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 dates. After a very warm start to March, air temperatures across Kansas declined this past week.
For the week of March 8-14, average weekly soil temperatures at 2 inches varied greatly among crop reporting districts, overall ranging from 39 to 53 degrees F (Fig. 1). For example, in the NE region, soil temperatures ranged from 45 to 46 F; while in the SW region, soil temperatures varied from 46 to 49 F, and even a bit warmer in a small area in the southernmost portion of the SC district (Fig. 1). Soil temperatures were below 45 F for the NW region.
Differences in soil temperature were primarily related to the large variations in air temperatures, from 33 F north of the state to 50 F for the SE portion of Kansas, experienced last week (Fig. 2).
Projections for the coming weeks are for increasing air temperatures – warmer than normal for the southern part of the state, which can increase soil temperatures (Fig. 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 vary more rapidly, matching air temperatures.
Current moisture status across the state is quite dry, with the largest weekly departure in precipitation in the NE corner of the state (Fig. 4). Projections for coming weeks are for precipitation to be above normal for the entire state (Fig. 5), which can also slow down soil warming conditions and potential plans for an early start to planting.
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 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.
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 (Fig. 6). The largest variability is from SE to NW Kansas; with the earliest last spring freeze date for the SE region (April 5-15) and latest for the NW area (>May 3). Corn planting dates before April 15 in the SE region would increase the likelihood of the crop suffering from a late spring freeze. Similar conditions can be projected for NW Kansas if corn is planted before May 3.
Think about all these factors when deciding on the optimal planting time. More information about 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