THE ROTATION OF CROPS. 69 



(3) This system of farming, embodying as it usually does 

 a number of crops, furnishes a variety of food for stock. 



(4) One crop tends to correct the faults of another and 

 often leaves land in better condition for the succeeding crop. 

 Thus the cowpea, when used as a green manure, naturally 

 adds to the humus and nitrogen content of the soil, and conse- 

 quently the soil is better able to support a crop like maize 

 than if the maize had followed another cultivated or humus- 

 destroying crop. 



(5) Rotative farming is one of the most effective means of 

 controlling weeds. Certain weeds thrive better when grown 

 'with "some crops than with others. Dodder is parasitic on 

 lucerne, and rooibloom (Striga lutea) on plants of the 

 Gramineae ; therefore , if crops are grown which are not hosts 

 of these parasites the latter are not so easily perpetuated. 



(6) The losse s due j Q_iQ££rt pests. axLd^ plant diseases are 

 lessened_j)y_^hi!llng_ tkeJocaliXY. _of the crop they'a"ffecrr"and 

 by' cliangmg'tlurhost on which they thrive they are deprived 

 of their means of subsistence. In this connection, it must be 

 remembered that insect pests and the organisms causing plant 

 diseases are more or less specific in their choice of host plants ; 

 thus those affecting solanaceous plants may not affect legumin- 

 ous crops. Decreased yields are too often caused by badly 

 infected soil — the cumulative result of the pathological condi- 

 tion arising from the continuous cultivation of the same crop. 



(7) It is the best and most economical method of main- 

 taining and increasing the productivity of the soil. 



(a) Crops differ in their requirements of plant food. Cer- 

 tain plants make greater dema.nds on certain plant foods than 

 do others ; therefore , a succession of crops requiring different 

 quantities of potash, nitrogen and phosphorus for each crop 

 will maintain a better balance between the quantities of these 

 plant-food elements in the soil. It is easily seen, therefore, 

 that an alternation of crops will retard the excessive reduction 

 of certain necessary compounds. Moreover, an opportunity is 

 afforded the soil to regain its normal content of the available 

 element reduced through the specific assimilation of certain 

 crops. On the other hand, the residues of some crops are 

 beneficial to a succeeding one, thus crops following a legume 

 are benefited by its nitrogen accumulation. If manures and 

 fertilisers are employed to guard against the exhaustion of 

 these elements this system of farming gives greater possibili- 

 ties of their profitable use. 



