Selection of the optimal planting date is one of the most critical factors in the farm decision-making process. 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-April 5, average weekly soil temperatures at 2 inches among crop reporting districts, overall ranged from 28 to 53 degrees F (Figure 1). For example, in the northeast region, soil temperatures ranged from 40 to 42 degrees F; while in the southwest region, soil temperatures varied from 58 to 48 degrees F (Figure 1). Soil temperatures were around 42-45 degrees F for the northwest region.
Differences in soil temperature were primarily related to the large variations in air temperatures experienced last week, from 33 degrees F in northern portions of the state to 50 degrees F for areas in southeast Kansas (Figure 2).
Projections for the coming weeks are for increasing air temperatures – warmer-than-normal for southern Kansas, which can increase soil temperatures (Figure 3). The actual change in soil temperatures in any given field will be affected by amount of residue cover, amount of 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.
Current moisture status across Kansas is quite dry, with the largest weekly departure in precipitation in the northeast corner (Figure 4). Projections for coming weeks are for precipitation to be above-normal for the eastern parts of Kansas and drier for the west (Figure 5), which can also slow down soil warming conditions and potential plans for an early start to planting.
Optimal soil temperature for 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.
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. Stay tuned!
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library