By: Paul Kassel
Iowa State University has conducted a survey of subsoil moisture since 1955. It is my understanding that there were some very dry conditions in the 1950s – and the ISU Agronomy department and ISU Extension responded by assessing soil moisture to help farmers make planting decisions for the next spring.
This soil moisture survey is not complicated. We sample some pre-selected farm fields to a depth of five foot in one-foot increments. Those samples are weighed wet, dried and re-weighed. Plant available moisture is then calculated. The amount of plant available moisture is then compared to long-term averages.
The amount of subsoil moisture is well above average at some northwest Iowa sites. Sites near Storm Lake, Pocahontas and Sac City were not sampled because rainfall at these sites ranged from 5 to 11 inches above normal since August 15 (table 2). Those levels of late fall rainfall will provide adequate amounts of reserve soil moisture for the 2016 crop season.
However, rainfall near Spirit Lake and Estherville was only slightly above normal since August 15. Areas of Emmet and Dickinson county also experienced drier weather in early and mid-summer.
Subsoil moisture levels were sampled at these locations on November 2. Plant available moisture at Spirit Lake and Estherville was 7.7 and 6.3 inches respectively. The soil moisture at these locations is slightly above normal, and is not at level that is considered an issue (table 1).
Table 1. County Subsoil Moisture – Fall 2015
County Fall Average, in. Location 2015 Crop Inches Moisture on Nov. 2, 2015
Dickinson 5.7 Spirit Lake soybean 7.7
Emmet 5.9 Estherville corn 6.3
Table 2. Rainfall from August 15 to November 2, 2015
Actual Normal Difference
--------inches ------------
Spirit Lake 7.8 7.2 +0.6
Estherville 8.7 7.1 +1.6
Spencer 8.9 7.0 +1.9
Storm Lake 13.1 7.7 +5.4
Pocahontas 18.2 7.4 +10.8
Sac City 14.4 7.7 +6.7
