28 SCIENCE AND SOIL 



Limestone is a constituent of good soils, and if limestone is not 

 present in a soil, then it should be applied; for, as already explained, 

 carbonic acid is the weakest of all acids, and, consequently, if the 

 soil contains limestone, it cannot be an acid soil, because the soil 

 acids will take the base away from calcium carbonate (and also 

 from any other carbonate), and the liberated carbonic acid is 

 broken up into water and carbon dioxid. 



Magnesium carbonate and iron carbonate are found in rock 

 deposits; while potassium carbonate (K 2 COg) is the lye (alkali) 

 obtained from wood ashes, and sodium carbonate (Na 2 COg) is the 

 most harmful alkali in alkali soils. The carbonic acid is so weak 

 that the carbonates of the strongest bases (potassium and sodium) 

 are almost as basic (alkaline) as the hydroxids of the same ele- 

 ments (KOH and NaOH). 



The term hydrate used in the name of a chemical compound 

 means that it contains water combined with some other constitu- 

 ent, or that hydrogen and oxygen are present in the same propor- 

 tion as in water (H 2 O). Thus, carbohydrates contain carbon and 

 water. This very important group of carbon compounds, includ- 

 ing sugar, starch, and cellulose (wood fiber) will be explained after 

 hydrogen has been discussed. 



Hydrogen. Hydrogen is the third most important element in 

 plants, constituting about 6.4 per cent of the corn kernel. Water 

 is the only abundant source of hydrogen, although the element is 

 found in the earth's crust in appreciable amount, chiefly in hy- 

 drated mineral compounds containing, as the name indicates, 

 water in combination with salts. In some cases the combined 

 water corresponds to the amount that might be held in hydroxid 

 form; as, for example, in the abundant mineral called gypsum, 

 or land plaster, which is calcium sulfate crystallized with two 

 molecules of water, CaSO 4 : 2 H 2 O, or CaO 2 S(OH) 4 . 



Hydrogen in the free state (H 2 ) is a gas. From Table i it will be 

 seen that the molecule of hydrogen (H^ is lighter than the atom of 

 any other element; and, according to the gas law, a given volume 

 will be filled by the same number of molecules of hydrogen as of 

 any other gas. Consequently, hydrogen gas is the lightest of all 

 known gases, so that a balloon filled with hydrogen easily floats 

 in the atmosphere of nitrogen (N.j) and oxygen (O 2 ), one of which 



