Sustainable Soil Management
Glossary
H
HARDPAN. A dense, hard, or compacted layer in soil that slows water percolation and movement of air and obstructs root growth. Pans may be caused by compaction, clay, or chemical cementation.
HIGHLY ERODIBLE SOIL. A soil-mapping unit with an erodibility index of 8 or more.
HISTOSOLS. An order in Soil Taxonomy. Soils formed from materials high in organic matter. Histosols with essentially no clay must have at least 20 percent organic matter by weight (about 78% by volume). This minimum organic matter content rises with increasing clay content to 30 percent (85% by volume) in soils with at least 60 percent clay.
HORIZON, SOIL. A horizontal layer of soil, created by soil-forming processes, that differs in physical or chemical properties from adjacent layers.
HUMUS. Highly decomposed organic matter that is dark-colored and highly colloidal.
HYDRAULIC CONDUCTIVITY. An expression of the readiness with which a liquid, such as water, flows through a solid, such as soil, in response to a given potential gradient.
HYDROLOGIC SOIL. Movement of water in and on the earth and atmosphere through processes such as precipitation, evaporation, runoff, and infiltration.
HYGROSCOPIC WATER. Water held tightly by adhesion to soil particles. Cannot be used by plants and remains in soil after air-drying. Can be driven off by heating.
