By John Sawyer
Department of Agronomy
Iowa State University
A comprehensive testing of soils for soil fertility and nutrient management planning for your crop, garden, or turf area is desirable to use as a basis for adjusting fertilizer, manure and limestone applications. Over or under application may reduce profits. The best way to determine nutrient application needs is through a soil test.
Different plants have different soil pH and nutrient requirements. Testing is inexpensive when compared to investments in your plants, amendments, time, and efforts. Fertilizer and liming guidelines provided with the soil test results are based on research results that are economically viable and environmentally-friendly.
A soil test will assess the level of plant available nutrients (typically phosphorus and potassium) and soil pH. Soil organic matter can also be determined, but does not need to be tested frequently. Recommendations should be based on calibrated soil tests and interpretations that have university research basis. Recommendations will include the amounts of limestone and fertilizer, if necessary, to meet the requirements of specific plants or crops being grown, and often take into account rotation of crops. If elevated or deficient nutrient levels are indicated, appropriate information will be included with your results to address the problem.
Soil samples are typically collected in the fall after harvest or in the spring before planting, and should not be collected when the ground is frozen. A soil test should be performed every two to four years to monitor test levels. However, if a large correction in soil pH is called for by an initial soil test, it may be a good idea to apply part of the limestone rate suggested into two applications, one the first year and another in a following year. This can avoid over-costly application in one year. Retesting in two to four year intervals will help determine if applications are keeping soil tests in optimal ranges. Soil sampling at approximately the same time each time samples are collected is suggested for better comparison between test results and for building a database of test results over time so trends can be monitored.
Standard soil sample information sheets and sample bags are available at no cost from your local ISU Extension and Outreach office or the ISU Soil and Plant Analysis Laboratory located in the Department of Agronomy.
Soil and Plant Analysis Laboratory
G501 Agronomy Hall
Iowa State University
Ames, IA 50011
Submission forms need to be filled out and sent with the sample to generate proper recommendations for your field, lawn, or garden. There are also many commercial laboratories that perform soil tests. A list of laboratories participating in the Iowa Department of Agriculture and Land Stewardship certified soil testing laboratories program is available at https://iowaagriculture.gov/sites/default/files/Cert%20Labs%20after%20Q2%20Update%232.pdf.
When submitting soil samples, place the sample bags in a sturdy cardboard box and fill the empty space with packing material so that the contents don't shift during shipment. Enclose your information sheet and payment in the box. If the soil is too wet, wait until another time to collect samples. Soil sample bags are wax-lined to resist moisture, but cannot be used for excessively wet samples and are not waterproof. Sample turn-around time will vary with the lab and season.
Most labs have standard test packages, but individual tests can also be requested. Typically, standard packages include soil test phosphorus, potassium and soil pH. Additional tests that are needed less frequently, such as soil organic matter, can be added.