Oxygen (O), 15.99 Atomic Weight
Oxygen is a major element in plant growth, being the basis for all life on earth as part of the DNA molecule. It is the most plentiful element in the earth’s crust, representing 46 - 49%. It is the second most prevalent element in the earth’s atmosphere at 21%, and in sea water, it makes up 89% by weight. Approximately two thirds of the human body is Oxygen.
Oxygen is very reactive and capable of combining with most of the other elements. It is a component of thousands of organic compounds, many of which are essential to plant growth.
In what is known as photosynthesis, plants utilize available CO2 in the presence of sunlight, from both the soil and the atmosphere, assimilating the Carbon element and releasing the free Oxygen back into the atmosphere. Almost all free Oxygen in the earth's atmosphere is due to photosynthesis. See “Soil Air” for more information.