62 



SOIL CONDITIONS AND PLANT GROWTH 



soil micro-organisms, the liability of the plants to disease and the 

 tractability of the land. Soils sufficiently supplied with calcium car- 

 bonate stand out in sharp contrast with those containing too little, 

 although they may be of similar composition in all other respects. So 

 great is the effect that the practical man has long since adopted the 

 special term " sour " to describe soil deficient in calcium carbonate, a 

 term we shall find it convenient to retain. Table XXVIII. shows pairs 

 of soils similar in constitution and general external conditions, tempera- 

 ture, water supply, etc., but very different in agricultural value because 

 of their different content of calcium carbonate, one being readily culti- 

 vated while the other is wet and sticky, and only suitable for pasture 

 land : — 



Table XXVIII. — Effect of Calcium Carbonate on the Texture of Soils. 



It is impossible to ascertain the amount of calcium carbonate neces- 

 sary for 'a soil except by actual field trials : in general, sandy soils 

 require only sufficient to prevent sourness, while clay soils need in 

 addition enough to keep the texture good. Sands well supplied with 

 calcareous water and under ordinary arable cultivation may get along 

 with O'l per cent, or even less calcium carbonate, while others that are 

 being heavily dunged respond to dressings of chalk, or ground limestone, 

 even though 0"2 or 0*3 per cent, is already present. It commonly 

 happens that 0-5 per cent, of calcium carbonate proves insufficient for 

 clay soils, and even i -o per cent, may not be enough in highly-farmed 

 districts, especially where cattle are fed on the land and tread the soil 

 into a somewhat sticky state. Further increases in calcium carbonate 

 over and above the critical amount are not known to have any effect 

 except to provide a margin of safety. 



Calcium carbonate is not a permanent constituent of the soil, but 



