NDSU Extension - Ramsey County

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October 17, 2011 Agriculture Column

Howdy from the Ramsey County Extension Service!!!  There has been much conversation about soil compaction and its effect on soil structure and crop yields.  Research has been going on for years and will continue into the future of the what, how’s and effects of soil compaction.  Universities across the country are studying the effects of soil compaction and how if any strides can be made to alleviate the problem.  An article published by the University of Minnesota Extension described the first part of soil compaction.  I used parts of that publication to talk about the problem and hopefully you understand more the issue we are facing. 

SOIL COMPACTION-CAUSES AND CONSEQUENCES

As farm tractors and field equipment become larger and heavier, there is a growing concern about soil compaction. Soil compaction can be associated with a majority of field operations that are often performed when soils are wet and more susceptible to compaction. Heavy equipment and tillage implements can cause damage to the soil structure. Soil structure is important because it determines the ability of a soil to hold and conduct water, nutrients, and air necessary for plant root activity. Although much research has been conducted on soil compaction and its effects on yield, it is difficult to estimate an economic impact because fields vary in soil types, crop rotations, and weather conditions.

WHAT IS SOIL COMPACTION?

Soil compaction occurs when soil particles are pressed together, reducing pore space between them. Heavily compacted soils contain few large pores and have a reduced rate of both water infiltration and drainage from the compacted layer. This occurs because large pores are the most effective in moving water through the soil when it is saturated. In addition, the exchange of gases slows down in compacted soils, causing an increase in the likelihood of aeration-related problems. Finally, while soil compaction increases soil strength-the ability of soil to resist being moved by an applied force-a compacted soil also means that roots must exert greater force to penetrate the compacted layer.

Soil compaction changes pore space size, distribution, and soil strength. One way to quantify the change is by measuring the bulk density. As the pore space is decreased within a soil, the bulk density is increased. Soils with a higher percentage of clay and silt, which naturally have more pore space, have a lower bulk density than sandier soils.

MYTHS ABOUT SOIL COMPACTION

There are two wide spread myths about compaction; 1) Freeze-thaw cycles will alleviate a majority of soil compaction created by machinery, and 2) What compaction "Mother Nature" does not take care of, deep tillage or subsoiling will alleviate.

Deep Tillage/Subsoiling

While deep tillage (greater than 10 inches) is capable of shattering hard pans created by wheel traffic, it has not been proven to increase yield consistently or for long periods of time. In Midwestern studies where plots with established compaction were split with a deep tillage treatment (14-16 inches), corn yields were either unaffected or reduced slightly (10 bushels per acre) compared to the non-subsoiled plot. The one possible exception would be on an irrigated loamy sand. A lack of consistent positive yield response may be due to:

·         Insufficient depth of subsoiling

·         Ineffective removal of compacted layer due to high soil moisture levels

·         Recompaction of loosened soil by subsequent wheel traffic

·         Deterioration of some soil properties by the subsoiling operation

·         Detrimental effects caused by compaction were no longer limiting crop yield

WHAT CAUSES SOIL COMPACTION?

There are several forces, natural and man-induced, that compact a soil. This force can be great, such as from a tractor, combine or tillage implement, or it can come from something as small as a raindrop (Figure 5). Listed below are several types of soil compaction and their causes.

Raindrop impact - This is certainly a natural cause of compaction, and we see it as a soil crust (usually less than 1/2 inch thick at the soil surface) that may prevent seedling emergence. Rotary hoeing can often alleviate this problem.

Tillage operations - Continuous moldboard plowing or disking at the same depth will cause serious tillage pans (compacted layers) just below the depth of tillage in some soils. This tillage pan is generally relatively thin (1-2 inches thick), may not have a significant effect on crop production, and can be alleviated by varying depth of tillage over time or by special tillage operations.

Wheel traffic - This is without a doubt the major cause of soil compaction. With increasing farm size, the window of time in which to get these operations done in a timely manner is often limited. The weight of tractors has increased from less than 3 tons in the 1940's to approximately 20 tons today for the big four-wheel-drive units. This is of special concern because spring planting is often done before the soil is dry enough to support the heavy planting equipment.

Minimal Crop Rotation - The trend towards a limited crop rotation has had two effects: 1.) Limiting different rooting systems and their beneficial effects on breaking subsoil compaction, and 2.) Increased potential for compaction early in the cropping season, due to more tillage activity and field traffic.

 

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