CHAPTER XXVI 

 TEE MAINTENANCE OF SOIL FERTILITY ^ 



The maintenance of a profitable and continuous soil pro- 

 ductivity is an intricate problem, since many variable factors 

 are involved. Weather conditions, moisture relations, soil or- 

 ganic matter and tilth, plant diseases, soil reaction, and avail- 

 able nutrients are only a few of the influences that function 

 continuously throughout the growing season. No scheme of 

 soil management and crop production is perfect, even though 

 it is fairly profitable. Except in special cases, every system is 

 open to improvement and modification as soil and plant 

 knowledge increases. 



The sources of knowledge regarding the profitable growing 

 of plants are numerous. Much data have arisen from expe- 

 rience and observation, much are empirical, while some are 

 confessedly conjectural. In spite of the large amount of 

 scientific information available regarding the soil and its 

 plant relationships, practical experience has contributed more 

 towards a profitable and continuous soil productivity. Soil 

 survey classification and mapping have contributed some- 

 thing. Field tests, both practical and technical, have added 

 to such information, while laboratory and greenhouse experi- 

 ments, although often arbitrary and artificial, are by no 

 means unimportant. These latter contributions, however, 

 always need practical confirmation under typical field con- 

 ditions over a period of years. 



318* Loss of plant nutrients from the soil. — A consid- 

 eration of the principles governing the rational management 



* Fertility is here used in the sense of continuous productivity. 



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