238 SOIL ACIDITY AND LIMING 



The Nature of Soil Acidity. — In practically all upland, acid 

 mineral soils, the acid characteristics are imparted to them mainly 

 by insoluble acid substances which in themselves are soil particles. 

 In such soils, therefore, reduction of acidity through leaching 

 is impossible. 



In some cases, upland soils may be acid because of the accumu- 

 lation of certain organic or mineral acids (Figs. 152 and 153). 



In undramed, cumulose soils, acidity is due, m a large measure, 

 to the presence of soluble organic acids which may be largely 

 leached out when the marsh is thoroughly drained. On low sand 

 islands in undrained acid marshes, the soils are usually strongly 

 acid because they are saturated with the organic acids from the 

 surrounding peat. When the marshes are drained these sand 

 islands become less acid. Permanent acidity in peat must neces- 

 sarily result from the presence of insoluble, acid organic substances. 

 Loss of lime increases the acidity of drained, acid peats. 



Kinds of Agricultural Lime. — ^Many materials are suitable for 



hming soils, such as: 



r a — Pulverized limestone. 

 6 — ^Air-slaked lime 

 c — Marl 



1. -Carbonates of Inne (CaO+CO.) | ^ZKn^^hel'ls 



/ — Marble dust 

 Q — ^Pulverized coral 

 h — Chalk. 



B. Lump or burnt lime (CaO) 



C. Hydrated hme (CaO 4- water) 



Pulverized Limestone. — Strictly speaking, limestone is calcium 

 carbonate (Fig. 154). When a rock consists of a combination of 

 calcium and magnesium carbonates it is called magnesian lime- 

 stone or dolonnte. The name '' limestone'' is commonly applied 

 to both rocks. Limestone may be distinguished from other rocks 

 by the fact that it gives ofif bubbles of gas when treated with dilute 

 acid (muriatic acid). This is a common test for any carbonate. 



Limestone is prepared for direct agricultural use by crushing 

 and pulverizing (Fig, 155). When it is pulverized so that about 

 fifty to sixty per cent of the material will pass through a sixty- 

 mesh sieve ^ it may be considered of standard fineness (Fig. 156). 

 The fine material produces immediate action in soils, while the 



«A 60-mesh sieve means a sieve havmg perforations }io of an mcb 

 in diameter. 



