Soil aeration is caused by tree roots grazing animals fertilizers earthworms

earthworms

Soil aeration is a phenomena that is related to the pores present in the soil, these pores allows the exchange of gases by the roots. Roots acquire oxygen from the soil pores from the atmosphere and released out carbon dioxide from the roots leaves the soil from these pores. The earthworms and other insects plays an important role in increasing the soil aeration as these worms are decomposers which feed upon the dead organic matter present in the soil these worms creates bores inside the soil in search of food. The bores helps in exchange of gases within the soil and the external environment.  

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im not intirely sure but i think the anwser is d relative humidity