94 AGRICULTURE. 



of its most important foods, and if we grow wheat year after 

 year we may soon exhaust the nitrates available; but if we grow 

 wheat one year and some other crop the next, the second 

 crop may be able to feed well and flourish upon food left by the 

 wheat. 



2. The plants have different methods or powers of getting 

 the same kind of food. Thus clover or peas will get nitrogen 

 by means of the little knots or tubercles (page 57) upon their 

 roots, whereas wheat has not this power to take up free nitrogen. 

 A clover crop will need more nitrogen than a crop of wheat, 

 and yet, because of the root tubercles, we do not apply nitrates 

 to a clover crop, but nitrates may be applied to wheat with 

 good results. 



3. The plants have different kinds of roots. Those of barley 

 are very short, those of wheat longer, those of red clover and 

 lucerne still longer. A deep-rooted crop feeds lower down than 

 a shallow-rooted crop. If, then, we grow clover this year and 

 wheat the next, we grow these crops, to a great extent, in 

 two different soils. We use surface soil for one and under-soil 

 or sub-soil for the other. By changing from a shallow-rooting 

 crop to a deep-rooting, or from a deep-rooting to a shallow- 

 rooting, we, as it were, change the soil from year to year. This 

 is one of the most important points to observe in rotating crops. 



4. By rotating crops we change the treatment of the same 

 soil, since we do not treat the soil exactly alike in preparing it 

 for different crops. Some crops, also, are cultivated and others 

 are not. We thus give the weeds different treatment. The 

 weeds differ as do the crops there are annuals, biennials and 

 perennials ; there are long-rooted and shallow-rooted ; there are 

 early seeding and late seeding weeds. The same treatment 

 year after year may be just the right treatment to encourage 

 certain weeds to grow and spread. The growing of wheat 

 year after year in the west is causing the spread of some very 

 noxious weeds. By changing the crops, and therefore the treat- 



