deep soil compaction

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Deep Soil Compaction

Deep soil compaction is defined as excessive soil compaction below the normal annual tillage depth, usually 6 to 10 inches. Soil compaction below tillage depths is of greater concern than surface compaction because it is a difficult problem to solve.

Deep compaction was discovered in drainage research plots on Hoytville soil in northwest Ohio. In its natural state, the Hoytville soil will allow 4.8 to 14.4 inches per day of water movement, hydraulic conductivity, below the plow layer. As shown in Table 1, the rate of water movement below the plow layer was reduced to 1.4 inches per day as a result of tillage and traffic. Research is in progress to determine if the rate of water movement will be improved by crops such as alfalfa and changing tillage practices.

Table 1: Hydraulic Conductivity of Hoytville Soil
Depth	Hydraulic conductivity, inches per day
(Inches)	Natural Soil (SCS Report)	Farmed Soil (research plot)
0 - 8	4.8 - 48	3.8
8 - 20	4.8 - 14.4	1.4
20 - 52	4.8 - 14.4	7.0
52 - 60	1.4 - 4.8	7.0

Deep tillage such as subsoiling or deep chiseling may be used to "break up" compacted layers, but the soil must be dry for the practice to be effective. The cost is high and the effect will have a very short life unless a change is made in farming practices that caused the compaction.

Heavy wheel loads are believed to be the cause of most deep soil compaction. The volume of soil compacted by a wheel pass varies with soil type, soil moisture, tire size, pressure and total load. Figure 3 shows that pressures are transmitted deeper into wet soil than in dry soil by the same tire size and wheel load.

A comparison of pressures in the subsoil shows that wide tires carrying a large load at the same pressure as narrow tires will cause compaction to a greater depth than the smaller tires with smaller loads, as shown in Figure 4.

The situation gets much more complex as one compares different tire size, load, soil condition and tire arrangement. Figure 5 gives four examples. The major point is that large tires with heavy loads transmit excessive pressures 16 to 24 inches into the soil when soils are moist or wet.