NDSU Crop and Pest Report

ISSUE 13  July 24, 2003


Questions regarding compaction and the benefits of deep tillage arise every year. Soil compaction is a byproduct of field work in the spring and sometimes in the fall. In our climate, and with many of our soils, freezing and thawing will alleviate some of the compaction so that crops do not suffer greatly the following year. Under heavy traffic and multiple field trips, especially under moist soil conditions, a traffic induced compacted layer may develop under the soil. In some parts of the US with mild winters, deep tillage (tillage below 6-8 inches) is used to break up the compacted layer and make the fields suitable for water infiltration and rooting.

A number of studies have been conducted in North Dakota and neighboring states which compared normal tillage with deep tillage under compacted conditions. Nearly all of these have shown little if any benefit to subsequent crops with deep tillage, even though a study at Morris, MN showed that soil bulk density differences following compaction lasted for eight years or more.

There are several reasons for this lack of crop response to deep tillage. First, in order to actually break up the traffic "pan", the soils must be dry. Deep tillage in moist to wet soils is recreational at best and does little to fracture the subsoil into smaller aggregates. Secondly, trips the spring following deep tillage pack down the more fluffy soil and the result is a soil with similar compaction compared to what was present prior to deep tillage. The third reason is that most crops seem to do best with a medium bulk density, not one which is really fluffy and not one which is really dense, so the physical activity of freeze/thaw, wet/dry in our soils seems to be enough in most cases to support maximum crop growth and development in our climate.

There are two situations that I have encountered which may respond to deep tillage. The first is a soil with a sodium induced restrictive layer, usually at about 4-8 inches in soils with a high sodium content. If these soils also have significant soluble salts (sodium SAR ratios and soluble salts can be determined from a soil sample), then deep tillage may result in mixing these calcium rich salts with the sodium layer, reestablish good flocculation and "heal" the soil for a few years before the problem reoccurs, as it will. The second responsive situation is a soil which has had excessive traffic over the years. Perhaps the end rows, or the depressions, which will tend to be wetter when the rest of the field is fit to work. Generally, whole fields are not heavily compacted, usually only small areas within fields.

In each situation, it is unlikely that whole fields will need deep tillage. The rest of the field probably already has good soil structure and will not benefit from this expensive procedure. Determine which areas of the field might benefit and which will not. If areas do not need it, donít spend the money to treat it.

There are drawbacks to deep tillage beyond the obvious one of wasting money on an expensive, unnecessary field operation. In the northern valley this year, a grower used deep tillage in strips across a field to test whether it would help his drainage. From observations this summer, it appears that the added water percolation and reduced runoff from the field resulted in significant crop damage to the deep tilled strips compared to areas with more compaction. Instead of running off these strips, the water penetrated the soil and has kept the strips wetter than the rest of the field to the point of being continually water-logged. Make sure the advantages and disadvantages are weighed before using deep tillage.

Serious compaction can be avoided by using controlled traffic within fields and keeping heavy trucks off fields as much as practical in the fall, but especially in the spring.

Dave Franzen
NDSU Extension Soil Specialist
(701) 231-8884

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