Managing against soil compaction
By KRISS NELSON
With the less than ideal soil conditions many producers are experiencing this harvest, what damage could they be causing in terms of soil compaction?
Mark Hanna, retired Iowa State University Extension and Outreach agricultural engineer said that when soil becomes compacted, the real damage that occurs is when you lose some of the larger soil pore spaces.
“About 50 percent of soil’s volume is mineral and the other 50 percent is pore space and that pore space is occupied by both water and air,” he said. “The water filled pores are kind of smaller ones and the air filled pores are the larger ones. When you mash that soil down when you compact it, it’s typically those larger macro pores that you lose first.”
Hanna said soils tend to be more prone to compaction just when a producer thinks they can get back into the field and not get stuck.
“The soil moisture tends to be what agronomists call field capacity – it’s just about the right mix of air and water to kind of rearrange the soil particles and compact them more,” he said. “There’s evidence with some of the ruts that you start to see.”
Hanna said the good news is, many producers were faced with a very similar situation last fall.
“The memories are still enough they kind of remember where those wet spots were last fall and I’m sure they are adjusting harvest to the extent that they can,” he said.
Particularly this time of year, there is a lot of traversing in the field with heavy axle loads.
Hanna said most combines may be carrying 20 to 25 tons per axle.
“That is with the grain tank loaded,” he said. “An empty grain tank is going to be quite a bit lighter. A lot of that grain tank weight is carried on the front axle as well as the weight of the head.”
Grain carts that have a 1,000 or 1,100 bushel capacity when filled could be carrying 40 to 45 tons per axle.
“We all want great crops out there. The more bushels that we raise the better for our profitability in many cases,” said Hanna. “But, for corn, a lot of folks would like to get at least 200 bushel to the acre if they could and frankly we would like to get 250 bushels to the acre. But, it turns out that 250 bushels of corn per acre weighs about 14,000 pounds or seven tons per acre. My point here is, these great crops that we are after, you have quite a bit of material you are trying to take off the fields, so the weight of a single axle grain cart comes with the territory a little bit with these yields.”
Regardless of yields, the crops must come out.
“We just don’t have any choice about that,” he said. “One potential solution would be to just say you will wait until the ground freezes and for those really wet spots there will probably be a few people that will do that, but in most cases, we can’t afford to do that. We have to get the crop out in as timely of a manner as we can.”
One way to potentially help mitigate the amount of compaction, Hanna said is the idea of “controlled traffic” – by trying to limit the area of your wheel tracks, could be advantageous.
Controlled traffic is basically lining up the wheels of you grain cart to the wheel tracks of where the combine has already ran.
“Kind of keep a path in the same place. We limit the amount of damage that way,” he said. “If you can line up your wheel tracks appropriately, driving the grain cart in the same area as the combine tracks can help limit the area of damage.”
Having the proper tire pressure could also help to limit soil compaction.
“I want to stress to people to use the correct inflation pressure in tires and to do that, you need to know basically what the load is the wheel is carrying how many pounds and check that with the tire manufacturer or with your implement dealer. They can help you out with that too,” he said.
Can you repair the soil?
If a producer could not avoid compaction and has some damage in their field, should they try to fix the problem with tillage by using a ripper or sub-soiler and loosen the compacted area back up again?
“If the soils were relatively dry, that potentially could help,” he said. “But, unfortunately, when soils are moist or wet, not every fall is a good fall to sub-soil. You can go out there and run that ripper or sub-soiler, but you tend to just make slots in the soil. You don’t really loosen the soil very well. About the best thing you can do at the moment is try to avoid being on there when it’s too wet as much as you can and try to manage your compaction.”
Mother Nature could possibly come to the rescue for repairing compacted soils.
“If we go through a winter where we’ve got all of this moisture in the ground, particularly if the soil surface stays bare for a little while and we get some good, deep frost into the ground, the freezing and thawing action of the ice tends to mitigate a bit of that compaction that way,” he said. “Also, next summer, when things get dry out there and the soil kind of cracks the clay in the soil shrinks that will tend to loosen the soil back up too. Mother Nature has a hand to play in here and often can mitigate things probably as well, or as better than trying to use some of that deep tillage when the soil is wet.”
Is there a true threat in terms of compaction?
Hanna said oftentimes, quite a few of the soils in Iowa can be fairly forgiving in terms of compaction.
“I’m not saying compaction is good, but you can have some crop seasons where research has shown you go out there and try to mash the heck out of the soil and compare to an area you did not do that in and sometimes you can’t find a difference,” he said. “Part of that could be Mother Nature helping out. Part of it could be the crop season.”
Hanna referred to research that was done several years ago with compaction plots throughout the state. Researchers compacted the soil with a five, 10 and 20 ton axle load – typical loads for that time. Yields were collected off of those plots for the next three years and three different site locations – which provided for nine chances to see a statistically significant difference. Only in one of those nine cases did they see a yield that was down enough they were able to declare that yield lag due to compaction.
“I don’t say compaction is a good deal, generally it is not, but it may not be quite as bad of a deal of what you are thinking it is and quite a bit of it depends on what the following crop year is like,” he said. “The best advice I have is try to be aware of the situation and limit the damage as much as you can, but we all understand the crop has to come out of the field. Realize some of the loads you are working with and hopefully that motivates one to be a little careful – or a little bit better manager in the situation.”
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