section_path: Mechanism
section_name: Mechanism
paragraph_text: Mechanism
  
  Fertilizers enhance the growth of plants. This goal is met in two ways, the traditional one being additives that provide nutrients. The second mode by which some fertilizers act is to enhance the effectiveness of the soil by modifying its water retention and aeration. This article, like many on fertilizers, emphasizes the nutritional aspect.Fertilizers typically provide, in varying proportions:
  
  The nutrients required for healthy plant life are classified according to the elements, but the elements are not used as fertilizers. Instead compounds containing these elements are the basis of fertilizers. The macro-nutrients are consumed in larger quantities and are present in plant tissue in quantities from 0.15% to 6.0% on a dry matter (DM) (0% moisture) basis. Plants are made up of four main elements: hydrogen, oxygen, carbon, and nitrogen. Carbon, hydrogen and oxygen are widely available respectively in carbon dioxide and in water. Although nitrogen makes up most of the atmosphere, it is in a form that is unavailable to plants. Nitrogen is the most important fertilizer since nitrogen is present in proteins (amide bond between amino-acids), DNA (puric and pyrimidic bases) and other components (e.g., tetrapyrrolic heme in chlorophyll). To be nutritious to plants, nitrogen must be made available in a "fixed" form. Only some bacteria and their host plants (notably legumes) can fix atmospheric nitrogen (N) by converting it to ammonia (NH). Phosphate (PO 3− 4) is required for the production of DNA (genetic code) and ATP, the main energy carrier in cells, as well as certain lipids (phospholipids, the main components of the lipidic double layer of the cell membranes).