NUTRITION: THE WORK AND THE MATERIALS. 15 



Principles of Manuring. The few remarks we have 

 thought it right to make as to the nature of the substances 

 absorbed with the water from the soil have an important 

 bearing on the theory and practice of manuring. The 

 nitrogenous and the saline substances are taken from the 

 soil, used up in the plant, and removed in the crop. The 

 annual produce of hay on unmanured land at Kothamsted 

 has been found to be about 23 cwts. per acre, over an average 

 of 25 years, the range of variation according to season 

 having been from 8 to 39 cwts. On the other hand, the most 

 highly manured plot has yielded for the same period an 

 average of 64 cwts. of hay per acre, varying in separate 

 years from 40 to 80 cwts., according to season. These 

 figures will suffice to illustrate the amount of food derived 

 from the soil and from the atmosphere, and the beneficial 

 effects of suitable climatal conditions. The decline not 

 only of produce, but also in mineral and nitrogenous in- 

 gredients in the soil, in the case of the continuously un- 

 manured plots at Rothamsted, is very marked. To insure 

 continued fertility, therefore, and obviate exhaustion, some 

 restitution must be made ; and this is effected by the 

 addition at the right time, in the right condition, and 

 in the right quantities, of an appropriate manure ; or the 

 exhaustion may be compensated by suitable rotation, or 

 the growth in alternate periods of plants having different 

 requirements, as wheat after potatoes or clover after wheat. 



Apparent Power of Selection, how Explained. The 



circumstance that certain crops are specially benefited by 

 particular manures, though they contain relatively little 

 of the substance in their composition, would seem to 

 indicate the existence of a power of selection, as also would 



