Iowa State University has completed their fall survey of subsoil moisture in northwest Iowa. The amount of subsoil moisture is sampled in the fall in six northwest Iowa counties.
Soil moisture is sampled to a depth of five foot in one-foot increments at six different pre-determined locations. 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 amount of subsoil moisture is well below average. The amount of subsoil moisture at the soil moisture sites in Emmet, Dickinson, Clay, Buena Vista, Pocahontas and Sac county ranges from 1.8 inches to 3.4 inches of plant available moisture (table 1).
Rainfall during the growing season was 3.6 inches below normal in Estherville to 8.3 inches below normal in Sac City (table 3). Additionally, rainfall from September 1 to November 8 was 1.8 inches below normal in Pocahontas to 2.8 inches below normal in Estherville (table 2). The water use during the growing season by the relatively good crop produced in 2020 and dry fall explains the current lack of subsoil moisture.
The concern now will be to recharge the subsoil moisture for 2021 crop production. However, these low levels of fall subsoil moisture and a dry early growing season would compromise crop yield potential for 2021.
Rainfall during November, March and April will also contribute to subsoil moisture. Typical rainfall for those months is about 6 inches (table 4). About 80% of that rainfall will contribute to subsoil moisture reserves.
Table 1. County subsoil moisture – fall 2020.
plant
available
County moisture
Fall inches
County average, in. location 2020 crop Nov. 3, 2020
Dickinson 5.7 Spirit Lake corn 2.2
Emmet 5.9 Estherville soybean 2.5
Clay 5.7 Rossie corn 3.2
Buena Vista 6.0 Newell soybean 2.9
Pocahontas 6.0 Rolfe soybean 3.4
Sac 5.9 Schaller corn 1.8
Table 2. Rainfall from September 1 to November 8, 2020 (info from ISU mesonet).
Actual normal difference
------------ inches -----------
Milford 2.8 5.5 -2.7
Estherville 2.7 5.5 -2.8
Spencer 2.8 5.4 -2.6
Storm Lake 3.7 5.7 -2.0
Pocahontas 3.7 5.5 -1.8
Sac City 3.9 5.8 -1.9
Table 3. Rainfall from April 25 to September 1, 2020.
Actual normal difference
---------- inches --------
Milford 10.8 16.3 -5.6
Estherville 12.8 16.4 -3.6
Spencer 11.2 16.3 -5.1
Storm Lake 13.5 18.3 -4.8
Pocahontas 10.7 17.6 -7.0
Sac City 9.4 17.7 -8.3
Table 3. Location of moisture in the soil profile, November 3, 2020.
Location Spirit Esther Ros- New- Rolfe Schal
Lake -ville sie ell ler
Depth, ft. ----------------------------inches --------------------
0- 1 1.0 1.3 1.4 1.4 1.6 1.6
1-2 0.0 0.5 0.8 0.0 0.7 0.2
2-3 0.0 0.1 0.7 0.0 0.3 0.0
3-4 0.3 0.3 0.3 0.3 0.1 0.0
4-5 0.9 0.3 0.0 1.2 0.7 0.0
Total 2.2 2.5 3.2 2.9 3.4 1.8
Table 4. average expected rainfall for November, March and April.
November March 1 to April 30 total
----------------------- inches ---------------------
Milford 1.4 4.5 5.9
Estherville 1.3 4.7 6.0
Spencer 1.3 4.7 6.0
Storm Lake 1.5 5.0 6.5
Pocahontas 1.5 5.0 6.5
Sac City 1.5 5.1 6.6
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