Selection of the optimal planting date is one of the most critical factors in the farming decision-making process. In making this decision, producers should consider soil temperatures rather than just calendar schedule. There has been a declining trend in air temperature across Kansas in the last two weeks of March.
For the week of March 16-22, average weekly soil temperatures at 4 inches varied greatly among crop reporting districts in Kansas (Fig. 1). For example, in the NE region, soil temperatures ranged from 46 to 49 F; while in the SW region, those temperatures varied from 50 to 56 F. Soil temperatures at 4 inches were below 50 F in most of Kansas, with the exception of western areas. Projections for the coming weeks are for increasing air temperatures, which can increase soil temperatures. The actual change in soil temperatures in any given field will be affected by amount of cover, amount of soil moisture, and landscape position. Wet soils in a no-till situation will be slower to warm. Dry soils will vary more rapidly, and match air temperatures more closely.
Each summer row crop has an optimal soil temperature for emergence. A minimum for corn is 50 F for germination and early growth. However, uniformity and synchrony in emergence is primarily achieved when soil temperatures are above 55 F. Uneven soil temperature around the seed zone can produce non-uniform crop germination and emergence. Lack of uniformity in emergence can greatly impact corn potential yields.
The average date of the last spring freeze is quite variable around the state (Fig. 2). The earliest dates are in southeast Kansas (April 5-15) and the latest dates are for the northwest area (May 3-8). Thus, planting dates for corn before April 15 in the southeast region would increase the likelihood of the crop to suffer from a late spring freeze (temperatures below 32 F). That would also be the case in northwest Kansas if corn is planted before May 8.
Low temperatures at planting can greatly impact the final number of plants through non-uniform emergence and early growth), which will consequently reduce final yields. This is particularly true for corn, since it is the earliest summer row crop planted. When soil temperatures remain at or below 50 degrees F after planting, the damage to germinating seed can be particularly severe.
Corn is also more likely than other summer crops to be affected by a hard freeze after emergence if it is planted too early. The impact of a hard freeze on emerged corn will vary depending on how low the temperature gets, the intensity and duration of the low temperatures, field variability and residue distribution, tillage systems, soil type and moisture conditions (injury is more severe under dry conditions), and the growth stage of the plant. Injury is most likely on very young seedlings or on plants beyond the V5-6 growth stage, when the growing point is above surface.
Think about all these factors when deciding the optimal planting time for corn and your other summer row crops. More information about planting status of summer row crops will be provided in coming issues of the Agronomy eUpdate newsletter. Stay tuned!
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library