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. After a very warm start to the month, there has been a declining trend in air temperature across Kansas in the last two weeks of March.
For the week of March 19-24, average weekly soil temperatures at 4 inches varied greatly among crop reporting districts in Kansas, overall ranging from 45 to 55 F (Fig. 1). For example, in the NE region, soil temperatures ranged from 50 to 55F; while in the SW region, those temperatures varied from 47 to 50 F. Soil temperatures at 4 inches were below 50 F in most of Kansas. Minimum soil temperatures were below 40 F for the NW region (Fig. 2).
The range between minimum and maximum soil temperatures during the past week varied from 10 (Central) to 30 F (Northeast). These differences were primarily related to the large variations in air temperatures experienced last week (Fig. 3). 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 temperatures around the seed zone can produce non-uniform crop germination and emergence. Lack of uniformity in emergence can greatly impact corn potential yields.
Overall average day for last spring freeze (32 F) is quite variable around the state (Fig. 4). 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 3-8). 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 8.
Low temperatures at planting can greatly impact the final number of plants through non-uniform emergence and early growth, consequently reducing 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 the soil surface.
Think about all these factors when deciding on 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 upcoming issues of the Agronomy eUpdate. Stay tuned!
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Mary Knapp, Weather Data Library