Should we be planting corn and soybeans by calendar day or by soil temperature? Planting early has benefits, such as maximizing yield, avoiding an early fall frost and possibly missing disease and insect cycles.
Corn requires a consistent temperature of 10 degrees Celsius or 50 degrees Fahrenheit, while soybeans require slightly more and above to germinate and grow evenly. Planting into colder soils causes the seed to sit in cold soil longer than needed and makes it more susceptible to disease, insects and a stress known as imbibitional chilling.
Imbibitional chilling occurs when seeds take in water prior to germination, a process that is needed to rehydrate the seed. If this first water is cold from the soil, a cold rain or snow, cell membranes can rupture, causing damage to the new emerging plant. This could also result in lower germination, delayed growth, more risk for entry of disease and pests as well as reduction of normal plant development and nutrient uptake. This new plant will also have less energy to emerge from depth or push through surface crusting, adding to early season stress.
Both corn and soybeans have different growing points. The soybean growing point is above ground and at risk to frost, whereas corn has the growing point below ground. Soybeans, however, can survive a short frost after emergence.
You should assess a crop after a frost, but the decision to replant should not be done hastily. Allow the crop two days to recover, also taking into account the original planting date.
Soil moisture is also an important factor. Corn should be planted at a two-inch depth, or to moisture. Dry soil will cause delayed germination and delayed emergence. Soybeans should be planted at one to 1.5-inch depth and into moisture, not on moisture. Planting into saturated soils results in reduced oxygen availability, inhibiting root growth and further reducing plant development.
Delayed plant growth creates a stress on the emerging and young plant. Photosynthesis will also be reduced resulting in slow or reduced growth.
Temperature or Calendar Date?
Obviously, every year is different. Planting into ideal soil temperature is the first thing to be considered. But calendar date is also a very important factor.
If you’re going to start planting and it’s on the early side of normal, then watching the weather forecast is the best option. Planting on the upside of warm weather trends is important for the following 48 hours after planting.
If the weather forecast is in a downward trend, or you’re experiencing rain, which is typical during a cold spring, waiting until after the rain or when the temperature is on the upcycle is important. If it means switching away from planting cereals to planting more sensitive crops like corn or soybeans, then switching over could be time well spent.