Iowa State University Extension and Outreach Field Agronomists have completed their spring survey of subsoil moisture in northwest Iowa. Subsoil moisture is sampled in mid-April in the spring and early November in the fall across northwest Iowa each year. The dates that the samples were collected are listed later in this article. Consistent sampling dates provide a baseline comparision of historical subsoil moisture records across NW Iowa. Rainfall that occurs after sampling is not accounted for in the data listed below but would contribute to subsoil moisture reserves before the crop utilizes the moisture.
Sites within this survey have been sampled for decades providing a historical perspective on subsoil moisture levels. Subsoil moisture sampling dates back to February 1954 with ten locations that were chosen for sampling. Today 18 locations in northwest Iowa are sampled. The sample locations are selected for no water run-on or ponding, with the same 40 feet by 40 feet area sampled each year.
Soil moisture is sampled to a depth of five foot in one-foot increments in sixteen Iowa counties in northwest Iowa. Soil moisture samples are weighed wet, dried for 48 hours, and weighed dry. Soil moisture is reported in inches of plant available moisture after calculations account for plant available and plant unavailable moisture. Iowa soils have the potential to hold from 10.0 to 11.0 inches of moisture in the top five feet of soil. The soil moisture information is based on actual soil samples.
Table 1: Northwest Iowa Subsoil Moisture Locations in reference to historical average
A comparison of the last four sample collections:
Values are reported as inches of plant available moisture.
Table 2: Comparison of last four sample collections
Image 1: Iowa Drought Monitor April 25, 2024
Subsoil moisture samples were taken in the eastern part of northwest Iowa on April 15th, those counties included Sac, Buena Vista, Clay, Dickinson, Emmet, Pocahontas, and Hancock counties. Samples in these counties were taken before recent rains of 3-4 inches.
Subsoil moisture samples taken in the remaining counties: Lyon, Osceola, Sioux, O’Brien, Plymouth, Cherokee, Woodbury, Ida and Monona were taken on April 21st. Rainfall accumulation in this region after the sample date of April 21st was around 2-3 inches.
Image 2: Northwest Iowa Counties sampled in a comparison of Spring 2023 VS Fall 2023
About 80% of the precipitation is expected to contribute to subsoil moisture. Weather stations in NW Iowa are reporting an improved current precipitation amount over the Climo precipitation value which is computed over the time since 1951.
Table 3: Precipitation November 3, 2023-May 2, 2024, per Iowa Environmental Mesonet
In summary, a corn or soybean crop needs about 20 inches of moisture from rainfall or subsoil moisture to produce a normal crop. A great resource is the Iowa Environmental Mesonet website to keep track of soil moisture, soil temperature, precipitation for your farm. The website can be located at https://mesonet.agron.iastate.edu/
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...
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