crops require about half, as 20–25 kg per hectare. Yara International is the world's largest producer of nitrogen-based fertilizers. 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, emphasises the nutritional aspect. Fertilizers typically provide, in varying proportions: three main macronutrients: Nitrogen (N): leaf growth Phosphorus (P): development of roots, flowers, seeds, fruit; Potassium (K): strong stem growth, movement of water in plants, promotion of flowering and fruiting; three secondary macronutrients: calcium (Ca), magnesium (Mg), and sulfur (S); micronutrients: copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), boron (B). Of occasional significance are silicon (Si), cobalt (Co), and vanadium (V). 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 as water and carbon dioxide. 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, DNA and other components (e.g., 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 (N2) by converting it to ammonia. Phosphate is required for the production of DNA and ATP, the main energy carrier in cells, as well as certain lipids. Microbiological considerations Two sets of enzymatic reactions are highly relevant to the efficiency of nitrogen-based fertilizers. Urease The first is the hydrolysis (reaction with water) of urea. Many soil bacteria possess the enzyme urease, which catalyzes conversion of urea to ammonium ion (NH4+) and bicarbonate ion (HCO3−). Ammonia oxidation Ammonia-oxidizing bacteria (AOB), such as species of Nitrosomonas, oxidize ammonia to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria, especially Nitrobacter, oxidize nitrite to nitrate, which is extremely mobile and is a major cause of