Sentence:  Greasy soils are hardly removed by washing in plain water ; and natural waters , in addition , often contain impurities such as <WSD>calcium</WSD> salts which can react with soils to make them more difficult to remove . 
Meaning of the word marked with <WSD> tags:  a white metallic element that burns with a brilliant light; the fifth most abundant element in the earth's crust; an important component of most plants and animals