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What Is Soil Compaction?

Simply put, soil compaction is the loss of pore space within a soil profile caused by the compression of soil particles due to natural, human, and animal activities. Soil compaction has been found to occur at any depth up to 24" and is generally categorized in three specific groups.

Surface Soil Compaction
(Hardened, cracked, soil found on the surface)
Sub-Surface Soil Compaction
(Compacted layers found 10" deep and above)
Deep Soil Compaction
(Compacted layers found below 10" deep)

Ideal, non-compacted, soils will be composed of 45% mineral particles, 5% organic matter, and 50% pore space (25% water and 25% air).⁶ As the soil is compacted by moving soil particles closer together, pore space is dramatically reduced. This restricts water flow within the soil, reduces air pockets, and inhibits proper root development.

The Causes of Compaction

As farming practices continue to evolve, compaction has become harder to reduce and prevent. The use of heavier equipment, multiple field operations, operating in less than ideal conditions due to time constrictions, and the consolidation of crops used in a rotation, all provide elements for "more extensive and deeper compaction"⁵ Additionally, since farmers cannot easily see if they have compacted soil, many are not aware that they have a problem;³ and some older methods believed to help solve compaction problems have been proven ineffective when tested.

Wheel compaction from heavy equipment.
Researchers agree that the primary cause of soil compaction remains tires from heavy equipment running through the field. Tractors alone have increased in weight over the past 70 years from less than 3 tons to about 20 tons.¹ Although heavier machines may not produce more compaction near the surface when compared to lighter machines, they produce compaction deeper within the soil structure.³ Additionally, dual tractor tires do not prevent compaction, they may reduce the depth of compaction, but they also double the area affected.⁶
Multiple Field Operations
Those growing corn on corn face a residue challenge that can take multiple tillage operations to overcome. Because of these heavy residue crops being planted year after year, tandem and offset disk use is increasing. These tillage processes create a thin layer of compaction with each pass you make through the field.³ "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."¹ Additionally, newer seed hybrids that are more resistant to disease and climate conditions often leave behind tougher residues that do not break down as easily and often require additional tillage. These extra passes through the field will cause additional compaction within a soil profile, but can be taken care of by varying the depth of tillage over time or through "special tillage operations."¹
Time limitations
The average number of acres a person is able to farm has dramatically increased over the past few years. Although larger machinery assists in making this possible, producers are finding that time constraints are forcing them to operate in a manner that promotes soil compaction.¹ The best example of this is operating on soils that are too wet because time allows no other choice. Soils are weaker when they are wet and "considerable compaction" often takes place⁶ due to this practice. The question of "how wet is too wet?" can be difficult to answer. There are varying maximum moisture contents for every soil type and infrastructure situation which, if exceeded can be "severely compacted with minimal effort."⁴
Less Diversification in Crop Rotations
The crops you plant can have a dramatic effect on the susceptibility your field has to developing compaction due to the organic matter and root penetration that they provide to the soil structure. Since organic matter is a key component in soil composition, having less of it will weaken the soil and cause compaction to develop more easily. Growing potatoes year after year is a good example of a practice that will decrease organic matter in the soil structure. Because potatoes are a low residue crop, organic matter from previous years will disappear and the potato crop will not provide enough residue to replenish it.⁶
"Organic matter is the fuel, short-term building blocks of soil structure, and supply warehouse for living things in the soil. As organic matter decomposes and mineralizes without adequate replacement, soil becomes more compacted. Bulk density increases and aggregate stability declines as organic matter is “burned “ out of the soil.""⁴
Lessened crop diversification has also eliminated the use of deep rooted, perennial crops in some areas. The root systems from these crops help to reduce the effects of compaction by recreating porous areas in the soil structure.⁵

The Effects of Compaction

Most are already aware that compaction effects on yields can be severe. Studies done in Indiana and Wisconsin prove as much. On Wisconsin silty clay soil, corn yields were reduced as much as 51 bushels per acre on compacted soil. In Indiana, yields were decreased by 48 bushels per acre in moderately compacted soil.³ The question more commonly asked is "why does compaction effect yeild". The answers can be found when we consider restricted water movement, limited aeration, and inhibited root development in a compacted soil profile.

Water Movement through the soil profile
"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.¹
Studies done on Hoytville soil in northwest Ohio showed that normal water movement through the soil profile of 4.8 to 14.4 inches per day was reduced to just 1.4 inches per day in compacted soils due to tillage and traffic.² When soil is compacted, water movement through the soil profile is slowed. This often causes excess water to be present, which means more water on the surface, more runoff, increased erosion, and longer drying times. This will lead to delayed planting and harvesting resulting in a decrease in crop harvesting.⁷
Aeration in compacted Soils
The presence of excess water within the soil profile also can cause aeration problems. The excess water will fill pore space that would normally be filled with oxygen necessary for good seed germination and plant growth.⁶ Water need not always be present for aeration problems to occur. Simply compacting the porous space between soil particles is enough to slow down the exchange of gasses within the soil and cause aeration-related problems.¹
Root Development in compacted soils
Root development can be restricted simply because soil strength, or "the ability of soil to resist being moved by an applied force". Roots systems in a compacted soil profile must exert more force to break through compaction.¹ Shallow root systems and malformed roots develop due to the excess effort needed to grow in compacted soils.⁶ Roots often will go down to a compacted soil layer and then spread horizontally, instead of breaking through the compaction and continuing down through the soil structure.
The root is prohibited from using the moisture and nutrients that are below the compacted layer.³ Dense soils will not provide adequate moisture and nutrients for plant roots to grow well.⁷ With crops that are grown underground, such as potatoes, tuber size can be reduced."In Minnesota, potato yields were reduced 35 percent becuase of wheel traffic compaction restricting the development of tubers"⁶
Other Problems found in compacted soils
In addition to the problems caused directly by compaction, other problems are heightened by the presence of compaction within a soil structure. Disease and low nutrient supply due to reduced root distribution are two challenges that producers will face more often with compacted layers in their fields.⁵ When compaction causes excess water to appear in a field, denitrification also occurs from the loss of stored nutrients within the soil.⁶

Avoiding Compaction

There are many recommendations made for producers looking to try and avoid compaction. As previously mentioned, some of these may become unrealistic as the number of acres a person farms increases; and as crop rotations become less diversified. This point becomes even more clear when one considers that the first pass of a wheel on loose soil causes 80 percent of the total compaction done over a total of four passes. "So don't think that driving over an area “just this once” isn't harmful."⁶

The following is a list of practices the University of Wisconsin Extension office recommends for attempting to avoid compaction:

Avoid Operations in Wet Soil
Avoid Carrying too much tractor weight.
Vary the depth of your primary tillage operation from year to year.
Keep your tillage equipment in top operating condition.
Try to combine field operations for fewer passes over the field.
(See BLU-JET LandTracker 9400)
Use the right tire size and type.
Consider drainage improvements for poorly drained fields.³

Solving Compaction Problems

There are several ways agronomists have recommend solving compaction problems throughout the years. These include Controlled traffic (compaction management), Natural Freezing and thawing, and deep tillage solutions. Over the course of several years, some of these methods have not yet proven to be realistic, others have proven to be ineffective, and others have been proven effective if done correctly.

Controlled traffic Compaction Management
This system requires producers to use the same traffic lanes every year in their fields. The theory is that this will limit the number of compacted areas one has over the total number of acres that he farms. However, researchers admit that "this system will require additional research, analysis, and machine design before it becomes practical for production agriculture."³
Soil Profiles Freezing and Thawing
Yearly Freezing and Thawing of a Soil Profile was long thought to be the answer to compacted soils. However, the University of Minnesota claims that even in Minnesota soils, where freeze depths can reach deeper than 3 feet, this cycle is not sufficient to combat layers of compaction found more than 6 inches deep.
"The belief that freeze-thaw cycles will loosen compacted soils may have developed years ago when compaction would have been relatively shallow because machinery weighed less and grass and legumes were grown in the rotation. Research conducted in 1960 at Lamberton reported that nine years of cropping and annual freezing and thawing did not remove a compacted soil layer at the bottom of the plow furrow in a Nicollet clay loam...In other studies at Lamberton, compaction due to wheel tracks also persisted over winter at depths of 6 to 18 inches."¹
Deep Tillage
Deep Tillage is effective in breaking up compacted layers and assisting in higher levels of water filtration, greater aeration, and proper root development. A good way to determine the severity of a compaction problem is to inspect crop roots. If root growth is hampered because compaction is present in the soil profile, and the compaction layer is below 8 to 10 inches, deep tillage may be warranted.⁵ The yield advantages deep tillage provides will vary from year to year.
"In years when soil moisture is plentiful, the impact on crop growth may not be obvious. In years of moisture shortage, plants on compacted soil stress more easily, and reduced growth and yield are noticeable."⁷
Operation of Deep Tillage Tools
When operating a deep tillage implement such as SubTiller, you should keep in mind that the objective is to shatter as much of the compacted zone as possible.Operating it one or two inches below the compaction layer when the soil is dry will achieve this objective.⁵ Additionally, a straight shank should be used, as "parabolic shanked subsoilers heave the soil surface too much...Soil heaving and the need for secondary tillage to smooth the field can be reduced by using a coulter in front of a straight legged subsoiler shank"⁵
The BLU-JET SubTiller has been built to maximize shattering of the compacted layer with its original shank design. The angle of the shank allows for full shattering of the compacted layer between the row without the use of parabolic shanks or winged points. Before buying just any subsoiler, be sure to ask whether it will completely fracture hardpan layers across the row; and whether the subsoiler will achieve that full shatter without using a winged point.
Summary
As the number of acres a person farms increases, compaction management plans and other methods used to limit the amount of compaction a field receives are sometimes unrealistic. Natural events thought to assist in minimizing or eliminating compaction, like freeze-thaw cycles, have also been shown to be ineffective. In order to truly solve compaction issues within a soil profile, some sort of tillage will need to be used to break up the compacted layers. For compacted layers below 10", subsoiling is the answer. Proper operation of the subsoiler in proper field conditions is critical. Consult your BLU-JET SubTiller owners manual for details on operating yours correctly.

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