III.] VALUE OF ORGANIC MANURES loi 



potash ; but, as has already been shown, the reserves of 

 such materials in the soil are so large that long- 

 continued cropping would be needed to deplete them 

 seriously. 



Some other source must be found for the farmer's 

 prejudice, and its true cause is probably the manner in 

 which organic manures improve the tilth of the soil by 

 maintaining the stock of humus, whereas sulphate of 

 ammonia, and particularly nitrate of soda, injure it. 

 The importance of this factor of tilth will be more 

 realised when we remember that nearly the whole of the 

 farmer's labour in spring is directed towards obtaining 

 a fine seed-bed for such crops as barley and roots. 

 Furthermore, if the weather conditions are adverse to 

 the start of the crop, the eventual yield will depend 

 more upon the condition of the seed-bed than upon any 

 other factor. 



The potent effect of organic manures in promoting 

 a good tilth is very clearly shown by the Rothamsted 

 experiments upon mangolds, where the nitrogenous 

 manures are nitrate of soda, sulphate of ammonia, and 

 rape cake respectively. In a good season the nitrate of 

 soda is the most effective manure ; but taking an average 

 over the whole period, rape cake shows a great superi- 

 ority, simply because of the difficulty of jetting a full 

 plant upon the other plots. Though all the plots are 

 cultivated in the same way and at the same time, the 

 condition of the soil has become so bad where purely 

 inorganic manures have been used, that only in favour- 

 able seasons is what a farmer would call a good plant 

 obtained on the nitrate and the ammonia plots, whereas 

 the rape cake plot starts regularly enough. On three 

 occasions the plant has completely failed on the 

 ammonia and nitrate plots. Even in the other years 

 there are great deficiencies, as shown by the average 



