Subsoil moisture levels in northwest/north central Iowa

November 10, 2015 10:30 AM
Blog Post

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