Iowa State University Extension and Outreach Field Agronomists have completed their fall survey of subsoil moisture in northwest Iowa. Subsoil moisture sampling in Iowa dates back to February 1954 when ten sites were chosen for soil moisture sampling. In 1959, the sampling increased to 22 locations, and in 1971, 61 locations were sampled. Today, 18 locations are sampled in northwest Iowa.
The sites were selected for being typical for the major soil type. Each site that was selected has little or no water run-on and no ponding. A 40-feet-by-40-feet area is designated for sampling at the locations, and GPS helps ensure sampling is done at the same location each spring and fall.
Fall 2024 Subsoil Moisture Survey Results
The table below compares the results of the last 5 sampling periods.
Table 1: Comparison of last five sample collections
The table shows that there was some recharge of soil moisture this fall after late October and early November rains. Soil moisture is reported in inches of plant available moisture after calculations account for plant available and plant unavailable moisture. A full profile in northwest Iowa can hold about 11 inches of plant available moisture.
Soil moisture is sampled to a depth of five feet in one-foot increments at predetermined locations in the spring and fall. Moisture samples are taken to a depth of 5 feet as corn and soybeans can root to this depth. In years with reduced rainfall, understanding ways to conserve soil moisture is important. Methods of moisture conservation include reducing surface runoff, reducing tillage applications, and increasing ground cover with crop biomass.
The following are the values of plant available moisture for the fall of 2024 with the fall of 2023 values denoted by ().
Image 1: Northwest Iowa Counties sampled in a comparison of Fall 2024 VS Fall 2023
2024 Growing Season Conditions
The 2024 growing season in northwest Iowa started with an exceptionally wet spring, ranking as the 3rd wettest spring on record for northwest Iowa. Challenges continued on June 21-22, when heavy rain fell on saturated soils, causing significant flooding across northwest Iowa. This summer ranked the 15th wettest on record for northwest Iowa. After an active weather pattern from March to August, the faucet seemed to turn off in September. In Spencer, Iowa, precipitation remained below average from August to November. Northwest Iowa achieved the 2nd driest September through October on record.
Image 2: Daily precipitation from March 1, 2024 to November 25, 2024 at the ISU Northwest Research Farm.
The drought monitor had significant changes from our sampling period this spring to our most recent sampling this fall. Rainfall received in northwest Iowa over the course of the growing season was variable but allowed some counties to be moved out of the D4 drought category, leaving most in the D0 to D1 categories. Eastern Iowa has increased in drought severity from last spring to this fall.
Image 3: Iowa Drought Monitor map
Image 4: Time series showing drought conditions in the Northwest Iowa Climate Division from January of 2023 to the end of November of 2024. Drought conditions in Northwest Iowa decreased from D3 to no drought, but drought began creeping back in during the late summer of 2024.
Winter conditions, like amount of precipitation and frost depth, will affect how much moisture is able to infiltrate the soil profile by spring. Subsoil moisture can also be replenished by rainfall when the ground thaws in the spring through the months of March and April. Typical rainfall for those months is three to five inches, and about 80 percent of that rainfall will contribute to subsoil moisture reserves. Corn and soybean crops require about 20 inches of moisture from rainfall and subsoil moisture to produce a crop. That number may increase to 25 inches per season when high temperatures and windy conditions are prevalent during the summer.
Look for updated survey results next spring, or for more information, contact Leah Ten Napel at 712-541-3493 or Gentry Sorenson at 641-430-6715.
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...
Leah Ten Napel Extension Field Agronomist
Leah Ten Napel is an extension field agronomist serving 9 counties in northwest Iowa. Her responsibilties include bringing research-based information to the grower level through educational programing and answering agronomic questions for landowners, farmers and ag retailers. Her areas of experti...