Soil temperature for planting corn and soybeans

What the decision depends on

Soil temperature and its direction. Below 50 °F, seed sits, and the longer it sits the more it is exposed to seedling pathogens and insects. Iowa State's rule of thumb is 50 °F at the 4 inch (10 cm) depth and warming; Ohio State's early-April guidance uses 2 inch readings and the same threshold. The 2 inch reading tells you what the seed feels today; the 4 inch reading changes more slowly and is a better guide to the trend.

The first 48 hours. Nebraska CropWatch describes the risk window as the first 48 hours after planting, when seed imbibes water. Seed that takes up water in the low 40s °F suffers membrane damage, which shows up as seed that never emerges, corkscrewed mesocotyls and seedlings leafing out underground. Once imbibition is done, the risk of this specific injury falls to near zero, though slow growth in cold soil still raises disease risk.

Heat units to emergence. Corn needs about 100 to 120 growing degree days (GDD, base 50 °F) from planting to emergence; soybeans need about 90 to 130. Iowa State translates that into roughly three weeks when soil is in the 40s °F, two weeks in the 50s and one week in the 60s. Calculate early-season GDD from soil temperature, not air temperature.

Seedbed moisture and condition. Purdue puts uneven moisture at seed depth as the most frequent cause of uneven emergence, worth 8 to 10% yield in the trial it cites. Target about 50% of available water capacity, not field capacity. Dark, drier, low-residue soil warms faster than wet, light-coloured or heavy-residue soil.

Calendar and insurance. Planting windows for full yield potential are wide: Iowa data put corn at 98% of yield potential from about mid-April to the first week of May (to the end of April in the north), and Iowa State's guidance covers April 11 to May 18 for corn and April 11 to May 20 for soybeans. The USDA Risk Management Agency sets an earliest planting date for each crop by state and county group; acres planted before it are not eligible for a replant payment. The dates were moved earlier in several states before 2023, so check your policy. Ontario and the Prairie provinces have their own crop insurance planting rules.

Step-by-step

1. Read your field, not just the map. Push a soil thermometer to seed depth (2 inches, 5 cm) in two or three representative spots, mid-morning, on consecutive days. Ontario's Agronomy Guide suggests reading at about 11:30 a.m.
2. Check the regional trend. The Iowa Environmental Mesonet publishes county 4 inch soil temperature maps, past three days and forecast; most states have a comparable mesonet. Manitoba Agriculture runs about 120 automated stations reporting soil temperature at 5, 20, 50 and 100 cm. Those sensors sit under grass, which runs cooler than a bare, tilled seedbed in spring.
3. Look at the 48 to 72 hour air forecast. Lows, rain and cloud cover. Rain at 40 °F onto freshly planted seed is the classic chilling event; a dry cool night is less of a problem.
4. Decide crop by crop. Both crops share the threshold. Corn's growing point stays below ground until about V6, so a frost after emergence usually costs leaves, not plants. Soybean's growing point is above ground from emergence, and a freeze below the cotyledons kills the plant, though the hypocotyl and cotyledons tolerate light frost (to about 30 °F, −1 °C) better than corn leaves do.
5. Adjust the inputs for an early start. For early soybeans, Iowa State recommends fungicide and insecticide seed treatments and raising the seeding rate 10 to 20%, with a goal of 100,000 plants per acre (250,000 per hectare) at harvest. For corn in a cool, wet forecast, Nebraska suggests a fungicide seed treatment on the earliest fields and a hybrid with a good cold-tolerance rating.
6. Set depth for the conditions. Corn at 1.5 to 2 inches (4 to 5 cm), deeper only to reach uniform moisture. Ontario advises no seed shallower than 3.8 cm (1.5 inches). Do not plant shallow to chase warmth; shallow seed dries out, swings in temperature and sets poor nodal roots.

Worked example with numbers

A central Iowa grower on April 22 reads 49 °F at 2 inches at 9 a.m. in a tilled field, up from 45 °F three days earlier. The mesonet 4 inch map shows 48 to 50 °F for the county and rising. The forecast is highs of 65 to 70 °F and lows of 45 °F for three days, then a front on day four with an inch of rain and a low of 36 °F.

Corn planted on April 22 or 23 imbibes during the warm 48 hours and the cold rain arrives after that window, so the chilling risk is acceptable; with soil in the low 50s °F, emergence will take about two weeks. Soybeans planted the same day face the same imbibition logic and little frost risk until they emerge two to three weeks later. If the front were forecast for day two instead of day four, the right call for both crops would be to park the planter for three days; the field is still inside the full-yield window on April 26.

Common mistakes

• Reading the thermometer at 3 p.m. Afternoon readings at 2 inches on a sunny day are higher than the morning readings the thresholds are built on. Use mid-morning readings for the decision.
• Treating 50 °F as a switch. Soil at 50 °F and falling into a cold rain is worse than soil at 48 °F and rising into a warm week. Nebraska's advice is to wait for 48 hour windows at or above 50 °F.
• Planting soybeans first because they tolerate frost. Seedling frost tolerance is not seed chilling tolerance; the imbibition rule applies to both crops. Ohio State puts the cost of soybean planting delay at 0.25 to 1 bu/ac per day after May 1, so there is some time in April, and Iowa State notes that early planting does not always translate into higher soybean yield.
• Mudding it in. Planting into wet soil to beat a front causes sidewall compaction and uneven depth, which Purdue ranks ahead of temperature as a cause of uneven stands.
• Ignoring residue. A no-till field under heavy corn residue runs cooler than a tilled field on the same morning. Check each field.

Regional differences

In the US Corn Belt the limiting factor in April is usually the soil temperature trend, not the calendar; the full-yield window runs into mid-May for corn and a few days later for soybeans. Insurance earliest planting dates are set by state and county group and are a hard line for replant coverage; confirm yours before an early start.

In Ontario, the Agronomy Guide uses 10 °C at 10 cm as the germination threshold and adds a calendar rule: after about April 26 in areas above 3,000 CHU, or May 1 in areas below it, plant as soil moisture allows rather than waiting on temperature.

On the Prairies, the Manitoba Ag Weather Program reports soil temperature at 5 cm, the depth that matters for seed. Follow your provincial guide for planting windows and crop insurance seeding deadlines rather than the Iowa dates above. Seed treatments sold in the United States are not automatically registered in Canada; use products with a Canadian PCP number and follow that label.