312 AMMONIA AND NITEIC ACID. 



which, by the operation of the causes above-mentioned, 

 the necessary surplus of nitrogenous food, formerly 

 present, is gradually expended. On recommencing the 

 cultivation of such soils, the employment of nitrogenous 

 manures will at first produce a remarkably beneficial 

 effect. Afterwards, these too require no further supply. 



There is one reason which excites in the farmer's 

 mind a prejudice in favour of introgenous manure, and 

 that is the great inequality in the appearance of the 

 young crops, when such manures are applied in com- 

 parative experiments. The cereal plants upon fields 

 manured with guano or nitrate of soda are distinguished 

 before others by a deep green colour, and by broader 

 and more numerous leaves ; but the harvest is generally 

 far from corresponding to the expectations raised by 

 this promising appearance. Upon a field excessively 

 rich in nitrogenous food, there is a kind of rankness in 

 the early growth like that produced by a hot-bed : the 

 leaves and stalks are watery and weak, in consequence 

 of the want of time in their over-hasty growth to absorb 

 contemporaneously from the soil the necessary quantity 

 of substances, such as silicic acid and lime, capable of 

 communicating to their organs a certain solidity and 

 power of resistance against those external causes which 

 endanger their existence. The stalks fail to acquire the 

 necessary stiffness and strength, and are always liable 

 to be laid, especially on lime soils. 



This injurious influence of excess of nitrogenous food 

 is particularly remarkable in the case of the potato 

 plant ; for if it grows upon a soil excessively rich in 

 nitrogenous food, and the temperature should suddenly 

 fall and wet weather supervene, the plant is often at- 

 tacked by the so-called potato disease ; while a neigh- 

 bouring potato field merely manured with ashes shows 

 no trace of it. 



Among all the many experiments which have been 

 hitherto made by farmers to improve their land, there is 

 not one instituted for the purpose of ascertaining the 

 actual condition of their soil, or of seeking proofs for 

 the correctness of the notions which they had once 



