Selection of the optimal planting date is one of the most critical factors in the decision-making process for farmers. In making this decision, producers should consider soil temperatures rather than just calendar dates. After a very mild start to March, air temperatures across Kansas cooled this past week.
For the week of March 27-April 2, average weekly soil temperatures at 2 inches among crop reporting districts ranged from 47 oF in northern locations to 63 oF in southern locations (Figure 1). For example, in the northeast region, soil temperatures ranged from 51 to 55 oF; while in the southwest region, soil temperatures varied from 49 to 63 oF. Soil temperatures were around 47-51 oF for the northwest region (Figure 1).
Figure 1. Average soil temperatures at 2-inch soil depth for the week of March 27-April 2, 2020. (http://mesonet.k-state.edu/)
Differences in soil temperatures were related to the large variations in average air temperatures experienced last week, from 42 oF in northern portions of the state to 64 oF for areas in southeast Kansas (Figure 2).
Figure 2. Weekly mean air temperatures for the week of March 25-31, 2020.
Projections for the coming weeks call for warmer-than-normal temperatures to the southeast and cooler-than-normal temperatures to the northwest. Warm, dry conditions in the southwest will speed the process of warming up the soils. Cool air temperatures or wet conditions will slow the warmup. (Figure 3).
Figure 3. 8 to 14-day outlook temperature probability for April 9 – April 15, 2020. (NOAA)
The actual change in soil temperatures in any given field will be affected by amount of residue cover, soil moisture, and landscape position. Wet soils in a no-till situation will be slower to warm. Dry soils will fluctuate more rapidly, matching air temperatures, particularly if skies are clear. (Figure 4).
Figure 4. Soil moisture at approximately 2 inches as of April 2, 2020 (Kansas Mesonet).
Current moisture status across Kansas is quite wet, with the largest weekly departure in precipitation in the northwest corner, although the southeast continues to be well above normal (Figure 5). Projections for coming weeks are for precipitation to be above normal for the eastern regions and drier for the western parts of Kansas (Figure 6), slowing down soil warming and plans for an early start to planting.
Figure 5. Departure from normal precipitation for the week of March 25 – March 31, 2020.
Figure 6. 8 to 14-day outlook precipitation probability for April 9 – April 15, 2020. (NOAA)
Optimal soil temperature for emergence
Every summer row crop has an optimal soil temperature for emergence. A minimum for corn is 50 oF for germination and early growth. However, uniformity and synchrony in emergence is primarily achieved when soil temperatures are above 55 oF. Uneven soil temperatures around the seed zone can produce non-uniform crop germination and emergence. Lack of uniformity in emergence can greatly impact corn potential yields. This is particularly true for corn, since it is the earliest summer row crop planted. When soil temperatures remain at or below 50 oF after planting, the damage to germinating seed can be particularly severe.
Impact of a hard freeze on corn
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 (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 the soil surface.
The average day for last spring freeze (32 oF) is quite variable around the state (Figure 7). The largest variability is from southeast to northwest Kansas; with the earliest last spring freeze date for the southeast region (April 5-15) and latest for the northwest area (>May 3). Corn planting dates before April 15 in the southeast region would increase the likelihood of the crop suffering from a late spring freeze. Similar conditions can be projected for northwest Kansas if corn is planted before May 3.
Figure 7. Average day for the last spring freeze (32 oF) for Kansas.
Producers should consider all these factors when deciding on the optimal planting time.
More information about the planting status of summer row crops will be provided in upcoming issues of the Agronomy eUpdate. Stay tuned!
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library
Christopher “Chip” Redmond, Kansas Mesonet Network Manager