This spring consisted of wet field conditions for many regions across Iowa during planting season. When soil moisture is at or exceeds field capacity, there is an increased potential for soil compaction, particularly at topsoil depths. Soil compaction at planting time can impact root growth and development for the rest of the growing season, and can be a serious problem for Iowa farmers. However, with proper field management, compaction can be minimized.

The contribution of wet conditions to soil compaction is highly linked to soil texture. Well-drained, medium-textured soils (loams, clay loams, silt loams, and silty clay loams) are much less affected than fine-textured soils (silty clay and clay), where saturated conditions are likely to exist due to poor drainage. The wet soil conditions of poorly drained soils may persist longer, depending on drainage availability, causing significantly increasing potential soil surface compaction and sidewall compaction with tillage operations, which may lead to poor seed germination. If planting and other field operations are rushed, poor seed germination and root development may result.

Recognizing soil compaction in your field
Indications of soil compaction during and immediately following a normal rainfall include: slow water infiltration, water ponding, high surface runoff and soil erosion. Additionally, soil compaction can be diagnosed by stunted plant growth, poor root system development (Photo 1), and potential nutrient deficiencies (i.e., reduced potassium uptake). These soil compaction symptoms are a result of increased bulk densities that affect the ideal proportion of air and water in the soil.

The most efficient way to verify soil compaction is to use a tile probe, spade, or penetrometer to determine a relative soil density. Soil moisture conditions can have a significant effect on penetration resistance.

For example, in dry soil conditions soil penetration resistance is much higher than wet conditions because soil water acts as a lubricant for soil particles. Therefore, it is wise to determine soil compaction early in the season or compare observations and measurements from suspected areas with adjacent areas that have little chance of soil compaction due to traffic patterns.

Management decisions to minimize soil compaction
1. Check soil moisture. The most effective way to minimize soil compaction is to avoid field operations when soil moisture is at or near field capacity, and it is important to check for proper soil moisture conditions prior to implementing any field operation. Soil compaction will be less severe when soil tillage, fertilizer application and planting operations occur when the field is dry.

Most of Iowa's soils have medium textures. For these soils, a simple method of checking soil moisture is the "feel" method. Probing the top 3-4 feet with a hand soil probe to assess the field's soil moisture conditions is time well spent.

Check the soil moisture status by pushing a ribbon of soil from between the thumb and index finger. If it breaks off within one or two inches, the potential for creating compaction is less. However, if the ribbon stretches out to four or five inches, it is still too wet. The chances are good that being in the field under these conditions may cause more problems than it will solve.

Another method is to make a ball of soil two inches in diameter and toss it through the air. If it hangs together until impact, it has a lot of cohesiveness, is still fairly plastic and probably is too wet to work the ground.

2. Tire size. Properly adjusted tire size and correct air pressure for the axle load being carried is a second management tool. Larger tires with lower air pressure allow for better flotation and reduce pressure on the soil surface. Additionally, using larger tires that are properly inflated increases the "footprint" on the soil.

3. Same wheel tracks. Use the same wheel tracks to minimize the amount of land traveled across. Most damage occurs with the first pass of the implement. Using control traffic patterns can be done effectively by using implements that have matched wheel-tread configuration for soil preparation, planting, row cultivation, spraying and harvesting.

4. Conservation. Remember to hold off soil tillage operations until soil conditions are drier than field capacity and look into the benefits of conservation tillage systems.

In wet conditions, the best choice producers can make is to stay away from the field and avoid traffic on wet soil to reduce soil compaction. How you approach planting and fieldwork after a heavy rain event can impact your soil and your crop development for the rest of the season.

Learn more at the 2016 Farm Progress Show
Learn more about the effects of soil compaction on root growth and development and best management practices at the Iowa State University tent at the Farm Progress Show in Boone, Iowa Aug. 31-Sept. 1. The soil health table will have four demonstrations, one which will focus on soil compaction at three different bulk densities. Mahdi Al-Kaisi, professor and extension soil management specialist at Iowa State University, will be present to answer questions, provide resources, and discuss soil health management practices.

New soil health publications available
Now available to purchase online at the Extension Store is the Iowa Soil Health Field Guide, which highlights the relationships between soil characteristics and provides information about soil health and its importance to sustainable agriculture systems; the Iowa Soil Health Assessment Card, for field assessment and evaluation of soil health indicators; and the Iowa Soil Health Management Manual, which provides information about soil functions and services that are essential for sustainable agriculture systems. These three publications are products of the collaborative efforts of Iowa State University and the Natural Resources Conservation Service (NRCS) in Iowa.