By DOUG SCHMITZ
Iowa Correspondent

MUSCATINE, Iowa — Since Midwest farmers know soil compaction left from machinery used in muddy fields in last fall’s harvest changed such factors as nutrient intake and root growth, Iowa field agronomists said farmers would likely be confronting the same issues this fall.

“Farmers in my southern counties are concerned about their yields, the condition of the crop by harvest time and their ability to even get into the field at harvest time,” said Virgil Schmitt, Iowa State University extension field agronomist in southeastern Iowa.

“They have shown up, although not as severe as would have been the case if we would have had drier weather. I have been in several fields where the roots have been limited by encountering compacted soil.”

Last fall’s extremely wet and historically late harvest – coupled with this year’s rain-saturated spring and summer cropland that has had little chance to dry out – has renewed concerns over the impact 2009’s soil compaction will have on the 2010 harvest season.

As a result, Clarke McGrath, partner program manager at the Iowa Corn and Soybean Initiative and an ISU extension field agronomist in Harlan, said the top concern for farmers he’s talked to is what they will ultimately face once they do start harvesting – namely, the amount of damage that may have already been done to their crops.

“We have some possible evidence of compaction with some poor drainage issues, but in reality, in a wet year, compaction doesn’t seem to show up as much as a normal to dry year, when limited root growth shows up as drought stress,” he said.

In fact, Clarke said it’s been so wet this year that there is no really reliable way to tell if compaction is limiting drainage or if there is just so much water, it cannot get through the soil profile fast enough. “Now if it dries out, shallow root growth could be a problem, and again, no reliable way to tell if the shallow roots are from surplus water or 2009 compaction,” he added.

“Is even the good-looking corn suffering from the surplus water? Root, crown, stalk and leaf diseases will likely be a huge issue all season, and all can contribute to yield loss, lodging and poor harvest conditions.”

Most of the compaction questions extension agricultural engineer Mark Hanna said he’s been fielding have been from springtime compaction – especially regarding diagonal wheel tracks from spring tillage or fertilizer application.

“Although with heavy snow cover in much of the state, soil did not freeze below a few inches in many areas,” he said. “Residual compaction from fall harvest has been at least masked somewhat in fields this crop year by more recent wheel tracks.”

But compaction from springtime wheel tracks, or wheel tracks made in the same crop year in which the crop is growing, Hanna said, are often easier to see. “Visible symptoms noted may be shorter or discolored crop, or ponding of surface water as a result of slow surface infiltration rates,” he said.

While the soil moisture profile remained nearly full as of July 19, with rain predicted for that week, Hanna said the rainfall received in August could still determine whether soil conditions are wet or dry for harvest in late September and October.

“If we stay in a wet pattern with crop water using no more than available soil moisture that’s being received, soil will be more easily compacted during fall harvest,” he said. “If the weather turns dry, plant roots remove water and soil stays dry at harvest, soil will not be easily compacted.”

So, the question is: What are the best ways for farmers to handle soil compaction with harvest season just around the corner?

“If soil is wet and compactable at the time of harvest, using controlled traffic lanes for grain carts and other grain transport vehicles can help minimize the compacted area within the field,” Hanna said.

“Although larger harvests are good, use of larger equipment tends to cause heavier loads for the soil to support. Trying to keep wheel loads in defined areas allows some parts of the field to be used as roadways, while the rest of the field is maintained for subsequent planted crops.”

McGrath said what farmers decide to do about soil compaction, however, will largely depend on their individual operations and soil management, crop rotation, soil type and the extent and type of compaction.

“Tillage is not always the answer, but sometimes it is,” he said. “It is a case-by-case situation. Occasionally, what looks like a compaction issue in the fall is alleviated by the freeze/thaw cycle by the next spring.”

To help prevent future soil compaction, Mahdi Al-Kaisi, ISU associate professor of agronomy, said properly adjusting tire size and air pressure and using larger tires with lower air pressure, will allow for better flotation and reduce load on the soil surface.