Why farmers should know the 4-inch-deep soil temperature before planting

Soil temperature is one of the most important factors crop farmers use to guide their planting decisions. The rule of thumb is to wait until the upper four inches reach at least 50˚F., with a warming trend in the forecast.

One way Iowa farmers can keep track of soil temperature in their county and across the state is by using the soil temperatures map compiled by the Iowa Environmental Mesonet at Iowa State University. Updated daily in the spring, this map provides current and historical soil temperatures for each county in Iowa.

“The four-inch temperature is one of the key indicators that we use to indicate if soils are warm enough to plant corn and soybean,” says Mark Licht, Iowa State University agronomist. “The map provides averages that can help inform producer decisions.”

Licht says some fields may be warmer or cooler than what appears on the map, but he adds it's a good reference for making assumptions about different counties and regions of the state.

As of April 7, soil temperatures ranged from 46˚F. in southern Iowa, to as low as 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 to May 18, with a shorter window in the northern part of the state compared with the south. The risk for a heavy frost (temperature below 28˚F.) remains above the 50th percentile until about mid-April.

“We want to make sure that we have a soil temperature that is warm enough for the seed to germinate and also for the shoot to emerge, without having to worry about heavy frosts,” says Licht. Although farmers can plant whenever they like, doing so under 50˚F. increases the chances of damage to the seed and plant, and the potential that replanting may be necessary. Farmers who want to take advantage of the replanting provision in their crop insurance should not plant before April 11.

“Planting early ensures you will not be planting late, but it does not mean that you will not experience damage or have to replant,” said Licht.

In addition to the current soil temperature, Licht said farmers should pay attention to the forecast and where the trend is headed. Avoid planting when a cooling trend with a strong chance of precipitation is forecast. The forecast should call for temperatures to remain steady or to increase after planting.