Gentry Sorenson, Field Agronomist
Soil is an important resource in Iowa farm fields. Productivity for corn and soybeans can vary based on nutrient levels, pH, and organic matter content. Soil samples should be taken every two to four years to understand nutrient levels available in the soil for the upcoming crop.
Different crops have specific nutrient needs. A resource for soil fertility in the state of Iowa is publication PM 1688 “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa.” The publication is available online as a free download from Iowa State University Extension and Outreach, https://store.extension.iastate.edu/product/5232. The guide outlines nutrient recommendations for crops in the state of Iowa as well as lime recommendations.
Best practices for soil sampling include timing, sampling depth and methods of sampling. The ideal time for soil sampling is during the fall after harvest or early spring before planting. Allow enough time for lab results to be processed and returned before making a fertilizer application. For corn and soybeans, samples should generally be taken from the top six inches of soil. ISU Extension and Outreach recommendations are based upon the six-inch soil depth. Pulling cores that are consistent with this depth is important for obtaining consistent results.
Options for soil sampling include grid soil sampling, zone sampling, and whole farm sampling.

• Grid soil samples utilize a composite sample taken in each of the grids ranging from 1 to 10 acres. Grid samples are formed with a pattern of uniform sizes of grids across the field.
• Zone sampling factors in field history, yield maps, topography, and other management factors creating zones. Composite samples are then collected from each zone forming representative samples.
• The final method is whole farm sampling. This method of sampling gathers subsamples across large areas of the field. A drawback of this method is that not all areas of the field are represented in the sample. If your farm has variability, whole farm sampling would not be recommended.
  When taking composite samples, factor in past management strategies in your plan for sampling. Old feedlots or areas of the field that were managed another way may not be a representative sample area and should be treated differently than an area that has less variability. Other non-representative areas of a field may include depressions or wet spots, end rows, areas that are heavily traveled, and locations where manure or lime was stored prior to application.
  Each soil sample should consist of multiple subsamples collected from various locations within the designated area to ensure a representative analysis. Typically, a subsample of 12-15 cores is combined into one sample.
  Label the samples and keep records of the sampling locations and conditions. This information is invaluable for tracking changes over time. When fields are soil sampled it assesses soil health and nutrient availability, enhancing crop productivity, reducing costs, and contributing to sustainable agricultural practices.