Features Soil
Compaction effects on IDC and salinity

Compaction is often thought of as causing issues with root growth and crop development due to increased soil density and reduced pore space between soil particles. But other issues like Iron Deficiency Chlorosis (IDC) and salinity can become worse with compaction, further impacting crop productivity.

“IDC and salinity are both water-based problems. So a loss of water flow and movement through the soil means these problems can become larger with compaction,” says Marla Riekman, soil management specialist with Manitoba Agriculture.

Riekman points to areas of a field where compaction typically occurs. Wheel tracks and headlands commonly have pooling of water after rainfall where other areas of the field may not. This, in turn, can impact IDC and salinity.

Looking at salinity first, Riekman says it is a water problem, not a salt problem, and is caused by movement of water in the soil.  With a high water table, and when surface evaporation exceeds water infiltration into the soil, this upward movement of soil water can bring salts into the rooting zone. Salinity typically goes in cycles and generally follows changes in precipitation patterns.

Around 2010, Riekman started getting phone calls from farmers in the Red River Valley who were seeing salinity where they hadn’t seen it before. She says the heavy clay soils there typically have mild salinity, and it became noticeable when soybeans acreage increased.  

“A sensitive crop like soybean was being put into an area that has always had mild salinity,” says Riekman. “Other crops like wheat and canola weren’t showing the impact of that salinity on crop yield the way it was showing up on soybeans.”

In terms of compaction impacting salinity, the macropores in the soil are compressed, leaving micropores where water movement is restricted. When there is slower downward movement of water, leaching of salts from the rooting zone is reduced. Smaller pores also have the potential for upward capillary movement of water, bringing more salts into the rooting zone. 

Riekman, however, says compaction isn’t necessarily the only reason for salinity on headlands. The salinity may exist because of other factors at play, such as water movement from drainage ditches upwards into the headlands. “Compaction, then, just makes the problem that much worse,” she says. “And, as we see more corn and soybeans grown on the Prairies, we’re going to see more salinity issues show up, especially if there is some compaction.”

The question arises whether managing compaction can help reduce salinity problems. Subsoiling has been offered as a solution to break up compaction resulting in more macrospores. 

“This can go horribly wrong because it can encourage upward movement of water, and we have seen salinity increase,” says Riekman. “If there isn’t sufficient precipitation to wash the salts downward, subsoiling can actually make salinity worse.”

An older research study in Lethbridge, Alta., in the early 1980s found that subsoiling did not lessen salinity under non-irrigated conditions because there was insufficient precipitation to move the salts downward. However, under irrigation, sufficient irrigation could be applied to leach the salts downward out of the rooting zone. 

Solutions to manage salinity aim at controlling water movement in the soil. Saline areas can be seeded to a salt-tolerant forage. Alfalfa buffers along field edges can help to minimize roadside salinity. Manitoba research has found that a berseem clover cover crop increased the downward movement of water and reduced soil moisture. And, at the very least, let weeds like kochia grow to use up soil moisture, but mow the weeds down to keep them from going to seed in saline areas  — both to keep resistance problems from developing, but also to help keep something growing in the saline patches to utilize soil moisture.

Tile drainage can also be a way of drawing down soil moisture to enable precipitation to move salts back down in the soil profile. Manitoba data from AgVise Laboratories showed that over a 10-year period, EC salinity measurements can be reduced by one to 1.5 mmhos (1:1). 

“If you are thinking about trying to bring extreme salinity down, remember, this takes time,” says Riekman. “It takes time because you need precipitation to wash the salts through the soil profile and out of those tile lines.”

Another consideration for tile drainage is that if compaction exists, there can be a reduced flow of water into the drainage tile.

Compaction effects on IDC in soybeans
Iron Deficiency Chlorosis is influenced by a number of factors, such as high lime (calcium carbonate) content, wet soil conditions, salinity, elevated soil nitrate levels and herbicide stress. Any of these factors can increase the risk of IDC in soybeans. Often, IDC shows up in wetter areas of a field, but especially along headlands.

Compacted areas of the field often show up with the worst IDC symptoms. This is because there can be poor drainage, and the high soil moisture means calcium carbonate is dissolved in the soil water. This impacts the root’s ability to acidify the area around the root zone to increase iron availability and uptake.

Conversely, compacted wheel tracks may have reduced IDC and improved crop growth. Riekman says this may be because decreased aeration under wheel tracks can increase the rate of denitrification resulting in lower soil nitrate levels – if elevated soil nitrate levels were part of the cause of IDC. Additionally, wheel tracks may shed water more readily, which decreases the impact of bicarbonate solubility.

The best solution to address IDC is to grow a resistant variety. Other management tools include improving drainage, planting soybeans on low residual nitrogen fields and biologically dewatering soils with cover crops. Increased seeding rates can also help the crop use up more water in the seedrow. 

“If you know IDC is a problem that you have, and compaction may be compounding and increasing the risk of IDC in certain areas of the field, consider varietal choices as your first line of defence,” says Riekman.

Riekman cautions that managing compaction won’t make salinity and IDC issues disappear. But reducing compaction can help reduce the extra crop stress caused by minor salinity and IDC issues. 

“Salinity and IDC are both influenced by poor drainage, so decreasing compaction and improving soil drainage are good places to start,” she says.