Every year about this time we get anxious about getting in the field and planting crops. Except for spring planted small grains, most field crops have minimum soil temperatures below which they tend to germinate poorly and become increasingly susceptible to seedling diseases. Knowing how soil temperatures fluctuate and how best to measure them can help you optimize your crop production success.
In the first figure below, soil temperatures at depths of from 0 to 20 inches are noted throughout a sunny day. Although this experiment was conducted at the end of May on a bare moist soil, you can expect the temperature response in most tilled soils throughout most of the year to react similarly, although with the curves showing somewhat greater or lesser amplitudes. What is important to note about this figure is that the soil temperature at the 4-inch depth does not reach a peak until almost five hours past the surface temperature maximum. In other words, we expect the flow of heat from the surface to lower layers of soil to take several hours.
In the figure immediately below, soil temperature throughout a cloudy day is noted at depths of 0 to 20 inches on a bare moist soil. Although under cloudy conditions it takes considerably longer for the soil to reach its maximum temperature, what is again important to note is that transference of heat from surface soil to lower soil layers is a delayed effect.
So what can we learn from this?
If we remember that enzymatic action within seeds ‘begins’ at predetermined temperatures (for example 50 F for corn and 55 F for soybean), then we will want to determine when the soil temperature at planting depth and below is consistently above germination minimums. This suggests that a better measurement of the soil’s readiness for planting might be the overnight average taken early in the morning than would a peak temperature reading taken from 4 to 6 PM in the afternoon. Finally, remember that soil temperatures at planting depth is also determined by heat being drawn out of the top layer of soil by cooler soil beneath. A soil with a deep frost layer will take much longer to warm up.
Figures after Soil Temperature, Michael D. Novak, Micrometeorology in Agricultural Systems 2005 p.105-129