Cultivated agriculture involves growing and producing crops through irrigated and nonirrigated farming practices. Improper or excessive application of nutrient sources such as inorganic fertilizer or manure can lead to pollution of ground water and surface water. The two main components of fertilizer that are of greatest concern to source water quality are nitrogen and phosphorous. Pesticides are also applied to crops by aerial spraying, topsoil application, soil injection, soil incorporation, or irrigation. Aerial spraying and topsoil application pose the greatest risks for surface water contamination from runoff. Soil injection and incorporation pose the greatest likelihood of ground water contamination because the pesticides are subject to leaching. Applying pesticides through irrigation can also cause ground water contamination. Pesticides can also reach ground water through drains, sink holes, and other conduits. Excessive rain or irrigation water can wash pesticides from plants and soils and into streams or other surface water bodies. Pesticides can leach into soil if plants are watered or rainfall occurs soon after application. Some pesticides resist degradation by microbes in the soil and will eventually leach into ground water.