Taking Good Soil Samples, the First Step in a Good Fertilizer Decision

October 7, 2024 1:08 PM
Blog Post

This article is Part 1 in a series of 5 ICM blogs on how to best allocate fertilizer dollars with tight margins.

Soil samples are a helpful resource for farmers as they provide data to be used for recommendations for limestone, manure, and fertilizer. Soil samples should be taken every two to four years to understand nutrient levels available in the soil for the upcoming crop. Soil sample fields with the same crop as this reduces variability of soil test results. The publication, CROP3108, Take a Good Soil Sample to Help Make Good Decisions, provides detailed information on sampling methods and collecting representative samples. 

Fall 2024 has been exceptionally dry. In extremely dry conditions soil test results appear lower than they actually are. Delay soil sampling in extremely dry conditions, until rainfall has soaked the depth of sampling, usually 6 inches. An article titled Beware of Dry Conditions When Soil Sampling and Interpreting Test Results provides more details.

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. Iowa State University 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 trafficked, and locations where manure or lime was stored prior to application. 

Materials needed for soil sampling include a plastic bucket, soil probe, soil sample bags, and a field map denoting where soil samples were collected. Each soil sample should consist of multiple soil cores collected from various locations within the designated area to ensure a representative analysis. Typically, a sample of 12-15 cores is combined into one sample. 

Label the samples and keep records of the sampling locations and conditions. After soil samples have been collected send the samples to a certified laboratory.  The Iowa Department of Agriculture and Land Stewardship Commercial Feed and Fertilizer Bureau has a list of certified labs in the State of Iowa.  The link for this resource is found under the soil testing heading on this website.

Soil sampling is an upfront cost but may save you money or make you money in the long run. This information is invaluable for tracking changes over time.  When fields are soil sampled it assesses soil health and nutrient availability, enhancing crop productivity if levels are low and fertilizer is needed, or reducing input costs if soil test levels are high and fertilizer is not needed. 

Photo of a soil probe with a 6 inch sample of soil.
A soil sample should consist of 12-15 cores. 

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Gentry Sorenson Field Agronomist in NW Iowa

Gentry Sorenson is a field agronomist in Northwest Iowa for Iowa State University Extension and Outreach. He works closely with farmers to offer educational programming in crop management issues.  He also works with agribusinesses, pesticide applicators, certified crop advisors, and other in...