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Common planting advice, when starting many plants from seed, is to bury the seed some small distance under the soil. It seems like most plants (at least members of class Dicotyledonae) will grow a root, lift the seed casing out above the surface, then the cotyledons push off the seed coat.

My question then is: why is it recommended to put the seeds below ground at all? It would seem that dirt only serves to interfere with the first stages of growth.

I can guess at a few reasons:

  • To keep moisture around the seed
  • To make sure the root hits dirt no matter which way the seed is oriented
  • To prevent predators from eating the seed
  • To keep the seed in place

Absent these reasons, is there any reason to have soil sitting on top of a seed?

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Excellent question!

The main reason why us humans bury seeds seems to be to keep seed-eaters from finding them, though any historical evidence that could conclusively prove why we started doing that is probably lost to time, since we discovered agriculture before we began writing things down. Beyond that, you've rightly identified a number of advantages to burying seeds.

First, it may be helpful to note that no physical substrate is even necessary for plant nutrition. We can actually grow plants in mid-air, but for the fact that we would still need something to keep them still. Seeds will germinate if you simply spray nutrients at them - see aeroponics. This is because plants take in what would be soil nutrients via cation exchange and diffusion, as opposed to more a more active process like phagocytosis. By surrounding the seed with nutrient rich substrate, we inadvertently improved germination by exposing more of their surface area to nutrients.

As far as soil seeking behavior goes, Thomas Knight demonstrated that plants can actually find the ground by following gravity. He did this by sticking seeds onto a rotating disc to simulate gravity in another direction and germinating them there. The seeds responded by growing roots away from the center and sprouting stalk towards the center.

Of course, burying seeds too deep means that they may run out of energy before they can break the surface. However, it seems that many varieties of seed have a better chance of germinating in darkness.

Resources

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    $\begingroup$ Ahhh I hadn't thought about light coming into play $\endgroup$ – John Walthour Aug 15 '15 at 23:09

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