380 THE POPULAR SCIENCE MONTHLY 



to plant development; under irrigation, or where the situation is such 

 as to result in permanent water a short distance below the surface, fine 

 crops will be produced on sandy soils that would remain almost barren 

 if they only depended upon the rainfall for their water. In western 

 Europe large areas of heaths and waste land owe their character to the 

 coarse and open texture of the soil. At the opposite extreme we find 

 clays so heavy that their cultivation is unprofitable ; such soils, however, 

 will carry grass and are rarely left unoccupied. For example, in the 

 southeast of England there are a few commons, i. e., land which has 

 never been regarded as worth enclosing and bringing into particular 

 ownership, situated on heavy clay land; most of such land is pasture, 

 often of the poorest, or, if at any elevation, has been covered with forest 

 from time immemorial. 



One last factor in the soil is of the utmost importance to fertility 

 and that is the presence of lime — of calcium carbonate, to be more accu- 

 rate — in quantities sufficient to maintain the soil in a neutral condition. 

 Old as is the knowledge that lime is of value to the soil, we are only 

 now beginning to realize, as investigation into the minute organisms of 

 the soil proceeds, how fundamental is the presence of lime to fertility. 

 A survey of the farming of England or western Europe will show that 

 all the naturally rich soils are either definitely calcareous or contain 

 sufficient calcium carbonate to maintain them in a neutral condition even 

 after many centuries of cultivation. Examples are not lacking where the 

 supply of calcium carbonate by human agency has been the factor in 

 bringing and keeping land in cultivation. I have discussed one such 

 case on the Rothamsted estate and several others have come under my 

 notice. The amelioration of non-calcareous soils by treatment with 

 chalk or marl from some adjacent source has been a traditional usage 

 in England and the north of France: Pliny reports it as prevailing in 

 Gaul and Britain in his day, and the farmer of to-day often owes the 

 value of his land to his unknown predecessors who continuously chalked 

 or marled the land. Upon the presence of carbonate of lime depends the 

 type of biological reaction that will go on in the soil, the beneficial 

 bacterial processes that prepare the food for plants only take place in a 

 medium with a neutral reaction. The Rothamsted soils have provided 

 two leading cases. I have shown that the accumulation of fertility in 

 grass-land left to itself and neither grazed nor mown, so that virgin con- 

 ditions were being re-established, was due to the action of the organism 

 called Azotobacter, which fixes free nitrogen from the atmosphere, and 

 was indirectly determined by the presence of calcium carbonate in the 

 soil, without which the Azotobacter can not function. Examination of 

 typical examples of black soils from all parts of the world, the prairies 

 of North America, the steppes of Russia and the Argentine, New Zea- 

 land and Indian soils, showed in all of them the Azotobacter organism 



