Read the following text and answer number 26 – 30

    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 plan obtains carbon and hydrogen dioxide from the atmosphere; other nutrients are taken up from the soil. Althoughthe plant contains sodium, iodine, and cobalt, these are apparently not essential. This is also true of silicon and aluminum.

    Overall chemical analysis 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.

Why do farmers prefer considering the only available nutrients to the total quantities of nutrient found in the soil? Because ...