Applying large amounts of lime can result in fairly rapid and substantial increases in soil pH at shallow soil depths. Although the lime eventually moves deeper into the soil profile and the pH at the soil surface equilibrates and decreases somewhat, the temporary increase in pH can have consequences for the persistence and activity of herbicides applied this spring, says Mark Loux, Ohio State University weed specialist.
A number of herbicides can be more active as soil pH increases above 7, including atrazine, metribuzin and chlorimuron, Loux says. Increased activity can be a good thing relative to weed control, but can also increase the risk of crop injury where other factors are favorable for injury to occur.
"The increased carryover risk at high pH is the more important issue here," Loux says. "Our main caution is to possibly avoid use of chlorimuron-containing herbicides, like Canopy, Cloak, Valor XLT, Envive or Enlite, where lime was applied this winter and the pH at the soil surface is above 7."
The first several months after herbicide application are important for herbicide degradation, Loux reports. Degradation of chlorimuron will be inhibited at the high pH that can result from liming, and lack of substantial degradation during these first few months increases the carryover risk.
Loux says statements from herbicide labels regarding high soil pH include the following:
- Isoxaflutole (Balance Flexx, Corvus, Prequel) labels indicate the high soil pH is one of the conditions that can increase risk of crop injury
- Hornet, Python, SureStart should not be applied where soil pH is greater than 7.8
- Metribuzin (corn) should not be applied where soil pH is 7.0 or greater
- With Authority Assist, use the lower rates where soil pH is greater than 7. Do not apply Authority MTZ to soils with pH greater than 7.5
- Metribuzin (soybeans) injury risk increases where soil pH is more than 7.5
For producers with no-till cropping systems, the surface application of lime brings up additional management considerations before the spring corn planting season, says Keith Diedrick, an Ohio State University agronomist.
"Lime is not terribly soluble in water," Diedrick says. "Tillage is a good method for working lime into the soil, but a layer of high pH material may exist on the soil surface in no-till fields.
"High pH levels combine with ammonium and urea-based fertilizers in a chemical reaction converting ammonium to ammonia gas, which can escape to the air and provide no nutritional benefit to crops."
Diedrick says as air and soil temperatures increase in the spring and summer, this process runs faster and nitrogen losses increase. This volatilization process is most exacerbated with surface application of urea and 28% UAN on recently limed fields on a warm day.
He says this process also occurs to a smaller extent in ammonium sulfate, much smaller in MAP and DAP, and significantly even less with ammonium nitrate.
"To avoid surface volatilization losses with recently limed no-till fields, avoiding surface urea and UAN is key," Diedrick says. "Instead, band starter materials if UAN or urea is used, and knife in sidedress applications of UAN, being sure that the application equipment is set to inject the material in the slot through the lime layer and into the soil where it does the most good for crops."
Diedrick says this sort of application also has the benefit of reducing the possibility of surface losses or runoff in rainfall events.