The process by which chemicals are dissolved and transported through the soil by percolating water. Pesticides and nutrients from fertilizers or manures may leach from fields, areas of spills, or feedlots and thereby enter surface water, groundwater, or soil. Leaching from concentrated sources such as waste sites and loading areas vulnerable to spills can be prevented by paving or containment with a liner of relatively impermeable material designed to keep leachate inside a treatment pond, landfill, or a tailings disposal area. Liner materials include plastic and dense clay. Sometimes referred to as nitrogen-fixing plants, they can convert nitrogen from the air to build up nitrogen in the soil.
Legumes are an important rotation crop because of their nitrogen-fixing property.
The 'variable import levy' of the European Community is an example. The EC's levy on grains varied from day to day, depending on the offering price of third-country suppliers. In USDA's view the variable import levy is a nontariff trade barrier because, unlike a moderate customs duty or even a quota, it can completely bar imports. The Uruguay Round Agreement on Agriculture resulted in the replacement of variable levies by fixed tariffs. The short fibers that remain on cottonseed after ginning.