Iowa State University Extension and Outreach has completed their spring survey of subsoil moisture in northwest Iowa. The amount of subsoil moisture is sampled in the fall and spring across northwest Iowa each year. Sites within this survey have been sampled for decades providing a historical perspective on subsoil moisture levels.
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. The soil moisture information is based on actual soil samples.
Table 1: Northwest Iowa Subsoil Moisture Locations in reference to historical average
Iowa soils have the potential to hold from 10.0 to 11.0 inches of moisture in the top five feet of soil. Soil moisture readings from sites where Iowa State University Extension and Outreach personnel analyzed subsoil moisture in northwest Iowa ranged from 1.7 inches in Castana to 9.79 inches near Rossie of plant available moisture.
A comparison of the last three sample collections:
Values are reported as inches of plant available moisture
Table 2: Comparison of last three sample collections
Leah Ten Napel Field Agronomist
Counties in my area of Northwest Iowa still range from D0-D4 drought conditions, with the majority in D2(severe drought). The effects of the drought were very evident during the growing season last year. The soil moisture data tells us that we will be relying heavily on precipitation during the growing season as it takes about 20” of moisture to produce a crop.
Image 1: Iowa Drought Monitor
Gentry Sorenson Field Agronomist
Subsoil moisture levels are below historical inches of plant available moisture at several locations. Northern Pocahontas County had the highest deficient of plant available water (5.17 inches), and all areas except for Rossie and Newell were below historical averages. The site near Newell was at historical averages for spring available moisture and the site near Rossie was above historical averages. Most of the area that I serve is currently in D2 to D1 drought. Timely rainfall will be needed though the growing season in sites with deficits in subsoil moisture.
Image 2: Northwest Iowa Counties sampled in a comparion of Spring 2023 VS Fall 2022
Precipitation since November 3, 2022, through April 18, 2023 (Table 3), which included snow and rain ranged from 7.03 to 10.17 inches which was above normal for all but two locations. About 80% of the precipitation is expected to contribute to subsoil moisture. It is possible that some of the moisture was lost and was not able to infiltrate the profile due to frozen soils in early spring. This may have led to subsoil moisture levels at some sites that did not replenish to expected levels. Rainfall at locations is about ten inches from mid-April to June 30 and about 18 inches from mid-April to August 30.
Table 3: Precipitation November 3, 2022-April 18, 2023
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/