In total, the plant has need of at least 16 elements, of which the most important are carbon,

hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium.

The plant obtains carbon and hydrogen dioxide from the atmosphere; other nutrients are

taken up from the soil. Although the plant contains sodium, iodine, and cobalt, these are

apparently not essential. This is also true of silicon and aluminum

Overall chemical analyses indicate that the total supply of nutrients in soils is usually high

in comparison with the requirements of crop plants. Much of this potential supply, however, is

bound tightly in forms that are not released to crops fast enough to give satisfactory growth.

Because of this, the farmer is interested in measuring the available nutrient supply as contrasted

to the total quantities.

"The plant obtains

carbon and hydrogen dioxide ..." (paragraph 2). The underlined word means

...