VIII.] VALUE OF ROTATIONS 157 



always get an extremely strong hold on the land and 

 become very expensive to keep under. Diseases and 

 insect pests special to the crop tend to accumulate 

 until they may unfit the land to carry the crop. The 

 tilth of the land often suffers when the same crop is 

 grown continuously on it; for example, when wheat 

 follows wheat there is very little time to prepare the 

 land between the crops ; again, if swede turnips are 

 repeated for the second crop the land would miss the 

 autumn ploughing, which on many lands is a very 

 necessary element in the preparation of a seed bed for 

 turnips. And though soil exhaustion in any strict sense 

 is impossible, yet when any particular crop is repeated 

 there will always be one particular layer of soil in which 

 the roots of the plant chiefly reside, and this layer is 

 somewhat affected either by the removal of its plant 

 food, or possibly by the addition of some injurious 

 substance due to the plant. At any rate it is the deep- 

 rooted plants, wheat and mangolds, which seem to 

 flourish best when grown repeatedly on the same soil ; 

 whereas shallow-rooted plants like turnips, barley, and 

 specially clover, soon begin to fall off in yield if grown 

 continuously on the same land. The theory that plants 

 excrete some substances specifically poisonous to them- 

 selves and that a rotation is necessary to give it time to 

 decay, has been revived of late years, but the evidence 

 in its favour seems inconclusive, nor is it necessary to 

 set up so remote a hypothesis to explain the niain 

 factors in the virtues of a rotation. 



Before we can leave the question of rotations and 

 the amount of plant food in the soil, it is desirable to 

 arrive at some idea of what is removed from the land 

 during an ordinary course of cropping, for on that will, 

 to some extent, depend the nature and quantities of the 

 materials which must be returned as manure if the 



