THE MAIN TYPES OF SOIL 59 



carbonic acid gas, and for this reason, and also 

 because it is not much " absorbed " by soil, 

 it is rather easily washed out, and so removed 

 from the feeding range of plant roots. The 

 ease with which lime may be removed from 

 soil is well illustrated by the experiments at 

 Woburn and Rothamsted. At both these 

 stations sulphate of ammonia has been used 

 for many years on the same plots, with the 

 result that practically all the available lime 

 has been washed out of the land. It would 

 appear that the sulphuric acid of the sul- 

 phate of ammonia has combined with the 

 lime of the calcium carbonate to form gypsum, 

 and in this form the lime has disappeared 

 in the drains, or has been removed to the 

 subsoil beyond the reach of plants. This is 

 a danger that has to be borne in mind where 

 sulphate of ammonia is used liberally at short 

 intervals. 



Soil containing much carbonate of lime 

 possesses very definite properties. It fixes 

 energetically soluble compounds of phos- 

 phoric acid, such as are present in super- 

 phosphate of lime and dissolved bones. But 

 its power of fixing most other substances 

 is rather limited. As a rule such a soil 

 contains but little potash, nor does it gener- 

 ally show a high percentage of nitrogen. 



