60 The Rothamsted Wheat Experiments. 



the agricultural sense exhausted the more striking would 

 be the effect of exclusively nitrogenous as compared with 

 that of exclusively mineral manures. A given weight 

 of nitrogen, as nitric acid (or nitrate), has produced 

 more growth than the same weight of nitrogen as ammonia- 

 salts. The amount of nitrogen supplied in the manures is 

 very much in excess of the amount recovered in the increase 

 of the crops ; and, after a certain amount of growth has been 

 reached, each increase of crop requires a proportionately 

 larger application of manure. When the price of wheat is 

 high, larger crops can be grown more profitably than when 

 the price is low. In the form of farmyard manure, a con- 

 siderably larger amount of nitrogen is necessary to produce 

 a given increase of crop; but, though a given weight of 

 nitrogen, in the form of nitric acid, will produce more 

 growth than the same weight of nitrogen in dung, the 

 influence of the nitrate upon succeeding crops will be very 

 much less. There is no evidence to show whether the whole 

 available effect of the nitrogen in one manure is greater than 

 it is in the other. 



Finally, with regard to unexhausted manures, it appears 

 that, in the absence of vegetation, or when applied to crops in 

 excess of their requirements, both potash and phosphoric 

 acid form insoluble compounds with the soil, and become 

 available for future crops. Under similar circumstances, 

 nitrates and salts of ammonia do not appear to form 

 permanent compounds with the soil, but are liable to be 

 washed out by rain, or to be otherwise lost. The applica- 

 tion of more nitrogen, as nitrates or salts of ammonia, than 

 the crop can utilise, does not appear to interfere with the 

 nitrification of the organic nitrogen of the soil. The stock 

 of nitrogen of the soil itself, therefore, may be reduced, 

 although the annual application of nitrogen may be much in 

 excess of the amount of that substance removed in the crop. 

 When large wheat crops have been grown by the use of 

 nitrates or salts of ammonia, with mineral manures, the 



