153a ELEMENTARY AGRICULTURAL CHEMISTRY 



Rotation of Chops. 



In all countj-ies where farming has been long established, 

 the plan of alternating different crops on any particular portion 

 of land has become general. It is only in the first few years 

 after virgin soil has come into cultivation, that the same crop 

 is grown, year after year, on the same land. 



The practice of rotation of crops has many advantages; some 

 with respect to the practical operations of cultivation, seeding 

 and reaping, some in diminishing the ravages of plant diseases, 

 insect pests, weeds and other troubles, and some in connection 

 with the power of the soil to supply the needs of plants. The 

 obvious advantages of distributing the labour involved in the 

 ploughing, drilling, harrowing and reaping of the crops, more 

 evenly over the year, which a variety of crops affords, need not be 

 discussed here. So, too, the benefits derived from changing the 

 crop from time to time, in preventing the continuance of any 

 particular disease or blight which may attack a certain kind of 

 crop, or in destroying the weeds which may accompany it, 

 hardly require more than passing mention. 



With the efiect of rotation upon the soil's ability to supply 

 plant food, however, chemistry has more to deal, and a short 

 account of the advantages of rotation, from this aspect, may 

 appropriately be given. 



These advantages mainly depend upon the following : 



1. Differences in the root range. Some crops, e.g.j barley, 

 have only shallow roots which draw their sustenance from the 

 uppermost layers of soil, while others, e.g,, mangolds, depend 

 chiefly for their food, upon the matters present in the lower 

 parts of the soil. By alternating shallow-rooted and deep- 

 rooted plants, all parts of the soil, in turn, are called upon tc 

 contribute plant food. 



2. Utilisation of crop residues. The root debris^ stubble 

 and waste left in the field after the removal of any crop, afford, 

 on decay, good food for another kind of crop, while such 

 residues are often of little benefit, indeed, in some instances, 



