54 FARM DEVELOPMENT 



These soil areas, whether formed in situ of the least 

 easily decayed rock substance ; in homogeneous layers, as 

 of clay or sand assorted and deposited by some agency; 

 or composed of mixed materials, as a bowlder clay or a 

 sandy clay, are in areas irregular in outline, and usually 

 much overlapped and mixed. A general soil survey of 

 any locality, outlining the extent of its soil types and 

 mapping each area under an appropriate name, gives a 

 basis for studying the need of each soil area and for 

 developing good cultural systems suited to it. Follow- 

 ing the soil survey naturally come the fertilizer and crop 

 rotation surveys, and the general farm management sur- 

 vey for determining, by systematic field plot tests on 

 each soil type, the fertilizers needed, also the kinds of 

 rotation and systems of farm management which best 

 maintain high productivity and are permanently the most 

 profitable. 



All the activities breaking up the surface of the 

 earth's crust, mixing and piling some materials here, 

 assorting and spreading others there, leaving the under- 

 lying rocks deeply buried in one place and uncovered 

 in another, have given to one section good materials 

 for soils and left another a barren waste of rock or a 

 desert of sand. The surface of mixed material forming 

 loose earth, mainly sand and clay, is rapidly changed 

 into a productive soil. On the other hand, the changing 

 of inhospitable lava beds, the transformation of the dense 

 clay recently risen from the lake, the binding and bet- 

 tering of the drifting sand surface, or the filling up of a 

 gravel bed and making it into a hospitable soil, have all 

 taken ages of time, even under the many forces at work. 



Agencies in soil formation. The physical agencies 

 air, sun, wind, water and ice, heat and cold are in 

 some cases sufficient to crumble the lava. Spores of 

 minute lichens fall upon hard rock, germinate and form 

 small plants. These live mostly upon moisture and air, 



