100 EXPERIMENTS UPON ROOT-CROPS 



form of rape cake to lO'O tons. A further addition of 86 lb. 

 of nitrogen (as ammonium- salts) to the 98 lb. of nitrogen in 

 rape cake (Plot 8 AC) produces no more increase of crop, 

 which remains practically stationary at 9 '8 tons. However, 

 with 86 lb. of nitrogen in the shape of nitrate of soda the result 

 is somewhat higher than in any other case, for the crop 

 amounts to 10"2 tons, a result which will be more intelligible 

 later. In all these cases, however, the crop is a very indiffer- 

 ent one for the large amounts of nitrogen employed, and the 

 aspect of the plots clearly show that the plant has received 

 far more nitrogen than it can effectively utilise in the absence 

 of the other mineral constituents which go to make up a 

 complete plant food. 



We may now turn to Plots 4, where superphosphate, 

 sulphates of potash and magnesia, and common salt are used 

 in conjunction with nitrogenous manures, thus constituting a 

 complete manure which supplies all the elements of plant 

 nutrition. 



The diagram Fig. 14 shows on the left hand the average 

 results obtained with the varying amounts and compounds of 

 nitrogen on the Plots 4 in question, where there is an abundant 

 supply of mineral manure. The right-hand half of the diagram 

 shows the effect of the same nitrogenous manures when used 

 in conjunction with dung instead of complete minerals, for 

 an account of which, see page 108. 



Without nitrogen (Plot 4 O) a very small crop is grown, 

 5 "4 tons per acre, which is increased to 14 '9 tons per acre 

 by the addition of 86 lb. nitrogen in ammonium-salts, or to 

 18*0 tons per acre by the same amount of nitrogen applied in 

 the shape of nitrate of soda. The application of 98 lb. of 

 nitrogen per acre in rape cake increases the crop to 21 "3 tons 

 per acre, and when both rape cake and ammonium-salts 

 are used together, making an application of 184 lb. of 

 nitrogen per acre, the crop is raised to 25 "5 tons per acre. 

 Thus when all the other elements of a plant food are present, 

 the crop increases with each addition of nitrogen. The increase 



