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?
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.
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.
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.
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.
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 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.
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.
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:
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.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.