How the corn and soybean crops (and soil-applied herbicides) have responded to dry soil conditions in Crookston

Dry conditions: How dry has it been?

Something feels wrong about complaining about the dry soil conditions this spring, particularly as this allowed folks in northwest Minnesota to finish planting much more quickly than our neighbors to the south. Therefore only observations follow below.

Corn and soybean plots were planted on May 21 into very dry soils at the Northwest Research and Outreach Center (NWROC) in Crookston. Only 1.38 inches of precipitation fell in the 60 days prior to planting. This was 2.01 less than the 30-year average of 3.39 inches (Figure). Since planting an additional 1.9 inches of rain, or 1.58 inches less than the 30-year average, have fallen (data: NWROC). Fortunately more is forecast in the near-term (NWS-NOAA).

What happens to corn and soybean seedlings trying to emerge in dry and crusted soils?

An article titled “A risk associated with slow germination and emergence: crusted soil” posted on April 26 detailed how, particularly in soils with poor structure, slow germination and poorly timed rains can result in crusting, uneven emergence or even plant death.

While a drive-by look of the plots yesterday wouldn’t cause one to slam on the breaks, a closer is more revealing and troubling. There are large gaps within some of the soybean rows (Figure 1).

Figure 1. Large gaps with no plants in soybean rows (blue arrows).

Soybean stand counts revealed that per acre plant populations ranged from 98,000 to 156,000, 7 to 42% fewer plants than the 168,000 seeds that were planted. What was revealed when digging within the row to determine what caused these gaps were plants with largely healthy taproots, hypocotyls and cotyledons. However the problem is that the hypocotyls had broken in half and are therefore unable to uncurl to pull the cotyledons (seed leaves) out from below ground (Figure 2). Unfortunately, the growing point of these very young soybean seedlings is located between the cotyledons and so seedlings that have broken at the hypocotyl will develop no further and will never emerge from the soil. A dry crusty layer of soil approximately 1/8 to 3/16 of an inch thick, below which soil was more moist, was observed when digging seedlings. Dry soil conditions at planting followed by 1.25 inches of rain in the three days afterward likely crusted soil leading to this seedling loss.

Figure 2. Digging within the gaps to determine the fate of seeds/seedlings revealed healthy root tissue (red arrow) and a broken hypocotyl (blue arrow).

Although fewer and less shocking than in the soybean plots, there are also some gaps in corn plots (Figure 3, blue arrow). Corn stand counts ranged between 31,300 and 35,000 or up to 10% less than the 35,000 seeds that were planted. While the majority of the corn plants appeared to be healthy and developing normally, some are growing in an odd way (Figure 3, red arrow). These plants have leaves that are more or less tightly coiled and not unfurled and spread out like they would on a normally developing plant.

Figure 3. Gap in a row of corn (blue arrow) and a corn plant that's leaves have not unfurled properly (red arrow).

Digging these seedlings up to look at the roots reveals healthy root and mesocotyl tissue, or the tissue between the seed and the soil line (Figure 4). My working hypothesis is that cracks in the crusted soil exposed the growing coleoptile (outer protective sheath) and leaves to sunlight, but that the crust was thick enough to significantly slow seedling emergence that would have allowed leaves to develop and unfurl from within the whorl normally.

Figure 4. Corn seedling that had abnormal leaves (blue arrow) had a normal mesocotyl and root tissue (red arrow).

Have you had weed seedlings emerge despite using a soil-applied, residual herbicide? 

One danger of planting into very dry soils is that weed control can be less than ideal if a soil-applied, residual herbicide is applied pre-emergence and there is no rain that follows to activate it. According to an article written by Purdue University weed scientists Drs. Bill Johnson and Joe Ikley (Dr. Ikley is now at NDSU) these herbicides need to be “dissolved in the soil water (aka “solution”) so it is able to be taken up by the germinating weed seedling root or shoot”. The article (available here) details how much rain is needed to activate individual herbicide active ingredients and premixes, which range between 0.25 and 1 inch. Among other suggestions the weed scientists suggest planning for timely, and perhaps earlier than anticipated, post-emergence herbicide applications or using a rotary hoe to manage emerged weed seedlings while waiting for activation.

Additional information

Additional information about estimating plant populations and the effects of sub-optimal plant populations on yield potential can be found on the UMN Extension Crop Production website: https://extension.umn.edu/crop-production.