Ellen Phillips, University of Illinois
Soil testing is one of several diagnostic tools used to evaluate soil quality, specifically pH, and soil nutrient and organic matter levels. Soil tests and their interpretation for fertilizer recommendations are based on decades of research to correlate the soil test numbers with the amount of fertilizer applied and the resulting crop yield. Conventional chemical soil testing depends on taking a representative sample, then using appropriate soil nutrient extraction methods that have been calibrated to fertilizer rate studies that indicate the most economic rate of fertilizer applications to maximize yields. Because of the complexity of organic systems, as well as the dependence on the biological release of nutrients, utilization of traditional soil testing methodology and interpretations may need to be reconsidered within organic systems.
What are some of the potential benefits of utilizing chemical soil tests?
- Gathering baseline data on nutrient levels in new fields can help in making decisions on the use fertilizers, amendments, and cover crops to improve soil quality.
- Some of the basic soil chemical tests such as pH and organic matter, in combination with soil texture analysis, can indicate which crops will grow best on that soil.
- If concerns arise about nutrient deficiency symptoms or low yields while crops are growing, chemical soil tests can add pieces to the puzzle of trying to improve soil quality so crops will thrive.
- Organic systems often have a heavy reliance on compost or manure. Understanding nutrient cycling within these systems is important to avoid nutrient overloads and potential pollution. Chemical soil tests become a monitoring tool to avoid excessive additions of nutrients to your farm system.
- You may be required by the Organic Certification process to conduct soil tests in order to apply micronutrients or other fertilizers.
Soil Sampling Methods
The first step in conventional soil testing is to take one soil sample per approximately every 10 acres of cropland. This one sample is actually made up of 3 to 10 subsamples, to a depth of 7 inches, within the sampling area to try and get a representative sample. Currently, intensive grid sampling of large fields for precision applications has become common. In doing so, many unique areas, e.g., by gravel roads or wet areas, may be left out or sampled separately. Conventional soil samples are usually taken every three to four years, depending on the state, and are typically taken in the fall.
Organic System Considerations
In many cases, organic systems require more intensive soil sampling than conventional systems, since they often have a greater diversity of crops and rotations. Sampling each unique field or garden area with different crop rotations and amendments may result in a much larger number of samples being taken than one sample for 10 acres.
Depth of sampling may need to reflect depth of tillage, depth of amendment incorporation, or perhaps depth of rooting of the predominate crop (corn vs. lettuce) within the crop rotation. If no tillage is used, shallow soil sampling of 2 to 3 inches may be best to evaluate the distribution of nutrients in the surface soil. Farmers need to decide what sampling protocols will give the most information to answer their questions about how to modify their systems to increase nutrient availability.
Timing of sample collection may not be related to calendars. Instead, samples might be collected to correlate to a crop sequence within the rotation. If significant organic materials such as manure or compost are incorporated in the fall, sampling may not take place until the spring to evaluate the amount of nutrients released.
Because the initial sampling scheme establishes the baseline for comparisons of future soil tests and interpretations of how management decisions influence soil chemical, biological, and physical properties, serious consideration should be given to the initial sampling strategy for each field. Sampling timing and depth will probably differ from traditional sampling, therefore interpretation and fertilizer recommendations from conventional systems may not be directly applicable to your organic system. Thus, the year-to-year changes in soil test values of fields, when sampled consistently in the same manner, becomes the predominate value of chemical soil tests.
Soil Testing Methods
Choosing A Soil Testing Lab
Labs can run different soil tests depending on the type of soil, the chemical and physical properties of the soil, as well as the availability of calibration data for the interpretation of test results. Labs should participate in one of the available lab certification programs. The largest program is the North American Proficiency Testing (NAPT) program. It is a national program managed through the Soil Science Society of America. It is important to choose one lab that will be able to provide consistent results and services throughout the duration of your farming operation.
Conventional chemical soil test labs are almost a century in the making. The soil test methods, field calibration research, and interpretation for fertilizer recommendations are based on an abundance of research. Particularly, the calibration data and interpretation tend to be state specific. Therefore, it is important to become familiar with the methods a lab utilizes and what data they are basing their interpretations on. Standard soil test methods vary by region.
Traditional soil testing includes analyzing for pH, phosphorus, and potassium for a nominal fee ranging from $5.00 to $15.00 per sample. Additional soil tests for calcium, magnesium, sulfur and micronutrients are generally also available. Many labs offer other soil tests as well, such as organic matter, texture, cation exchange capacity, and others. The one nutrient that is often not analyzed is nitrogen, which transforms readily within soil making it difficult to measure and interpret results. See Soil Microbial Nitrogen Cycling for Organic Farms for more information.
Organic System Considerations
Traditional chemical soil tests can be one tool for organic farmers to use to assess soil quality within their organic system. Since organic farmers often sample fields and utilize soil test results in a non-traditional manner, it is important to identify someone who can assist in interpreting changes in soil test levels through the years. Ask the lab's agronomist or horticulturalist about their background in working with organic systems. Understanding the mineralization process of organic fertilizers and amendments is crucial in interpreting changes in chemical soil tests levels over time.
Soil Testing Calibration
The soil nutrient extraction methods utilized in labs would not be very valuable if they were not calibrated with field conditions. Traditionally this has meant conducting field research utilizing varying fertilizer rates (0, 15, 150, 200 pounds of “X” nutrient). The resulting change in soil test values and economic analysis of maximum yields result in soil test interpretation information. These studies are repeated on different types of soils, with varying weather conditions, crops, etc. Most of these studies overlooked the importance of soil biological contributions to nutrient cycling, however. Studies also focused on quick release fertilizers, rather than slow release amendments and long-term changes to soil organic matter.
Organic System Considerations
Organic systems have a limited number of fertilizer products available and most of these would be considered slow release. In addition, organic systems often add large amounts of organic materials or incorporate cover crops. The organic additions result in a slow release of nutrients that is highly dependent on soil biology and weather conditions. Therefore, most studies calibrating soil chemical tests to fertilizer rates are not useful within organic systems. An abundance of research is taking place and new data sets for interpretation of soil tests for organic systems are emerging.
Soil Testing Interpretations
The interpretation of conventional soil test results relies on years of research calibrating soil test methods to specific soil types, crops, and fertilizer rates. The result are fertilizer calculators where you enter your type of soil, expected yield, and soil test level, and out comes the rate of fertilizer you should apply. There is little consideration for the type of fertilizer you will choose and how quickly the nutrients will become available in the soil or for the impacts of soil physical and biological properties or weather on nutrient availability.
In an attempt to serve the organic community, some soil test labs have offered to give fertilizer rates for organic fertilizers. These often are straight conversions based on the grade of nutrients and do not account for soil-fertilizer interactions. For additional information on converting conventional fertilizer recommendations, see How to Convert an Inorganic Fertilizer Recommendation to an Organic One from the University of Georgia Cooperative Extension.
Organic System Considerations
Developing a relationship with the agronomist or horticulturalist at the lab of your choice is important in interpreting the chemical soil tests and evaluating your options for fertilizers and amendments. Simple substitution of organic fertilizers into fertilizer calculators may not lead to similar results. Most organic fertilizers are slow release fertilizers and may be present in the soil many years longer than traditional synthetic fertilizers. Currently there are many different theories on how to interpret soil test results within organic systems:
- Nutrient budgeting: This system focuses on what crops are removing to estimate how much nutrients should be replaced.
- Sufficiency approach: Utilizing conventional soil tests ranges of low, medium and high, additions of fertilizers and amendments would only be added when a soil test level is low or medium.
- Cation balance: Cation balance strategy focuses on maintaining ratios of base cations of calcium, magnesium and potassium within the soil.
Conventional chemical soil testing strategies were not designed to address nutrient management questions in organic production systems. Despite some limitations in the calibration and interpretation of results for organic systems, the test levels over time can be a useful tool for organic farmers to evaluate the impact of their management decisions on the chemical properties of their soils.
In addition to conventional chemical soil tests there are a growing number of other diagnostic tools to help interpret soil quality within an organic system.
- A Soil Health Card can be carried in your shirt pocket to make rapid checks on changes in soil quality within a field.
- Soil Quality test kits are quick hands-on tests to assess critical soil processes such as infiltration and aggregate stability.
- During the growing season plant tissue analysis, especially for perennial fruit crops, is a useful tool to assess the actual nutrient content of the plant. For more information, see Diagnosing Nutrient Disorders in Fruit and Vegetable Crops from University of Minnesota Cooperative Extension, Sampling for Plant Tissue Analysis from New Mexico State University Cooperative Extension, and Plant Analysis: a Diagnostic Tool from Purdue University Cooperative Extension.
- Some soil physical properties such as texture have been readily available from public labs for some time. Some labs are beginning to offer a wider spectrum of tests for soil physical properties such as aggregate stability and macroporosity. As more research is done to understand how to interpret these tests within organic systems, this trend may grow.
- Biological testing is an area that is quickly growing within the organic community. Research continues to expand our understanding of how to interpret these tests and their changes over time as impacted by management. For more information on laboratories that conduct biological soil tests, see the ATTRA (National Sustainable Agriculture Information Service) publication Alternative Soil Testing Laboratories.
- Bierman, P. M., and C. J. Rosen. 2005. Diagnosing nutrient disorders in fruit and vegetable crops. University of Minnesota Cooperative Extension, St. Paul. (Available online at: http://www.extension.umn.edu/garden/fruit-vegetable/diagnosing-nutrient-...) (verified 4 Mar 2014).
- Flynn, R., S. T. Ball, and R. D. Baker. 1999. Sampling for plant tissue analysis. Guide A-123. New Mexico State University Cooperative Extension, Las Cruces. (Available online at: https://aces.nmsu.edu/pubs/_a/A123/) (verified 2 May 2019).
- McLaurin, W., and W. Reeves. 2011. How to convert an inorganic fertilizer recommendation to an organic one. Circular 853. University of Georgia Cooperative Extension, Athens. (Available online at: http://extension.uga.edu/publications/detail.html?number=C853) (verified 5 Mar 2018).
- North American Proficiency Testing (NAPT) program [Online]. Available at: http://www.naptprogram.org/ (verified 4 Mar 2014).
- Pressman, A. 2012. Alternative soil testing laboratories [Online]. National Sustainable Agriculture Information Service. Available at: https://attra.ncat.org/attra-pub/soil_testing/ (verified 4 Mar 2014).
- Schulte, E. E. and K. A. Kelling. 1991. Plant analysis: a diagnostic tool. Purdue University Cooperative Extension Service, West Lafayette. (Available online at: https://www.extension.purdue.edu/extmedia/nch/nch-46.html) (verified 10 Jul 2014).
- Soil health card [Online]. U.S. Department of Agriculture, Natural Resources Conservation Service. Available at: http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?cid=nrcs... (verified 4 Mar 2014).
- Soil quality test kit guide [Online]. U.S. Department of Agriculture, Natural Resources Conservation Service. Available at: http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/home/?cid=nrcs1... (verified 4 Mar 2014).