56 THE CHEMISTRY OF THE FAKM. 



produce their principal effect at once, and are of little 

 benefit to subsequent crops. Sparingly soluble manures, 

 and those which must suffer decomposition in the soil 

 before they are of service to the plant, as farmyard 

 manure and bones, will on the contrary continue to pro- 

 duce an effect over many years. Farmers have a preju- 

 dice in favour of the latter class of manures, but it is clear 

 that the quickest return for capital invested is afforded by 

 the former class. 



Nitrogen applied as ammonium salts or nitrates will 

 give all its effect during the first year ; 45 to 50 per cent, 

 of the nitrogen applied in this form to wheat and barley is, 

 according to Lawes and Gilbert, recovered on an average 

 in the increase. In the case of farmyard manure, applied 

 on the heavy land at Rothamsted to wheat and barley, 

 only about 10 to 15 per cent, of the nitrogen was recovered 

 in the increase, but the effect on the barley continued 

 many years after the application of the manure ceased. 

 It is evident that a small quantity of an active manure 

 will accomplish the same work as a large quantity of one 

 less active. 



The residues of phosphatic and potassic manures are 

 available for subsequent crops, but are distinctly less active 

 than fresh applications of the same manures. 



