24 



In two series of Maercker's experiments, also made with oats, the 

 following were the proportions of nitrogen recovered in the crop for 100 

 supplied in the manure. As the nitrate of sodium gave the same return 

 in each series of experiments, we assume that the conditions were, alike 

 throughout. 



Nitrate of Sodium - 57.5 Ox Dun g - 4.5 



Sheep Dung - 19'9 Wheat Straw - 



Farmyard Manure (4 kinds) 3'0-9*8 



The influence of the decomposable organic matter in a manure is 

 twofold: it affects the process both of nitrification and of denitrification. 



If the conditions within the soil are suitable for nitrification, any 

 addition to the soil of decomposable carbonaceous matter will 

 tend to diminish the rate at which nitrogen is oxidised, and 

 may cause nitrification to cease altogether. The decomposition of an 

 organic manure, and its partial oxidation, must precede its nitrification. 

 If much of this preliminary work has to be done, the commencement of 

 nitrification is greatly delayed; the products of the decomposition of car- 

 bonaceous matter are, indeed, inimical to nitrification. 



When the quantity of organic manure applied exceeds a certain 

 proportion, the conditions prevailing in the soil may be entirely changed, 

 and a nitrifying medium, converted for a time into a denitri- 

 fying medium, the oxygen demauded by the decomposing organic mat- 

 ter being now obtained by the destruction of the nitrates in the soil. An 

 organic manure which is effective when applied in small quantity may 

 thus become injurious when made use of in excess. 



An illustration of the decrease in efficiency which follows an in- 

 crease in the quantity of an organic manure is furnished by one of Wag- 

 ner's experiments: 157 grams of pasture grass were incorporated with 

 the soil of one pot, and double this quantity. 314 grams, was applied to 

 another pot. The return in the crop for 100 of nitrogen applied was in 

 the first case 43, in the second 36 ; or, in other words, the crop 

 instead of being doubled was increased by 69 per cent, by 

 doubling the quantity of the manure. As the total amount of nitrogen 

 assimilated with the heaviest manuring was far less than that taken up 

 when nitrate of soda was applied, the smaller return from the green 

 manure was clearly not due to its supplying an excess of nitrogen, but 

 simply to the incapacity of the soil to bring this nitrogen into a con- 

 dition available for the crop. 



That the addition to the soil of large doses of fermentable organic 

 matter will retard the nitrification of other easily nitrified nitrogenous 

 manures is probably the principal reason of the ill-effect resulting in the 

 German experiments from applying horse dung with sulphate o ammo- 

 nium, with urine, and with green manures. Wagner and Maercker ap- 

 plied these nitrogenous manures, both alone and with the addition of 

 dun<: or farmyard manure ; the following is a selection from their re- 

 sults. The produce given by dung alone is in every case substracted 

 from the produce of dung with other nitrugenous manures and the re- 

 maining increase is credited to the respective manures. 



