Over the last 10 days, precipitation totals across the state have varied with less precipitation falling in the western and northern part of the state and heavier amounts falling in the east-central and southeast part of the state.
With the recent precipitation, we are seeing improvements in drought conditions across the state. Below is a comparison of the U.S. Drought Monitor on March 5 (left) to April 4 (right).
While we are seeing improvements in the drought conditions, what exactly does this mean for soil moisture for row crops as we head into the upcoming growing season.
When we talk about soil moisture, some terms that are important to know are:
- Saturation is defined as the threshold when all the pores are filled with water.
- Field capacity is when water in the larger pores has drained away due to the force of gravity.
- Permanent wilting point is when plant roots can no longer extract water from the soil because the water that is in the soil is held so tightly to the soil particles and unavailable for plant uptake.
The amount of available water for crop uptake is the difference between field capacity and the permanent wilting point. Soil texture plays an important role in the amount of available water for crop uptake. The graphic below does a nice job of demonstrating the general relationship between soil moisture and soil texture.
One resource that collects real-time soil moisture data is the ISU Soil Moisture Network. At several weather stations across Iowa, the ISU Soil Moisture Network monitors the volumetric soil moisture at the 12-inch, 24-inch, and 50-inch depth.
For a quick comparison on the status of soil moisture across the state, let’s take a closer look at data from the Southeast Research and Demonstration Farm near Crawfordsville, IA and the Armstrong Research Farm near Lewis, IA.
Below is the time series graph for the station at the Southeast Research and Demonstration near Crawfordsville, IA from April 3, 2023 – April 8, 2024.
To put this data into perspective, it is helpful to know where field capacity is considered, which as mentioned above varies by soil type. The soil types at the Southeast Research Farm primarily have a silty, clay, loam soil texture.
Field capacity at the Southeast Research and Demonstration Farm at the 12-, 24-, and 50-inch depths would be estimated as follows:
- 12-inch depth: 45-48%
- 24-inch depth: 45-48%
- 50-inch depth: 40-42%
Looking at the graph above, the current volumetric soil moisture at the 12-, 24-, and 50-inch depth would all be close or right at field capacity. Compared to the end of the 2023 growing season, we have seen significant improve in soil moisture at the Southeast Research Farm, and we are sitting pretty close to where we were last spring at this time.
In contrast the graph below shows the volumetric soil moisture at the Armstrong Research Farm in southwest Iowa near Lewis, IA for April 3, 2023 to April 8, 2024.
Based on the soil type at the Southwest Research Farm, which is also primarily a silty clay loam, field capacity would be estimated at the following volumetric soil moisture levels:
- 12-inch depth 37-39%
- 24-inch depth: 37-39%
- 50-inch depth: 41-43%
Looking at the graph for the Southwest Research Farm, while there have been significant improvements in soil moisture, at this farm current soil moisture levels are still below field capacity but better than where we were a year ago.
There are also resources like the NASA/SPoRT that use data to model soil moisture conditions.
When looking at the map above, values of 0% indicate moisture can no longer be extracted from the soil and 100% indicates complete saturation.
What does this mean?
While the recent precipitation has helped to improve soil moisture conditions as we head into the 2024 growing season, there are still concerns about ground water, wells, rivers/streams, and pond levels because of the long-term precipation deficeit.
Iowa is currently in its 197th week of drought somewhere in the state, and just looking at precipation departures from normal (1991-2020) a good percentage of the state is at least 10 inches below normal.
There are some things farmers can do to help conserve soil moisture this spring, including earlier cover crop termination and how spring tillage. Read more in the ICM blog “Do we need to think about conserving soil moisture prior to planting?”
Source:
Barker, D., Beuerlein, J., Dorrance, A., Eckert, D., Eisley, B., Hammon, R., Lentz, E., Lipps, P., Louz, M., Mullen, R., Sulc, M., Thomison, P., and Watson, M. 2015. 14th edition of the Ohio Agronomy Guide. https://agcrops.osu.edu/sites/agcrops/files/imce/fertility/Ohio_Agronom…
A special thanks to Meaghan Anderson, Angie Rieck-Hinz, Aaron Sauegling, and Madelynn Wuestenberg for revieiwng.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 9, 2024. The information contained within may not be the most current and accurate depending on when it is accessed.
- Field Agronomist in EC/SE Iowa