as fields need to be flooded or drained at different stages of the crop cycle. The Netherlands has also led the way in this type of drainage, not only to drain lowland along the shore, but actually pushing back the sea until the original nation has been greatly enlarged. In moist climates, soils may be adequate for cropping with the exception that they become waterlogged for brief periods each year, from snow melt or from heavy rains. Soils that are predominantly clay will pass water very slowly downward, meanwhile plant roots suffocate because the excessive water around the roots eliminates air movement through the soil. Other soils may have an impervious layer of mineralized soil, called a hardpan or relatively impervious rock layers may underlie shallow soils. Drainage is especially important in tree fruit production. Soils that are otherwise excellent may be waterlogged for a week of the year, which is sufficient to kill fruit trees and cost the productivity of the land until replacements can be established. In each of these cases appropriate drainage carries off temporary flushes of water to prevent damage to annual or perennial crops. Drier areas are often farmed by irrigation, and one would not consider drainage necessary. However, irrigation water always contains minerals and salts, which can be concentrated to toxic levels by evapotranspiration. Irrigated land may need periodic flushes with excessive irrigation water and drainage to control soil salinity. See also Bar ditch Building construction Deep drainage Drain commissioner Drain (plumbing) Drainage basin or watershed Drainage by wells Drainage divide or watershed Drainage equation Drainage research Drainage system (agriculture) Drainage system (geomorphology) Geomorphology Hydrologic Evaluation of Landfill Performance (HELP) Hydrology John Johnston, who introduced land drainage to the United States. Plumbing Potable cold and hot water supply Septic systems Sewage traps, drains, and vents Rain gutter Retaining wall Sewage collection and disposal Soil salinity control by subsurface drainage Tile drainage Trench drain Trencher (machine) Urban exploration Watertable control References External links Water management