266 SOILS AND MANURES 



to its action, to give a larger return than an equal 

 weight of nitrate of soda. For the same quantity of 

 nitrogen there is often very little to choose between them, 

 but nitrate of soda is the more soluble, acts more quickly, 

 and for grass and cereal crops at least, the advantage, 

 if any, will generally be found to lie with it. For 

 turnips and potatoes, on very light soils and in wet 

 climates, sulphate of ammonia may prove superior. The 

 prices of both manures fluctuate so much from time to 

 time that it is difficult to compare them on the basis of 

 equal money values. Sulphate of ammonia generally 

 costs rather more than nitrate of soda at present the 

 difference is about sixpence per unit of nitrogen but it 

 should be kept in view that 1^ cwt. of the former contains 

 approximately the same quantity of nitrogen as 2 cwts. of 

 the latter. 



Exhaustive Effects. Nitrogenous manures, and especi- 

 ally nitrate of soda, are sometimes said to have an ex- 

 haustive effect on the soil. A state of exhaustion, it has 

 been shown, is due to removal of available plant foods 

 which are abstracted by the growing crops. It is obvious 

 therefore that inasmuch as nitrate of soda stimulates the 

 growth of the crops it must tend to produce such effects. 

 It does not follow, however, that it is inadvisable to use 

 nitrate of soda or other nitrogenous manures, but they 

 must be used with discretion. The available phosphoric 

 acid and other plant foods which are abstracted in conse- 

 quence of the action of the nitrogenous manures, must 

 be replaced. In other words, phosphatic manures, etc., 

 must be used in conjunction with nitrate of soda. It is 

 only when used alone and for several years in succession 

 that exhaustive effects are produced by the action of 

 nitrogenous manures, and any such effects are not usually 

 apparent until the application is discontinued. 



On the grass land at Kothamsted, plot 17, which has 



