A risk associated with slow germination and emergence: Crusted soil

Soil crusting. 

The longer the delay between planting and seedling emergence, particularly in soils with poor structure, the greater the chance that weather can interfere with emergence.  An unexpected rain event followed by warm temperatures that quickly dry soils can cause soil to form a crust.  Crusted soil creates a barrier that is more difficult for emerging seedlings to penetrate.  For soybean this can result in plant death if emerging stems break below the emerging cotyledons (seed leaves, Figure).  Crusting can also lead to uneven emergence.

Figure. Blue arrows are pointing to soybean cotyledons or 'seed leaves'. If emerging seedlings break off below the cotyledons the plant is unable to recover.

Uneven emergence in corn.

Figure. Crusted soil can lead to uneven corn emergence.

It was long-ago established in corn that yield loss can occur when some plants emerge significantly later than their neighbors. Those plants that emerge earlier are often taller, have larger root system and more photosynthetic capacity due to their later stage of development. Later emerging plants must compete with those that have already emerged for finite sunlight, moisture and nutrient resources and are often barren.

Researchers in Illinois and Wisconsin compared yield of corn rows planted and emerging uniformly with that of corn rows planted and emerging at staggered time periods (Nafziger et al. 1991) and found that yields were between 6 and 9 percent lower or between 10 to 22 percent lower when some plants emerged 1.5 weeks later or 3 week later than their neighbors, respectively. Yield losses associated with 50 to 75 percent of plants emerging 3 weeks later than the rest of the stand were actually greater than just holding off planting until 3 weeks later (Carter et al 1989).

According to Purdue University corn agronomist Dr. Bob Nielsen, drops in soil temperature below 45 to 50 degrees can also "decrease the uniformity of germination (1991). 

Uneven emergence in soybean. 

Figure. Crusted soil leads to uneven soybean emergence.

More recently researchers have investigated the effects of uneven emergence on soybean yield (Masino et al. 2018).  Experiments were designed that compared soybean yields in plots in which plants within a row emerged at a uniform time to plots in which seed for some of the plants within the row was sown when the first trifoliate leaves of neighboring plants had already developed.  The plots in which plants emerged uniformly yielded between 7 and 8 percent more than plots in which plants emerged non-uniformly.  The yield reductions associated with uneven emergence were attributed to variability in plant growth rates and fewer resources going into setting seed in those plants with a faster growth rate.

References.

Carter, P.R., Nafziger, E.D. and Lauer, J.G. 1989. Uneven emergence in corn. NCR-344.

Masino, A., Rugeroni, P., Borras, L. and Rotundo, J.L. 2018. Spatial and temporal plant-to-plant variability effects on soybean yield. European Journal of Agronomy. 98:14-24.

Nafziger, E.D., Carter, P.R. and Graham, E.E. 1991. Response of corn to uneven emergence. Crop Science. 31:811-815.

Nielsen, R.L. 1991. Stand establishment variability in corn. Agronomy Dept. pub No AGRY-91-01. Purdue University.