348 SOIL FERTILITY 



begin to suffer first. It is not difficult to imagine how the drying-out of the soil affects 

 the soil solution and its surface tension. It is obvious that soils rich in soluble 

 salts might develop so concentrated a solution as to cause plasmolysis of bacterial 

 cells. Irrigation water and its content of soluble salts, the local application of fertiliz- 

 er salts, the removal of the excess of water by drainage, the rainfall-evaporation ratio 

 and moisture conservation by tillage and other methods, must all find a place in the 

 study of the interrelations of soil moisture, micro-organisms, plants, and soil fertility. 

 A more accurate knowledge of these relations will indicate a better control of micro- 

 organisms as a factor of soil fertility. 



THE MOBILIZATION OF PLANT FOOD 



The foregoing discussion has dealt with the significance of the various internal 

 and external factors in the complex relations of micro-organisms to soils and crops. 

 It has been shown that the soil solution receives contributions from the inorganic and 

 organic soil constituents. It has been shown, Ukewise, that micro-organisms are essen- 

 tial for the normal changes in the soil, that is, the mobilization of an important portion 

 of the plant nutrients. In new agricultural regions the farmer is chiefly concerned 

 with tillage as a means of mobilizing adequate amounts of available plant food. Later 

 on, tillage alone is found to be ineffective for maintaining the crop yields, and so- 

 called "bare fallows" are resorted to. These were still the basis of the three-field 

 system of husbandry prevalent in Western Europe and in North America well toward 

 the end of the eighteenth century. But bare fallows were only a temporary expedient, 

 for they allowed the sources of food and energy of the soil micro-organisms to run low. 

 The satisfactory mobilization of plant food became impossible. Agricultural chem- 

 istry, and later agricultural microbiology, pointed the way to more rational methods. 

 A suitable soil reaction, an adequate supply of organic matter, a sufficiency of certain 

 inorganic constituents, and a reasonably good control of the environmental factors 

 of temperature, moisture, and aeration were indicated as desirable for the proper use 

 of micro-organisms in the mobilization of plant food and the production of large crops. 

 But not content with pointing out the principles that must govern soil practices the 

 microbiologist has contributed certain specific information and methods that are al- 

 ready of large practical value to agriculture. 



SPECIAL METHODS AND PRACTICES 



It was observed long ago that the continued production of certain crops on the 

 same land led to falling off in yields and not infrequently to complete crop failures. 

 Soils became "sick" for certain crops and, under given conditions, were said to be 

 "flax sick," "clover sick," "tobacco sick," "potato sick," etc. We know now that 

 pathogenic fungi and bacteria, nematodes and plant-lice, are the cause of most of these 

 soil ills. The continued growing of the favorite host plant allows the pathogens or 

 parasites to survive and flourish. Hence, methods had to be devised for ridding the 

 land of them. Partial soil sterilization has become a well-established practice in 

 greenhouses, and is occasionally resorted to under field conditions. Heat and volatile 

 germicides are employed. Among the latter, carbon bisulphide and formaldehyde are 

 the most popular. Salts of mercury, copper, and nickel are also used, but primarily 



