Chapter II. 



SOIL MANAGEMENT OR CONTROL. 



Aside from such devices as greenhouses, wind-breaks, etc., 

 which have a local application only, there are three general 

 methods of soil control: tillage methods, such as plowing and 

 harrowing; rotation of crops; and the use of soil amendments 

 or "fertilizers." 



The existing knowledge regarding tillage methods is generally 

 considered to be fairly satisfactory. The purposes are well 

 understood, namely, to break up and "fine" the soil, 1 to keep 

 down weeds, and by forming mulches to decrease the loss of 

 water by evaporation. Not much increase is being made in our 

 theoretical knowledge of this subject, although mechanical 

 improvements in the implements of tillage are being and will 

 undoubtedly continue to be made. 



The existing knowledge concerning crop rotations is fairly 

 extensive, but it is almost entirely empirical. Some at least of 

 the purposes served by a rotation of crops are fairly well known, 

 such as the elimination of weeds or lower types of parasitic 

 growth associated with particular crops; the introduction of 

 humus by a grass crop or a green manure crop, especially by the 

 Leguminosae with their symbiotic Azobacteria; the improve- 

 ment in the structure or arrangement of the soil particles by 

 alternating deep-rooted and shallow-rooted crops; the avoidance 

 of continually growing a crop in the presence of its own excreta, 

 products of decay, etc.; and lastly, economic and market 

 considerations. 



The existing knowledge of fertilizers, in spite of a vast amount 

 of work and an enormous literature, is still very meagre and it 

 also is almost entirely empirical ; and this because studies on the 

 subject have been dominated for three-quarters of a century 

 by one theory almost to the exclusion of any other. The 

 exponents of this theory have generally assumed that the action 



1 Actually, to granulate the soil. "Fine" would seem to be a mis- 

 nomer, but its agricultural significance is well understood, and it has the 

 sanction of long usage in the literature. 



