses. Take advantage of soil testing to ensure your plants thrive. Visit the STPAL website for more information about the services provided.
Also, take note of the improved aesthetics of the brand-new soil test report from the LSU AgCenter Soil Testing and Plant Analysis Laboratory (Figure 4). STPAL uses research-proven protocols to analyze acidity and nutrient availability in the soil (Mehlich-3 extraction method).
Figure 4. The old (left) versus the new (right) soil test report template from the LSU AgCenter Soil Testing and Plant Analysis Laboratory. Illustration provided by Dr. Leandro Vieira
Havlin, J.L., Tisdale, S.L., Nelson, W.L., and Beaton, J.D. (2014). Soil fertility and fertilizers: An introduction to nutrient management. 8th ed. Pearson, Inc., Upper Saddle River, New Jersey (USA). 516 p.
Mehlich, A. (1984). Mehlich 3 Soil Test Extractant: A Modification of Mehlich 2 Extractant. Communications in Soil Science and Plant Analysis, 15, 1409–1416.
Parvej, M.R. (2024). Soil Acidity, Liming Materials and Lime Recommendations. Louisiana State University Agricultural Center, P3939.
Vieira, L.O., Gauthier, T., Olson, G.M. (2026). Don’t guess: Soil Test! Louisiana State University Agricultural Center, P4062.
Vieira, L.O., Pereira-Neto, J.D., Twidwell, E.K. (2025). Fertilizer recommendations for pastures in Louisiana: N-P-K-S. Louisiana State University Agricultural Center, P3950-E.
Vieira, L.O., Tubana, B., Padgett, G.B., Moseley, D., Gravois, K., Levy, R.J. (2025). Fertilizer recommendations for field crops in Louisiana: N-P-K-S. Louisiana State University Agricultural Center, P4004.
P4072-A
8/26/26
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