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Does Deep Tillage Pay?

Emerson D. Nafziger Emerson D. Nafziger
Department of Crop Sciences
(217) 333-4424
  Bryan D. Young Bryan D. Young
Graduate Research Assistant
Department of Crop Sciences
(217) 244-2464

Most of the soils in Illinois are productive, with silt loam to silty clay loam soil texture in the surface. Subsurface textures are usually heaver than the surface, however, and the high clay content of the lower layers can be a barrier to root penetration, thus restricting rooting depth and decreasing the amount of water available to the crop.

Heavier subsoil textures tend to stay wet, thus keeping oxygen content low and making it difficult for roots to grow and function. This problem can be relieved by subsurface tile drainage. Heavy equipment, especially when operated when the soils are moderately wet, also tends to reduce soil air content and to increase the physical resistance to penetration of the soil by roots. Such soil compaction has increased as equipment axle weights have increased in recent decades.

The use of deep tillage—tillage at depths of more than 12 inches designed to shatter soil at that depth—has increased greatly in Illinois in recent years. Much of this has come about as producers have realized that their tractors and combines are heavy enough to cause compaction at greater depths, and they now have tractors large enough to pull tillage implements deep enough to break up some of the compacted layer. While we have no data on how much cropland is deep tilled each year, many producers are deep tilling many of their fields, either on a cycle of every two years (or more often) or after their observations lead them to conclude that compaction is a serious problem in a field.

Deep tillage is expensive in terms of energy use, equipment wear, and labor cost. Current cost estimates for using a "disk subsoiler" available at the website http://www.farmdoc.illinois.edu/manage/machinery/field_operations.asp total $15.60 per acre. We would expect costs for deep tilling using other "ripper"-type implements to be at least this high and probably higher, given that power requirements for implements designed to go 15 to 18 inches deep are often high, and that fuel costs have risen recently. If the cost is $20.00 per acre and deep tilling is done once in two years, it requires the equivalent of 5 additional bushels of corn and 2 additional bushels of soybean to pay for the operation.

In order to see what yield increase we might get from deep tilling, we are conducting a study at six University of Illinois research centers, all with somewhat different soil types. While it is somewhat speculative as to what we might expect deep tillage to accomplish in a given soil, most of the soils in these locations are probably only moderately compacted. Depending on soil characteristics, we also can guess that different soils will respond differently to deep tillage, though we don't have a good way to predict this.

Tillage treatments include deep tillage done using equipment manufactured for this purpose, chisel plowing, and no-till. The first tillage treatments were applied in the fall of 1999, following both corn and soybean if possible, but only one of the two crops in some cases. Both corn and soybean were planted in 2000.

The cone penetrometer is an accepted device for measuring soil resistance to penetration. We used it at most locations in the spring of 2000 and again in the fall, following harvest. We also measured yields of both crops.

At Urbana, the chisel and deep tillage both decreased soil penetration resistance, as shown in Figure 1 from the measurements made in the spring.

Spring 2000 penetrometer readings at Urbana
Figure 1. Spring 2000 penetrometer readings at Urbana.

By the fall of 2000, a year after the tillage treatments, soils were dry and considerably more difficult to penetrate, and the effect of tillage had decreased on a relative basis, but not on an absolute basis, as shown in Figure 2 (note that the scale on the bottom axis in have good way to predict this. Figure 2 is twice that of Figure 1).

Fall 2000 penetrometer readings at Urbana
Figure 2. Fall 2000 penetrometer readings at Urbana.

In the first year of this study, tillage produced little effect on yields of either corn or soybean, as shown in the table below.

Location No-Till Chisel Deep Till No-Till Chisel Deep Till
Brownstown 50.0 47.9 50.1
DeKalb 177 165 176 49.4 48.9 50.1
Dixon Springs 130 127 128 41.3 40.0 41.0
Perry 188 198 199 56.2 53.7 56.2
Monmouth 188 183 189 52.6 51.9 53.8
Urbana 169 176 170 43.1 45.5 46.8
Average 170 170 172 48.8 48.0 49.7

The 2000 season was relatively dry from mid-July on, and we might have expected an increase in rooting depth to have increased yields. Because we did not measure root growth, we do not know if roots grew deeper or not. We are not willing to draw strong conclusions about deep tillage until we get more results, but even one year's datasuggest that this practice may not be producing large returns in all fields where it is being done. In addition to the treatments listed above, this ongoing study includes deep tillage done only in alternate years and deep tillage done once (in 1999) and not again for several years, so we can see how long any effect might last.

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