I usually think of compaction as a spring problem, however, after looking back at past articles I found I have written a fall compaction press release for 2003 and 2004 and now 2005.

Before this past rainfall, most of the state had a majority of the soybeans harvested, sugar beets are behind the 5 year average, and some producers were just getting started on harvesting the corn. Most of the cropping regions of Minnesota received over 2 inches of rain last weekend with some areas receiving 4 inches in already saturated soil conditions.

Soil compaction is the reduction of pore space. An ideal soil situation has half of the pores filled with water and the other half filled with air. A saturated soil has all of its pore space filled with water. You can not compact water. Instead it will smear, displace and clod soil particles, but technically it won’t compact.

The soil is most vulnerable to compaction about 3-4 days after a nice soaking rain (i.e. at field capacity). This is the time when producers can get their equipment back out in the field. So what do you do? Harvest and risk significant compaction or stay off the field? Easy, get the crop off.

With that being said, there are three ways to minimize the extent of the compaction you may create. Look at axle loads, properly inflate tires of field equipment, and control the field traffic.

Heavy axle loads and wet soil conditions will increase the depth of compaction in the soil profile. As loads become greater than 10 tons an axle there is the potential to compact the soil down to two to three feet. Full combines, slurry tankers, and grain carts can weigh between 20 and 40 tons an axle and whether equipped with tracks or tires, create deep compaction. Tracks and duals have better floatation than single tires. If you can equip your combine or grain carts with either of these choices it would decrease the depth of compaction and allow the combine a few extra passes before it gets stuck.

Before using any equipment in the field make sure to check your tire pressure. Not only does this help reduce soil compaction, it also improves tractor efficiency. Studies have shown that given the same axle load, inflation of the tires (psi) will determine the depth and severity of the compaction.

An Ohio study looked at a loaded, 12 row combine and different tire configurations. The worst and most severe compaction happened with the single tires (30.5 L32) at 34 psi. From worse to least compaction were the experimental half-track (calculated at 10 psi), then dual tires (18.4 R38) at 26 psi, and then very wide tires (68x50.0-32) over-inflated at 24 psi. The least amount of compaction occurred with the very wide tires (68x50.0-32) properly inflated to 15 psi (Graph 1). The conclusion appears to be clear: for a certain axle load, the lower the inflation pressure, the better it is for the soil. Note that the half-track had an average calculated pressure on the soil of about 10 psi; but it gave results that appear to make it equal to a tire with about 26- 30 psi. This may be due to the psi of the tire on the rear of the combine

Graph 1. Effect of tires size and inflation on soil porosity.

Check with your tire manufacturer for proper tire size and inflation rate for the carrying capacity of your equipment.

Controlled traffic is not a new concept, but in the past was difficult to implement, unless the producer used a ridge till or a no till system. With the discovery of auto-steering and global positioning systems, a producer can use the same wheel tracks within a few inches of accuracy from year to year.

The theory behind controlled traffic is that 80% of the compaction happens on the first pass, so use this to your advantage. While it may take awhile to replace equipment that will use the same wheel tracks, there is one piece of equipment that should receive special attention; the grain cart. The grain cart has the highest potential to compact the soil due to the large carrying capacity (up past 1,000 bushels) and a single axle on which to carry that weight.

When using a grain cart try to use the same paths across the field. When unloading the combine, use the combine’s previous wheel tracks. After loading, follow those tracks down the field and take the headlands back to the semi or field entrance. Never diagonally cross the field. This will create multiple wheel traffic patterns at 80% compaction. If you can’t park the semi trucks on the adjoining road, keep them on the headlands. Semi’s and gravity wagons may have a lower axle load, but the tire inflation is quite high.

Your soil is one of the most important factors when growing a healthy crop. Preventing soil compaction or decreasing the affected depth will increase water infiltration and storage capacity, timeliness of field operations, decrease the stress on plant roots, and decrease disease potential.

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