LIMING THE SOIL 363 



tion with calcium. Calcium is employed because it is not only 

 effective with all types of acidity but because it is compara- 

 tively cheap and plentiful. Potassium in active form is too 

 expensive, sodium is likely to generate harmful compounds 

 in the soil, while magnesium in large amounts is sometimes 

 harmful. Calcium compounds may be applied in excess and 

 yet no harmful effects on plant growth are ordinarily likely 

 to result.^ 



197. Forms of lime.— The term lime correctly used re- 

 fers only to calcium oxide (CaO). In a popular and agri- 

 cultural sense the scope of the word has been broadened to 

 include all of the commercial compounds of calcium and mag- 

 nesium commonly applied to the soil to correct the so-called 

 acidity. The term in its agricultural sense refers to the fol- 

 lowing compounds either alone or in mixture : calcium oxide 

 (CaO), magnesium oxide (MgO), calcium hydroxide (Ca- 

 (011)2), magnesium hydroxide (Mg(0H)2), calcium car- 

 bonate (CaCOa), and magnesium carbonate (MgCOs). Such 

 compounds as gypsum (CaS04.2H20), mono-calcium phos- 

 phate (0^114(1*04^)2), and calcium silicate (Ca^SiO^), insofar 

 as they are carriers of calcium, also might be spoken of as lime. 



As might be expected, liming materials do not appear on the 

 market as single compounds of magnesium or calcium, nor 

 are they by any means pure. The better grades of the oxides 

 and hydroxides are generally used in the trades, the more im- 

 pure materials having an outlet as agricultural lime. The car- 

 bonated forms of lime have a number of different sources and 

 vary to a marked degree in purity. Lime, in whatever form 

 it may appear on the market, almost always carries magnesium 

 as well as calcium, the latter usually predominating. 



Three general groups of lime, as it is commercially handled, 



^ Floyd, B. F., Some Cases of Injury to Citrus Trees Apparently 

 Induced hy Ground Limestone; Fla. Agr. Exp. Sta., Bui. 137, 1917, 



Wyatt, F. A., Influence of Calcium and Magnesium Compounds on 

 PUnt Growth; Jour. Agr. Bes., Vol. VI, No. 16, pp. 589^619, 1916. 



