64 SOIL CONDITIONS AND PLANT GROWTH 



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 

 changes into the soluble bicarbonate and washes out into the drainage 

 water ; the average loss per acre per annum throughout England and 

 Wales has been estimated at 500 lb., and at Rothamsted on the arable 

 land at 800 to 1000 lb. (118). The rate of loss is influenced by the 

 treatment, being increased by the use of ammonium sulphate and 

 decreased by dung and by the crop ; it is much less on pasture than 

 on arable land. Repeated additions of calcium carbonate to the soil 

 are, therefore, necessary : indeed chalk and lime are among the oldest 

 of manures. Soils lying immediately above chalk and limestone 

 are no exceptions and in wet regions they may become thoroughly 

 decalcified. 



On chalk soils the percentage of calcium carbonate may rise very 

 high, and then a wholly new set of properties comes in. It is im- 



