Iowa State University Extension and Outreach has completed their fall survey of subsoil moisture in northwest Iowa. The survey provides a current and historical perspective on subsoil moisture levels. Soil moisture is sampled to a depth of five foot in one-foot increments at pre-determined locations in the spring and fall. 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 following are the values of plant available moisture for the fall of 2022 collected by Iowa State University Extension and Outreach field agronomists.
The amount of subsoil moisture is well below average at all northwest Iowa sites. This summer much of NW Iowa was in a drought classification with the drought intensifying by September 27th per the Iowa drought monitor. The D3 drought then extended into southern Palo Alto, northern Pocahontas, northern Buena Vista, and southern Clay Counties. D3 drought is considered Extreme Drought and defined as major crop/pasture loss, extreme fire danger and widespread water shortages.
Rainfall for much of northwest Iowa has been below average. A table from the Iowa Environmental Mesonet has been provided for the period of October 1, 2021, to November 14, 2022. ("Climo" is the climatology value, which is computed over the period of 1951-2015 and departure is the amount of rainfall that was received subtracted from the Climo value). The rainfall data from the IEM Climodat Station Monitor can be found at https://mesonet.agron.iastate.edu/climodat/
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 .36 inches in Lawton to 2.96 inches in Estherville of plant available moisture. The moisture readings collected in the fall of 2022 is well below the historical plant available moisture average, and below the averages of plant available moisture reading collected in the fall of 2021. Rainfall is needed to replenish subsoil moisture across much of northwest Iowa.
Subsoil moisture can be replenished from 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. 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.