By Sjoerd Willem Duiker
Growers are monitoring the weather to start planting. Early morning soil temps at a 2 in. depth must be above 50°F for corn to germinate and 55°F for soybeans.
The Northeast Regional Climate Center includes simulated soil temperature at 2 inch depth under a sod surface. The temperature under sod is lower than that under bare soil but reflects the temperature under high residue conditions or under a cover crop. While a map of simulated average daily soil temperatures at 2 inch depth can give us a first impression there is nothing like checking it yourself under your own field conditions.
Figure 1. Soil temperatures are approaching 50°F in the southeastern part of the state, indicating that conditions for corn planting will be here soon. Image taken from Cornell Network for Environment and Weather Applications.
Corn needs soil temperatures of at least 50°F for germination, so on April 22 we were almost there in the southeastern part of Pennsylvania (Figure 1), and indeed corn planters have been spotted in the field there. Corn vegetative growth is governed by soil temperatures for about 6 weeks after planting when the growing point rises about the soil surface. Soybeans need soil temperatures of 55°-60°F to germinate, explaining why soybeans are typically planted after corn.
While this map provides us with a statewide view of soil temperature, actual soil temperature will vary from field to field depending on soil and surface conditions. Soil moisture content has a big impact on soil temperature. A soil that is poorly drained will stay cooler than a soil that is well drained. The reason for this is that the evaporation of water takes a lot of energy and water conducts heat much better than air, so the heat penetrates to a greater depth causing the surface to warm up slower. Surface residue also affects soil temperature, as it acts like insulation reflecting solar radiation and slowing down the conductance of heat into the soil.
Figure 2. Soil temperature dynamics on a sunny day in May under different tillage systems.
Soil temperature should be measured in the early morning to determine if the soil is fit for planting. The reason being that soil temperature fluctuate during the day and tend to be at its minimum just before sunrise and at its maximum in the early afternoon as shown in Figure 2. The measurements were taken on a sunny day in the spring and also show how tillage affects soil temperature.
The slower warming of no-till soils under crop residue mean that a slight delay in planting is justified compared to soils that are tilled. While this is a disadvantage for no-till farmers it is well compensated by the benefits of soil residue cover such as erosion control, organic matter improvement and moisture conservation in summer.