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Seeding depth - from a seed's perspective

By Dr. Sheri Strydhorst, Agronomy Research Specialist 

Germinating wheat seeds with first roots and coleoptile that protects the young emerging first shoot. 

As we rush into the busy seeding season, it is important to think about the basics. There are many things farmers can do to properly set seeding depth such as checking openers for plugs and wear, front-to-back and side-to-side leveling of the frame and more. However, it is also helpful to understand seed physiology and how the correct seeding depth can set your crop off to a strong start.

In natural systems, seeds fall on the soil surface and some eventually germinate, but those seeds are subject to predation from birds, mammals or insects, temperature extremes and moisture variation. All farmers know that seeds on the soil surface can grow and produce viable plants as evidenced by volunteer canola, wheat and barley plants. However, placing seeds in the soil at the correct depth will increase uniform crop emergence.

One of the great agricultural innovations of modern agriculture was the invention of the seed drill in 1701 by Jethro Tull. The seed drill placed seeds at a uniform depth in the soil.    

While it can be tempting to seed shallow to speed emergence, shallow sowing is risky. The first step in seed germination is water imbibition and seeds typically imbibe 1-2 times their weight in water. Low water content in the soil will reduce the speed and percent germination. In our prairie environment, low and variable water content often occurs at, or near, the soil surface. Hence, shallow seeding creates an increased risk of delayed and uneven emergence. In dry soils, good seed-soil contact is important to ensure efficient water uptake. Even if there is good soil moisture, when soils are cloddy or fluffy, germination can be poor as there is poor moisture transfer between the soil and seed. This can be especially true in fields where manure application and or tillage create clumpy soil conditions.  

Once the seed has imbibed enough water, the first root (radicle) emerges. Then the seed pushes a delicate, young shoot (plumule) up through the soil to begin photosynthesis. The first shoot (plumule) is protected by the coleoptile. Once the coleoptile breaks through the soil it stops growing and the first shoot elongates and starts photosynthesis. The seed energy reserve in the form of carbohydrates nourishes the growth of the first root and shoot until photosynthesis can provide energy.  

From imbibition to emergence, the respiring seed sees a rapid use and depletion of stored energy reserves. Larger seeds have more energy reserves than smaller seeds which is why a large seeded faba bean can be seeded up to 3” (7.5 cm) deep while a small canola seed should be planted around 1” (2.5 cm) deep. In short, larger seeds have more energy reserves and can be sown deeper than smaller seeds.  

Semi-dwarf wheat varieties have shorter coleoptiles. If the coleoptile is shorter than the depth of planting, the first shoot is often unable to push through the soil surface to start photosynthesis. Plant height and coleoptile length are correlated, with taller varieties having longer coleoptiles. With semi-dwarf varieties of wheat, a shallower planting depth becomes crucial.  Barley also tends to have shorter coleoptiles than wheat. 

The optimum seeding depth for spring cereal crops (wheat, barley and oats) is between 1.5 to 2” (3.75 to 5 cm) deep. The goal is to place the seed deep enough to access adequate moisture for uniform water uptake of all seeds but shallow enough to ensure quick emergence.

References: 

Forbes, J.C. and Watson, R.D. 1992.  Plants in Agriculture. Cambridge University Press. 355 pp. Wiersma, J., Randson, J., and Hofman, V. 2018.  Seeding Depth.  University of Minnesota Extension.  https://extension.umn.edu/planting-small-grains/how-deep-do-i-drill-wheat-barley