138 THE CULTURE OF FARM CROPS. 



it has been going on for ages, in the constant accretion of 

 extensive beds of salt; borax; soda; and other saline sub- 

 stances which have been deposited on the surface, by the 

 long continued evaporation in the dry arid climate, of the 

 water of the soil which has held them in solution. The 

 niter beds of Peru and Chili, which are many feet in thick- 

 ness have been deposited in the same manner ; the vastness 

 of the accumulations showing the great amount of the evap- 

 orations which have been going on. 



The contraction of the soil by drying is a property which 

 is exerted in proportion to the power of absorbing water. 

 Some soils, such as pure clays and peat diminish in bulk by 

 drying, very considerably. A sample of compact black 

 peat tested by the author, weighing 8 Ibs. when taken from 

 the swamp, shrank when perfectly dry to 1 pound in weight 

 thus losing 87 2 per cent, of water, and to one half its bulk. 

 Clay soil shrinks about 25 per cent, in its bulk in drying, 

 while a sandy loam loses scarcely anything. The more 

 clay or vegetable matter the soil contains the more it con- 

 tracts, and the cracking of the surface thus occasioned is 

 often seriously injurious to the roots of the crops; ruptur- 

 ing the roots as the fissures open. A strong clay soil has 

 been known to be fissured in this manner down to the 

 drains at a depth of four feet, so that the next rain poured 

 down into the drains in floods without soaking the surface. 

 This property of clay land is one of its disadvantages which 

 should be noted and should encourage its improvement by 

 a considerable admixture of vegetable matter which will 

 add to its porosity and make it less subject to an injurious 

 evaporation. 



The relation of the soil to the atmosphere, in regard to 

 its physical properties, is a part of this subject which should 

 not be neglected : for the power of absorbing gaseous sub- 

 stances from the air is of great importance to the growth of 

 crops. The absorption of oxygen by porous substances has 

 already been referred to, and the more porous soils exert 

 this power to the largest extent. A supply of oxygen is H 

 required for the germination of a seed as well as for the 



