SOILS AND FERTILIZERS 515 



others where slaked lime is employed, it is important that it be 

 harrowed into the soil immediately after spreading. In no case 

 should it be exposed long to the air before harrowing, as it is liable 

 to cake and form a sort of mortar to such an extent that it is impos- 

 sible to mix it as thoroughly with the soil as before. 



FORMS OF LIME USED FOE AGRICULTURAL PURPOSES. 



Caustic (quick or burnt) lime obtained by burning oyster shells, 

 limestone, etc., is the most economical form in which lime can be 

 bought, in all cases where the distance of railway transportation or 

 of cartage is great. One hundred pounds of such lime usually con- 

 tain about 95 pounds of actual lime. Sometimes burned lime con- 

 tains considerable magnesia. 



Gypsum, or land plaster, is a combination of lime with sul- 

 phuric acid (oil of vitriol) and water. Upoi? heating, gypsum losea 

 its water and is changed into plaster of paris or calcined plaster, 

 which is used in making casts and for many other industrial 

 purposes. 



In case a soil is seriously deficient in lime, gypsum may act as 

 a direct manure ; usually, however, its beneficial effect upon soils is 

 attributed to its indirect action in liberating potash, and possibly 

 other substances, which were locked up in the soil in such combina- 

 tions that plants could not make use of them. Gypsum may be 

 helpful to a limited extent on clayey soils by flocculating the fine 

 particles, on account of which the soil is less likely to become water- 

 logged and to cake, and hence interfere with the operations of till- 

 age. In the last-mentioned respect water-slaked lime or the car- 

 bonate is said to be much more efficacious than gypsum, though as a 

 liberator of potash gypsum is claimed to lead. 



It is stated on good authority that, in the presence of decaying 

 organic matter, gypsum may be changed into carbonate of lime. 

 While this may be true under certain circumstances, in experiments 

 at the Rhode Island Station on a soil exceptionally rich in humus 

 and containing a moderate amount of plant residues which were 

 undergoing decomposition, such a change did not result, if at all, to 

 a practical extent. For this reason and on account of the fact that 

 gypsum contains only about one-third as much lime as burned lime, 

 and usually costs as much or more per ton, it can not take the place 

 of the latter for most of the purposes for which lime is applied to 

 land. 



For use in renovating black alkali (sodium carbonate) soils in 

 the arid regions, gypsum, as already explained, performs a valuable 

 function which cannot be filled by any of the other compounds of 

 lime. Chalk is a naturally occurring form of carbonate of lime 

 which is exceptionally pure. It is quite soft, and is frequently re- 

 ferred to as marl. 



Marl is a name which is applied to earthy deposits usually more 

 or less friable in their character and containing carbonate of lime in 

 quantities ranging usually from 5 to 95 pounds per 100 pounds of 

 me material. It must be evident, therefore, that if one intends to 

 make use of a given deposit of marl for the lime contained in it, he 



