Selection of the optimal planting date is one of the most critical factors in the decision-making process for producers. In making this decision, producers should consider soil temperatures rather than just calendar dates. After a very warm start to March, air temperatures across Kansas declined this past week.
For the week of March 30 to April 5, average weekly soil temperatures at 2 inches among crop reporting districts overall ranged from 45 to 53 degrees F (Figure 1). For example, in the northeast region, soil temperatures ranged from 45 to 47 degrees F; while in the southwest region, soil temperatures varied from 53 to 47 degrees F (Figure 1). Soil temperatures were around 44-47 degrees F for the northwest region.
Figure 1. Average soil temperatures at 2-inch depth for the week of March 30 - April 5, 2019.
Differences in soil temperature were related to the large variations in air temperatures experienced last week, from 38 degrees F in northern portions of the state to 58 degrees F for areas in southern Kansas (Figure 2).
Figure 2. Weekly mean air temperatures for the week of March 30 - April 5, 2019.
Projections for the coming weeks are for increasing air temperatures – but cooler-than-normal state wind, which will slow soil temperature increases (Figure 3).
Figure 3. 8 - 14 day outlook temperature probability for April 12-18, 2018 (NOAA).
Current soil moisture status across Kansas is quite wet, despite the relative low precipitation for the week (Figure 4). If saturation is above 50% at the 2-inch soil depth, this reflects wet soil conditions with a low probability for field work. Projections for coming weeks are for precipitation to be above-normal for all of Kansas (Figure 5), which will slow down soil warming and impact potential plans for an early start to planting.
Figure 4. Percent saturation at 2 inches as of April 5-2019 (KS Mesonet).
Figure 5. 8-14 Day outlook precipitation probability for April 12 - 18. (NOAA).
Optimal soil temperature for crop emergence
Every summer row crop has an optimal soil temperature for emergence. A minimum for corn is 50 degrees F for germination and early growth. However, uniformity and synchrony in emergence is primarily achieved when soil temperatures are above 55 degrees F. 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 degrees F 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 F) is quite variable around the state (Figure 6). 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 6. Average last spring freeze (32 degrees F) for Kansas.
Think about 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.