94 Physics of the Soil 



plasmic lining, p, began to shrink away from the cell wall 

 h, as shown at 3, and when the strength of the solution was 

 made 10 per cent., the conditions shown in 4 are produced. 

 When the cells of plants are affected in this way they wilt 

 and growth ceases. 



A soil containing 20 per cent, of water and also 2,000 

 Ibs. of water soluble salts per million of dry soil would 

 contain 2,000 Ibs. in 200,000 Ibs. of water, or 1 part in 100, 

 which is 1 per cent. If the soluble salts constitute 2 per 

 cent, of the dry weight of the soil then with 20 per cent, of 

 moisture present the strength of the soil solution would be 

 equal to that which De Vries found fatal to plants, or 10 

 per cent. 



The salts in the surface three inches of soil upon which 

 Hilgard found barley to grow four feet high were 1.2 per 

 cent., while they were 2. 44 per cent* in the same level where 

 the barley died. With 20 per cent, of moisture in the soil, 

 and all the salts dissolved, the soil solution in the first 

 case would represent a strength of 6 per cent, and in the 

 second case 12.2 per cent, which is larger than the amount 

 De Vries found fatal. 



107. Concentration of Salts in Zones. Where long contin- 

 ued drought has occurred in soils rich in soluble salts the 

 tendency is for the salts to collect in the surface two or 

 three inches and in this way become injurious to plants 

 when they would not be so with an abundance of water in 

 the soil. 



When heavy rains follow such a concentration of salts 

 at the surf ace, or if the land is irrigated so as to produce 

 percolation, the result is to wash the salts down in a body 

 to the depth reached by percolation, and hence it may hap- 

 pen that a layer of soil very rich in salts may occur at the 

 surface at one time and later at a distance of 12, 18, 24 or 

 30 or more inches below, determined by the depth of per- 

 colation. 



108. Origin of Soluble Salts. The excessive amounts of 

 Baits found in alkali lands are usually the result of long 



