Calcium – soft grey alkaline earth metal; the fifth most abundant element by mass in the Earth's crust that is essential for living organisms, particularly in cell physiology
Chemical weathering – breakdown of rocks, soils and minerals by atmospheric chemicals or biologically produced chemicals; also known as biological weathering
Clay – soil made of very fine particles, usually silicates of aluminum and/or iron and magnesium; absorbs water slowly and retains it for a long time
Erosion – the natural process by which the surface of the earth is worn away by the action of water, glaciers, winds, waves, etc
Horizon (A, B, C, R) – specific layer in the soil which measures parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath; horizon formation is a function of a range of geological, chemical and biological processes and occurs over long time periods
Humus – dark organic part of soil formed from decaying plants and animals, often called topsoil, which improves the soil’s fertility and water-holding ability
Legume – plant in the pea family (Fabaceae or Leguminosae) or a fruit of these plants; legume fruit usually opens along a seam on two sides; common name for this type of fruit is a "pod,” includes alfalfa, clover, peas, beans, lentils, lupins, mesquite, carob and peanuts
Loam – a rich, fertile soil containing clay, sand, silt and humus
Magnesium – alkaline earth metal that constitutes about 2 percent of the Earth's crust by mass, making it the eighth most abundant element in the crust; a common additive to fertilizers
Mechanical weathering – breakdown of rocks and minerals by frost, wind and tree roots with no chemical alteration
Nitrogen – nonmetallic element that constitutes nearly four-fifths of the air, occurring as a colorless, odorless, almost inert diatomic gas in various minerals and in all proteins and used in important manufactured materials
Percolate – permeate or penetrate gradually
pH scale – measure of a liquid’s or solid’s alkalinity, represented on a scale of 0 to 14 with 7 being a neutral state, 0 the most acidic and 14 the most alkaline
Phosphorous – nonmetallic chemical element that has the symbol P and atomic number 15; normally a white phosphorescent, waxy solid, becoming yellow when exposed to light; poisonous and unites easily with oxygen so that it ignites spontaneously at room temperature
Potassium – light, soft, silver-white metallic element of the alkali metal group; oxidizes rapidly in air and reacts violently with water
Rhizobium – nitrogen-fixing bacteria of the genus Rhizobium that form nodules on the roots of leguminous plants, such as clover and beans
Sandy soil – soil with large particles that drain quickly and hold nutrients poorly; has a gritty texture formed from weathered rocks such as limestone, quartz, granite and shale
Sulfur – pale yellow, nonmetallic element occurring widely in nature in several free and combined allotropic forms; used in black gunpowder, rubber vulcanization, the manufacture of insecticides and pharmaceuticals and the preparation of sulfur compounds such as hydrogen sulfide and sulfuric acid