<P> These elements stay beneath soil as salt . So plants consume these elements as ion . The macronutrients are consumed in larger quantities; hydrogen, oxygen, nitrogen and carbon contribute to over 95% of a plants' entire biomass on a dry matter weight basis . Micronutrients are present in plant tissue in quantities measured in parts per million, ranging from 0.1 to 200 ppm, or less than 0.02% dry weight . </P> <P> Most soil conditions across the world can provide plants adapted to that climate and soil with sufficient nutrition for a complete life cycle, without the addition of nutrients as fertilizer . However, if the soil is cropped it is necessary to artificially modify soil fertility through the addition of fertilizer to promote vigorous growth and increase or sustain yield . This is done because, even with adequate water and light, nutrient deficiency can limit growth and crop yield . </P> <P> Plants take up essential elements from the soil through their roots and from the air (mainly consisting of nitrogen and oxygen) through their leaves . Nutrient uptake in the soil is achieved by cation exchange, wherein root hairs pump hydrogen ions (H) into the soil through proton pumps . These hydrogen ions displace cations attached to negatively charged soil particles so that the cations are available for uptake by the root . In the leaves, stomata open to take in carbon dioxide and expel oxygen . The carbon dioxide molecules are used as the carbon source in photosynthesis . </P> <P> The root, especially the root hair, is the essential organ for the uptake of nutrients . The structure and architecture of the root can alter the rate of nutrient uptake . Nutrient ions are transported to the center of the root, the stele, in order for the nutrients to reach the conducting tissues, xylem and phloem . The Casparian strip, a cell wall outside the stele but within the root, prevents passive flow of water and nutrients, helping to regulate the uptake of nutrients and water . Xylem moves water and mineral ions within the plant and phloem accounts for organic molecule transportation . Water potential plays a key role in a plant's nutrient uptake . If the water potential is more negative within the plant than the surrounding soils, the nutrients will move from the region of higher solute concentration--in the soil--to the area of lower solute concentration - in the plant . </P>

Where do plants obtain the chemicals that they need to sustain life