Planting depth is a big decision made each year that has no input cost, but will have a significant impact on final yields. Planting depth decisions need to be fluid. Practical Farm Research (PFR)® multi-year, multi-location data gives us confident starting points; 2.0 inches in corn and 1.5 in. in soybeans. What factors lead to the proper seeding depth?

Uneven soil moisture throughout the seed trench is the primary cause of uneven emergence. To resume active embryo growth, 30% of a corn kernel’s weight and 50% of a soybean kernel’s soybean kernel’s weight in water needs to be absorbed. Be sure that the seed trench moisture is adequate and consistent. Placing seed deeper may be needed to achieve this. SmartFirmer® from Precision Planting® is a tool that identifies the soil moisture on the fly, allowing you to make depth decisions without leaving the cab. If soil moisture is below the target of 30%, planting depth adjustment is needed.

Temperatures in the top 1 in. of the soil can fluctuate throughout the day, but deeper soil temperatures tend to stay consistent. Cold injury to seedlings often occurs after germination starts, but before emergence happens. Kernels placed in the fluctuating soils are more at risk of these injuries than those placed deeper, even if soil is cooler in the depths. A temperature of 50°F or greater is needed for germination to begin.

Crop development is influenced by planting depth and is the most important reason to plant at the proper depth. In corn, a planting depth of 2 in. or deeper allows the nodal roots to develop below the soil surface. Nodal roots are vital for structural support and are responsible for most of the water and nutrients the plant needs. Five sets of nodal roots are optimal for maximizing potential, and 2 in. or deeper enables proper development. In soybeans, nodulation occurs at or below planting depth. Nodules and the role they play in symbiotic relationships with soil biology are most efficient at consistent temperatures around 72°F. A planting depth of 1.5 in. or deeper places future nodules at the depth where soil biology is most active.

Soil conditions like soil being too fluffy, heavy residue, or hard surface crusting impact the performance of our equipment designed to place seed consistently. Fluffy soils causing gauge wheels to drag and row cleaners set too aggressively are the two most common soil condition problems I observe. There are many considerations pertaining to soil type, tillage practice, downforce settings, and more. Set the depth needed for optimal equipment performance.

Weather events such as cold rains or even snow can change the soil temperatures or cause soils to crust. Heavy rains or mechanical rolling can compress the topsoil, changing the depth in which the seed is in relation to the surface. In corn, this prevents proper nodal root development and in soybeans, it places nodulation closer to the surface where temperatures exceed optimal biological activity. Finally, cultivating or hilling can also affect the depth in which these crucial plant developments take place.

When making this very important, zero-input cost decision this coming spring, lean on these five factors and our PFR data of 2.0 in. or deeper in corn and 1.5 in. or deeper in soybeans. Equip yourself with the technology to assist this, or be sure to stop at least every planter fill to identify if a change is needed. For a deep dive presentation into “A Fresh Look at Planting Depth,” watch our 2021 Virtual Field Show presentation on our YouTube channel.