By Sjoerd Willem Duiker and Daniela Carrijo et.al
Air temperatures are creeping up, and so is soil temperature. The soil acts as a buffer, so the deeper you go into the soil, the smaller the fluctuation of the temperature. The base temperature for corn is 50°F, and it is recommended that corn be planted only when the soil temperature at a 2-inch depth is consistently above that base. Soil temperature typically peaks at mid-afternoon and is at its lowest early in the morning. Therefore, the best time to check soil temperature is at 6 or 7 am. At that time, the soil should be consistently warmer than 50°F for corn germination and growth.
According to USDA-NRCS' National Water and Climate Center reports, which summarize data collected from two stations in central Pennsylvania, soil temperatures are rising and starting to approach 50°F early in the morning (Figures 1 and 2). Soil temperature in your fields may be different, and therefore, it pays to check your own soil temperature. At low soil temperatures, the corn seed may be damaged due to cold water imbibition. The seed is most sensitive in the first 48 hours after planting, and therefore you want to avoid lower soil temperatures during this period. That said, delaying corn planting past the optimum window will also lead to yield penalty; thus, the decision on when to start planting is affected by many factors, such as how many days it takes you to plant all your corn and how many days suitable for fieldwork you have because the other factor that plays into the corn planting game is soil moisture. In some areas of the state, the soil is still way too wet to be planted, but this varies a lot depending on where you are and the soil types you have to work with. To determine if the soil is fit for planting, you can use the 'ball test,' which involves grabbing a handful of soil from the top 2-3 inches, kneading it in your hand, and if you can make a ball out of the soil, it is still too wet, and you will likely cause sidewall compaction, which will compromise germination and early growth as well.
In addition to soil temperature and moisture, the third important consideration for planting is the weather forecast following planting. After emergence, corn seedlings can die if the air temperature is below freezing for a prolonged period. For example, studies have shown that 50% mortality occurs when corn seedlings are exposed to 28°F air temperature for 48 hours, although plant injury occurs at higher temperatures. Therefore, do not plant corn if the forecast shows sustained hours of below-freezing temperatures.
In a nutshell, to determine the optimal time to start planting, make sure that 2-inch soil temperatures are consistently above 50°F early in the morning, make sure the soil is fit (not too wet), and that the forecast shows above-freezing temperatures for the week(s) following planting.
On-line resources to check soil temperature
USDA-NRCS National Water and Climate Center
Cornell Growing Degree Calculator
Source : psu.edu