INTRODUCTORY 5 



that most of the operations upon the farm have received 

 a new light from the knowledge that has been acquired 

 in the last few years of the living processes taking place 

 in the soil. In this direction also new developments 

 of agriculture seem to be possible, and though the 

 progress is only small as yet, we see indications that 

 the productive capacity of the land may be per- 

 manently increased by the introduction of certain 

 organisms capable of assisting the work of the higher 

 plants. 



On the biological side we have also to study the 

 association of certain plants with particular soils ; an 

 examination of the natural flora of any district will 

 show that some species are almost confined to sandy 

 soils, others to soils containing chalk, rarely wandering 

 on to different types of soil ; again, particular weeds 

 are characteristic of clay land, others of sand ; and 

 some even of our cultivated crops show a marked 

 intolerance for particular soils. 



The full story of the soil cannot yet be told ; small 

 wonder that in the course of the many centuries man 

 has been cultivating the face of the earth, he has found 

 out much which science can barely explain, still less 

 improve upon. Nor are the problems simple — the 

 food, the water, the temperature, the living organisms 

 in the soil are all variables, affected by cultivation and 

 climate, themselves also variable ; they all act and 

 react upon one another and upon the crops ; hence we 

 can easily understand that the smallest farm may 

 present problems beyond the furthest stretch of our 

 knowledge. 



