172 AGRICULTURAL ECONOMICS 



the carbon dioxide have the power of attacking insoluble mineral com- 

 pounds in the soil and transforming them into soluble forms, available 

 for plant food. Thus insoluble phosphates and potash compounds 

 are acted upon and changed into soluble forms by the soil water carry- 

 ing the carbon dioxide and organic acids in solution. Again we see 

 that bacterial activities bring about the preparation of plant food. 



The demands of various crops for sulphur has been the subject of 

 recent investigations and it has been estimated that the supply of 

 sulphates in the soil may be insufficient in many cases for the proper 

 feeding of certain crops. A group of organisms occurring in the soil 

 and known as the sulphur bacteria come into prominence here as the 

 agency keeping up the proper supply of sulphates. When proteins 

 decay, hydrogen sulphide gas is set free. This is taken up by the 

 sulphur bacteria and oxidized to free sulphur, which is in turn oxidized 

 to sulphates. Increased decay therefore leads to increased hydrogen 

 sulphide and this in turn to increased sulphates for plant food. 



We may conclude, therefore, that all methods which increase the 

 activities of the decay bacteria lead directly to an increased supply 

 of available nitrogen, and indirectly to larger amounts of phosphorus, 

 potassium, and sulphur becoming available for plant food. 



In short, the relation between bacteria and soil fertility is very 

 close and very vital, and systems of permanent agriculture must rest 

 firmly on a bacteriological basis to be of any value. If chemical 

 analyses have shown sufficient amounts of the necessary mineral 

 plant food constituents, the bacteria under the optimum conditions 

 which can be maintained by any intelligent farmer will transform it 

 into an available form to satisfy the needs of the growing crops. 



47. THE REAL MEANING OF SOIL FERTILITY AND SOIL 

 EXHAUSTION 1 



BY EDWARD J. RUSSELL 



Fertility is not an absolute property of soils, but has meaning only 

 in relation to particular plants. Plant requirements vary; a soil may 

 be fertile for one plant and not for another; every soil might con- 

 ceivably prove fertile for something. But in practice the agriculturist 

 can find use for only a very limited number of plants; he therefore 

 has to select those combining the double features of salability in his 



From Soil Conditions and Plant Growth (new edition), pp. 150-52. (Long- 

 Orccn, & Co., Txindoa Used by permission of the publishers.) 



