Corn germination and emergence rely heavily on soil temperature. The warmer the soils, the faster emergence occurs. Emergence requires roughly 115 Growing Degree Days (GDDs) accumulated from planting. That GDD estimate is most accurate when GDDs are calculated using soil temperatures rather than air temperatures early in the season. To have corn emerge 10 days after planting, it requires daily average SOIL temperatures of approximately 61-62F. Most of Indiana’s soils are not quite at that point yet.
The recent spate of days with unusually warm air temperatures has certainly “added fuel to the fire” for those who are “chomping at the bit” to begin planting corn. Those air temperatures, combined with sunshine and fairly dry surface soils, have indeed resulted in a dramatic warming of the soils in recent days (Fig. 1), with maximum daily SOIL temperatures in some areas in southern Indiana reaching nearly 70F. However, air temperatures began to decline over the weekend, while cloudy weather and rain moved through the state. In response, soil temperatures have also decreased since Friday (Fig. 1) and will likely continue to do so considering the current (Apr 12) 6-10 and 8-14 day cool temperature forecasts from the National Weather Service.
Remember that the pace of germination, emergence, and initial stand establishment of corn relies heavily on temperature……. Specifically, SOIL temperature. The time-honored “rule of thumb” traditionally used as a guide to help determine when it is safe to plant corn is 50F (10C) soil temperature at planting depth. In reality, it is more accurate to think in terms of how many soil-temperature-based Growing Degree Days (GDDs) does it take for corn germination and emergence to occur? The GDD concept is a way of quantifying the accumulation of heat in a way that correlates nicely with the rate of corn development (Nielsen, 2020).
Corn emergence occurs in roughly 115 GDDs after planting, give or take 5 GDDs. If your goal is to achieve corn emergence in, say, 10 days after planting, that translates to the need to accumulate 11-12 GDDs per day FROM PLANTING UNTIL EMERGENCE of the crop. In general terms, that equals 10 days of soil temperatures AVERAGING 61 to 62F per day. That average soil temperature is a far cry from the “rule of thumb” soil temperature of 50F.
The time in April when daily soil-GDD accumulations begin to consistently average 10 or more per day is roughly the second week of April in the southern third of the state, the third week of April in the central part of the state, and the last week of April in the northern third of the state. Coincidently, those timings in April agree nicely with the typical start of serious corn planting in those areas of the state.
Bottom Line: Clearly, we are approaching the time when soils begin to warm consistently, but are still subject to the short-term whims of Mother Nature. Pay attention to SOIL temperature, not AIR temperature when assessing current conditions AND pay attention to the short term (6-10, 8-14 day) temperature forecasts when making decisions about planting in mid-April. The more quickly germination and emergence occur, the more likely emergence and initial seedling development will be uniform and vigorous. Speed of development, uniformity, and health of the plants greatly influence the resilience of the young corn crop to other stresses.
Read On Only If You’re Interested
So……. You might be wondering where you can find these estimates of daily GDDs. One source is the Useful to Usable (U2U) Corn GDD Decision Support Tool (https://mrcc.illinois.edu/U2U/gdd). This tool allows you to select your location, the planting date, the relative maturity of the hybrid, etc. It then provides an estimate of accumulated GDDs from planting to today and an estimate of future GDD accumulations based on 30-year historical normals.
This GDD Tool is a great tool…. EXCEPT for the fact that it calculates GDDs based on AIR temperatures, not SOIL temperatures. Once corn develops beyond the V5-V6 stages, that is no problem. However, prior to V5-V6, while the apical meristem (growing point) is below ground, corn plant development responds to soil temperature more than air temperature. Consequently, the GDD Tool estimates often do not agree with soil-based GDD estimates early in the season. In recent days, with unusually warm air temperatures, estimates of daily GDD accumulation with the GDD Tool have ranged as high as 20 GDD per day (Tippecanoe Co, Apr 8), while GDD estimates based on 4-inch soil temperatures obtained from the Indiana State Climate Office were no higher than 8 GDD per day (Purdue Agronomy Farm, Apr 8).
Unfortunately, it is difficult to find an online source that provides estimates of daily GDD accumulations based on SOIL temperature. There may be some commercial ag-related sources for these estimates that I am not aware of (someone please tell me if there are). The alternatives are 1) the Guess ‘n by Golly method and 2) the Brute Force method. Both rely on soil temperature data obtained from the Purdue University – Indiana Mesonet system, which you can find through the Indiana State Climate Office.
TIP: From the main page of the Indiana State Climate Office Web site, click “Data” in the menu, then “Purdue Mesonet”, then “Purdue Mesonet Dashboard Page”. Once on that page, you can select either select a weather station from the map (Firefox, Chrome browsers but not Safari) or click “Stations” in left menu and then select a weather station from the dropdown list. Once on a weather station’s dashboard, scroll down to find the “Soil Temperature” graph. Choose “Bare Soil” if you practice conventional tillage or “Grass Covered Soil” if you practice no-tillage. You can customize the dates to your interest.
The “Guess ‘n by Golly” Method is one where you simply estimate and track average daily soil temperatures by visually examining the soil temperature graph on the Purdue Mesonet Web site. Remember, you are looking for daily AVERAGE soil temperatures of roughly 60F.
The “Brute Force” Method involves 1) selecting “bare soil” or “grass covered soil” temperatures, 2) customizing the dates of interest, and 3) then clicking the “ellipse” icon in the upper right of the graph (looks like 3 horizontal lines) and choosing to download the data in a .csv file format. You can then open that file in Microsoft Excel or similar spreadsheet and create your own calculations of daily soil-based GDD accumulations (Nielsen, 2020).
References Mentioned in this Article
Nielsen, R.L. (Bob). 2020. Heat Unit Concepts Related to Corn Development. Corny News Network, Purdue Univ. Agronomy Extension. http://www.kingcorn.org/news/timeless/HeatUnits.html [accessed Apr 2021].
Indiana State Climate Office. Purdue University. https://ag.purdue.edu/indiana-state-climate [accessed Apr 2021].
U2U@MRCC: Corn GDD Tool. Midwest Regional Climate Center. https://mrcc.illinois.edu/U2U/gdd [accessed Apr 2021].
Fig. 1. Hourly Soil Temperatures (4-inches) at selected Purdue Ag Centers from Apr 1 – 12.
