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Every so often there is interest about subsoiling that is initiated by a favorable report from some locality.  But can that local report be applied to different regions of the US and, in particular, to our Minnesota glacial till soils?  This article will address the concepts behind subsoiling and the research that has been conducted.

Subsoiling is a very aggressive tillage operation that breaks up the soil usually to a depth of 12-18 inches.  The theory behind subsoiling is to shatter a compacted layer deep in the soil to allow increased water movement, better aeration of the roots, and access to additional nutrients for plant growth. 

Soil compaction can be associated with a majority of field operations that are often performed when soils are wet and, thus, more susceptible to compaction.  Heavy equipment and tillage implements can damage 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 growth and is the number one defense against soil compaction. 

There has been a great deal of research conducted on deep plowing with the goal of alleviating subsoil compaction.  The results are mixed.  It is difficult to accurately predict the effects subsoiling has on crop yield because of multiple factors, such as: the level and depth of compaction, soil water content, subsequent traffic, variable weather conditions, the crop grown, and tillage methods. 

Research in Michigan and Ohio has reported increased yields of 3-4% for the following crop year.  However, Michigan and Ohio’s soils were formed from lake deposits and not formed by glacial till (though this research was conducted on soil formed by glacial deposits).  Most of the soils in Minnesota were formed by glacial till.   Therefore, data from Michigan and Ohio does not necessarily apply to Minnesota conditions.  A majority of research conducted in the Midwest, on glacial till soils, has reported no change in yield or a decrease in yield due to the effects of subsoiling.  As early as the 1950’s, Midwest researchers were seeing no effect or negative effects from subsoiling.  Research in Iowa reported no meaningful changes in corn production.  They found that subsoiling at a depth of 24 inches decreased the corn yield by 9.7 bushels the first year and 6.4 bushels per acre the following year.

There has been extensive soil compaction and subsequent deep ripping research conducted in Southern Minnesota by Dr. Ward Voorhees and Jane Johnson of the USDA/ARS North Central Soil Conservation Research Laboratory.  Details of this research effort are in an MS thesis by Jane Johnson titled,”Soil and Plant Responses to High Axle Load Wheel Traffic and Subsoil Tillage”.  Research results of this study reported that subsoiling to a depth of 16 inches failed to increase yields for neither corn or soybeans and decreased corn yield 11 bushels per acre in one of the two years. 

One reason why subsoiling fails to increase crop yield, may be due to unfavorable soil moisture conditions at the time of subsoiling.   If the soil is wet, subsoiling will be ineffective.  To achieve effective subsoiling the operator must be certain the soil is fracturing to the depth of the shank.  If the shank is reaching a depth of 18 inches it is very difficult to determine if the soil is shattering at that depth. 

Another reason for the failure of subsoiling to increase crop yield is that subsequent wheel traffic can recompact the loosened soil.  Loosened subsoil has very little bearing capacity, meaning it can’t support much weight.  Johnson and Voorhees discovered that an ordinary 2-wheel drive tractor, that weighed less than 5 ton an axle, was sufficiently heavy enough to recompact the loosened soil down to a depth of 16 inches.  Similarly, Randell Reeder, an Ohio State University’s Agricultural Engineer, has shown that within the tire tracks, two passes of a tractor recompacted the soil to a condition as bad as or worse than before subsoiling.  For that reason, controlled traffic becomes an important management tool.