Soil temperature map can help guide farmers’ planting decisions
AMES, Iowa — Soil temperature is one of the most important factors farmers use to guide their planting decisions. The general rule is to wait for the top 4 inches to reach at least 50 degrees Fahrenheit, with a warming trend in the forecast.
In particular, farmers can track soil temperature in their county and across the state using the Soil Temperature Map compiled by Iowa State University’s Iowa Environmental Mesonet. Updated daily in the spring, this map provides current and historical ground temperatures for every county in Iowa.
“The 4-inch temperature is one of the key indicators we use to tell if the soils are warm enough to plant corn and soybeans,” said Mark Licht, assistant professor of agronomy and cropping systems specialist at the ‘Iowa State University Extension and Outreach. “The map provides averages that can help inform grower decisions.”
Licht said some fields may be hotter or colder than what appears on the map, but he said it’s a good reference for making assumptions about different counties and regions of the state. As of April 7, ground temperatures ranged from 46 F in southern Iowa to 33 F in the north.
Daily fluctuations are common, especially in early spring. Historically, the optimal planting window for Iowa corn has been April 11 through May 18, with a shorter window in the northern part of the state compared to the south. The risk of severe frost (temperature below 28 F) remains above the 50th percentile until about mid-April.
“We want to make sure the soil temperature is warm enough for the seed to germinate and also for the sprout to emerge, without having to worry about severe frosts,” Licht said.
Although farmers can plant whenever they want, doing so below 50 F increases the risk of seed and plant damage, and the possibility that replanting will be necessary. Farmers who wish to take advantage of the replanting clause of their crop insurance must not plant before April 11.
“Planting early ensures you don’t plant late, but that doesn’t mean you won’t suffer damage or have to replant,” Licht said.
In addition to the current soil temperature, Licht said farmers should pay attention to forecasts and where the trend is headed. A cooling trend with a high risk of precipitation should be avoided. Forecasts should indicate that temperatures will remain stable or increase after planting.
Shareable photo: Planting soybeans in Iowa.