The following is a view of the moisture status for six locations in northwest Iowa where the soil was analyzed for spring subsoil moisture. This discussion may or may not reflect the current soil moisture conditions in your area. This article does attempt to account for stored soil moisture, 80% of spring rainfall and expected average rainfall for the rest of the growing season.
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.
The concern is that the current levels of soil moisture plus 80% of expected total rainfall is below the needed 20 inches of rainfall to produce normal corn and soybean grain yields (table 2).
Table 1. crop moisture use balance sheet as of June 11, 2021.
plant crop current
available moisture soil moisture
moisture rainfall use status - inches
inches since since plant available
location 4/17/21 4/17/21 (dep) 5/31/21 moisture 6/11/21
Spirit Lake 4.1 3.6 (-3.2) 1.9 5.1
Estherville 5.4 3.2 (-3.5) 1.9 6.1
Rossie 8.2 4.7 (-2.1) 1.9 10.1
Newell 7.3 3.9 (-3.6) 1.9 8.5
Rolfe 6.1 3.2 (-3.8) 1.9 6.8
Schaller 6.8 3.6 (-3.8) 1.9 7.8
Table 2. Expected average rainfall - for June 11 to September 1, inches. The last column is the total- or the first column plus the info in the last column in table 1.
6/11- 9/1 moisture
Milford 10.5 13.5
Estherville 10.5 14.5
Sioux Rapids 11.0 18.9
Storm Lake 11.8 18.0
Pocahontas 11.6 16.0
Sac City 11.4 17.0
Plant available moisture - subsoil moisture levels - sampled by April 17, 2021
rainfall - ISU Mesonet
location weather station used
Spirit Lake Milford
Rossie Sioux Rapids
Newell Storm Lake
Schaller Sac City
Crop moisture use - from University of Nebraska Lincoln - data from Concord NE which is based on the evapotranspiration from the past seven days from the table, or 0.17/inches of moisture/day.
Current moisture status
- subsoil moisture as 4/17/21 plus 80% of the rainfall that has occurred since 4/17/21 minus crop moisture use. Rainfall is considered to be about 80% efficient since evaporation may reduce the total amount of moisture supplied to the crop.