23O SOILS AND FERTILIZERS 



fair application. Salt also improves the physical con- 

 dition of the soil by increasing the surface tension of 

 the soil water. It should not be used on a tobacco or 

 potato crop, because it injures the quality of the product. 

 Salt is beneficial in preventing some forms of fungous 

 diseases from becoming established in soils. 



273. Magnesium Salts. Magnesium is present in the 

 ash of all plants, and is an element essential for plant 

 growth. Usually soils are so well stocked with mag- 

 nesium that it is not necessary to apply it in ferti- 

 lizers. Some of the magnesium salts, as the chloride, 

 are injurious to vegetation, but when associated with 

 lime as carbonate, magnesia imparts fertility. In many 

 of the Stassfurt salts, magnesium is found. 



274. Soot. The deposits formed in boiler flues and 

 chimneys when wood and soft coal are burned contain 

 small amounts of potash and phosphoric acid. Soot is 

 valuable mainly as a mechanical fertilizer and is slow 

 in decomposing. It contains but little plant food as 

 shown by the following analysis : 



275. Seaweeds. Seaweeds are rich in potash and 

 near the seacoast are extensively used for fertilizer. 



