80 THE PRINCIPLES OF AGRICULTURE 



manure crop, or the stubble or sod, occurring 

 regularly once every few years, in alternation 

 with wheat, potatoes and other staple crops. 

 This, however, is possible only with general or 

 mixed husbandry (4rt). In market- gardening, 

 and other intensive farming, catch- crops are 

 often used. In fruit-growing, cover -crops are 

 frequently used. 



113. But even in intensive farming, the land 

 sometimes becomes unproductive from too con- 

 tinuous cropping with one thing, and the too 

 persistent use of one kind of fertilizer. It is 

 then often "rested" by seeding it to clover; but 

 the good effects are not the result of a rest, but 

 of rotation or change of crop. 



114. It is necessary to distinguish between 

 the effects of green -crops in improving soil 

 texture and their effects in enriching the soil ; 

 for soils which may need improving in texture 

 may not need enriching. In fruit-growing this 

 is often true ; and the heavy addition of nitro- 

 gen (which conduces to growth of wood) may 

 cause the plants to gi^ow too heavily and to 

 bear little, and to be too susceptible to dis- 

 ease and to cold. In such cases, the nitrogen - 

 consumers are the better crops. One must be 

 careful not to induce an over- growth in gi-apes, 

 peaches, apricots, and pears. 



115. On hard and poor lands, it is often 



