Any substance that is added to soil in order to increase its productivity. Fertilizers can be of natural origin, such as composts, or they can be made up of synthetic chemicals, particularly nitrates and phosphates. Synthetic fertilizers can increase crop yields dramatically, but when leached from the soil by rain, which runs into lakes, they also increase the process of eutrophication (see algal bloom; eutrophic). The inoculation of soil with beneficial bacteria, e.g. nitrogen-fixing bacteria, has potential to increase its fertility. For example, in tropical countries Azolla ferns, with their endosymbiotic nitrogen-fixing cyanobacteria (Anabaena sp.), are used to increase the fertility of rice paddies.