Small seeds, which often have limited energy reserves in their embryos, require light for several reasons:
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Germination Trigger: Some small seeds are photoblastic, meaning that exposure to light is a crucial factor for initiating their germination process. Light acts as a signal that conditions are favorable for growth, indicating that the seed is at or near the soil surface where it can access light for photosynthesis once it starts to grow.
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Energy Needs: Small seeds typically have insufficient energy reserves to support a long period of growth before they can begin photosynthesis. If they are planted too deep in the soil, they may exhaust their limited reserves before they reach the surface to access light.
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Optimal Growth Environment: Shallow planting ensures that the seedlings can emerge quickly and access the light they need. This quick emergence is critical because small seedlings are often at a competitive disadvantage against larger plants. Access to light allows them to begin photosynthesizing and growing rapidly.
In summary, small seeds should not be planted deep in the soil because they rely on light for germination and growth. Planting them too deep can prevent them from emerging successfully, leading to poor germination rates and reduced chances of survival.