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Soil temperatures throughout the day
April is the month to monitor soil temperature to determine when time is ripe for corn planting. Two inch soil temperature is provided on the PA-PIPE website. The system provides average daily soil temperature (calculated as the average of daily minimum and maximum temperature) and minimum daily soil temperature.
Soil temperature follows a typical sinusoidal rhythm every day as shown in Figure 1. Minimum soil temperature is typically reached at about 6 or 7 am, while maximum soil temperature is typically reached at about 2 or 3 pm. When determining if soils are warm enough to start planting corn, monitor the minimum soil temperature at 2 inches depth.
This year soil temperatures have been low. In fact, last week minimum soil temperatures hardly inched above freezing in the Commonwealth. But PA-PIPE shows that yesterday (March 31st) minimum soil temperatures were already in the mid-fifties in southeastern PA, although in the northwest they were still below freezing. Soil temperature is affected by management and moisture, too. Under crop residue soil stays cooler due to the higher moisture content and the insulation provided by the crop residue. The effect is most pronounced for maximum soil temperatures. Living cover crops can dry out the soil faster compared with fallow soil, which would lead to faster warming. Corn planting can start when minimum soil temperatures are above 50F and the weather forecast calls for seasonal temperatures for the next 4-5 days. PA-PIPE can give general information, but it is important to check your own fields with a soil thermometer to capture the effects of local soil conditions.
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