A few weeks ago a CropWatch article discussed chilling injury in soybean. To follow up on that discussion, we have been tracking air and soil temperatures, and germination in no-till and strip-till fields planted just prior to the May 1 cold spell. (These areas are part of a larger UNL research project being funded by the Nebraska Soybean Board.) 

To briefly recap the prior CropWatch article on chilling injury in soybean, this injury occurs when the soybean seed is planted into soils that are at or below 40°F. The chilling injury can be particularly severe if the soil temperature is well below 40°F, and especially if low moisture content seeds (i.e., less than 13%) are put into cold ground. The potential for chilling injury substantively increases when seed moisture is low. When planting into cold soils, if the moisture content of your seed is low (much below 13%, say as low as 7%), it may be a good idea to enhance the seed moisture content.

Chilling injury occurs only during the first (imbibitional) phase of water uptake by soybean seed. Imbibitional water uptake is very rapid during the first few hours when water is needed to hydrate cell membranes. This phase is usually completed within 24 hours, provided that the soil zone into which the soybean seed was placed was sufficiently moist for imbibitional water uptake to begin immediately. Obviously, seed planted into a dry soil zone would not be able to take up any imbibitional water until the next rainfall.

The second (osmotic) phase of water uptake begins as soon as the cell membranes in the seed are hydrated. Cold soil temperature is not a problem during this second stage because the probability of chilling injury goes to near-zero. However, while cold temperatures during this stage do not cause direct injury, they do slow germination to the extent that seedling emergence (defined as cotyledons thrust to a position above the soil surface – VE stage) will be delayed. And, when cold soil temperature is coupled with protracted soggy soil moisture conditions during this phase, significant losses of germinating seeds and seedlings can occur due to soil pathogen infection. This is why fungicide-treated seed is always recommended for very early soybean planting.

Temperature Data Collected in the Plots

As part of some research we are conducting on behalf of the Nebraska Soybean Board, yield and agronomic performance related to soybean planting depth (eight depths from 1.0 to 2.75 inches) and seeding rate (30,000 to 180,000 seed per acre) are being evaluated in no-till and strip-till conditions (Figures 3 and 4). This experiment was planted between 2 and 5 p.m. on April 29, 2013 (as per the recommendation in a previous CropWatch article to “plant soybean early for higher yield.") The soil temperatures were warm (>50°F) at planting in both no-till and strip-till sections, and the seed was placed into moist soil in both cases. (Remember that soybean seed requires 50% more water to germinate than does corn seed.)

The soil temperatures remained warm (above 50°F) for at least 36 hours after planting. Because of an interest in soil temperatures at the various planting depths in no-till vs. strip-till, we placed soil temperature probes at each sowing depth and measured the air temperatures at 6 inches above the soil surface. Due to time constraints, the temperature probes (and data loggers for these) were not installed until about 6 p.m. on April 30. Then, about 6 p.m. May 1, came the very cold rain, followed by snow later that evening. (Figure 2 shows a snowy field on May 2 just a few miles away from the experiment site.)

In Figure 1 the lines show air temperature at 6 inches above the soil surface and at 2 inches below the soil surface in the no-till and strip-till sections of the experiment. The temperatures were measured at 30-minute intervals starting about 24 hours after planting (green diamond in figure). There was little difference in soil temperatures measured at the various sowing depths of 1.0 to 2.75 inches within either the no-till or strip-till sections. To avoid cluttering the figures with eight lines, only the 2-inch soil depth temperatures are presented.

Note that on the night of May 1, about 2 days (48 hours) after planting, the air temperature plunged to 32°F and stayed there until dawn on May 2. (This is the first time I have ever had snowfall on a newly planted soybean field.) This plunge occurred again the next night. Then, about 10 days later on the night of May 11, the air temperature plunged again to 32°F and stayed there until dawn on May 12. The soil temperatures those same evenings, of course, remained above freezing because soils with a high water content (such as after a rain) will not freeze unless air temperatures plunge below 32°F and stay at levels much below freezing for more than 24 hours. (Soil temperatures don’t drop as quickly as air temperatures due to latent energy exchange requirements for water to transform from a liquid to a solid [ice] state.)

The 24-hour fluctuation in soil temperature indicated that the nighttime low temperatures (maintained until dawn of the next day) were slightly colder for the strip-till section than for the no-till section. Apparently, the surface corn residue in the no-till section provided some buffering to the nighttime drop in temperature. In contrast, the strip-till section exhibited substantively warmer soil temperature peaks than the no-till section during the solar day. The daily temperature differential, which peaked after solar noon, resulted in an accumulation every day of warmer daily growing-degree minutes for the strip-till section. As a result, the growing seedlings were more advanced in the strip-till section than in the no-till section. This is evident in the photo of three seedlings (Figure 5) collected at 6 p.m. on Friday May 10 from each section of the study. The three on the left are from the strip-till plots. (Note that all seeds were fungicide-treated before planting, thus the green seed coats.) On Wednesday evening May 15, I stopped by the field and observed seedlings in the strip-till section pushing up the soil layer just above them. In the no-till section on the same date seedlings were not yet visible and there was no evidence of soil bulging in that section.

We will continue to monitor the emergence in this experiment during the next few days. Our plan is to get counts of emerged seedlings per foot of row in the various experimental seeding depths and rates. This information will be reported in CropWatch later this spring.

James Specht
Professor, Department of Agronomy and Horticulture