50 EXPERIMENTS UPON WHEAT 



pared with Plot 7, although its soil must be becoming exhausted 

 of these constituents by their constant removal in the crops. 

 Doubtless the effect of the sulphates of magnesia and soda on 

 Plot 7 is due to their action as soluble salts, maintaining in 

 a more soluble condition the other manurial constituents 

 necessary to the crop. 



D. Retention of Manures by the Soil 



It has already been stated that, as a rule, 100 lb. of the 

 ammonium-salts are applied in the autumn when the seed is 

 sown, the rest being reserved for a top-dressing in the 

 spring. On one of the plots, however. Plot 15, the whole 

 400 lb. of ammonium-salts is applied in the autumn, 

 otherwise the manuring is identical with that of Plot 7. 

 The crop, however, on Plot 15 is on the average below that 

 of Plot 7, showing that some loss takes place when the 

 ammonium-salts are applied before the plant is able to utilise 

 them. Although the ammonium-salts are soluble in water 

 they are caught by the soil and held very near to the surface, 

 so that the loss does not arise by the washing out of the 

 ammonium-salts themselves. They are, however, rapidly con- 

 verted into nitrates when the land is warm and moist, 

 especially after it has been recently stirred by the autumn 

 cultivations. The nitrates thus produced are not retained by 

 the soil, and wash out very readily if heavy rain falls during 

 the early winter. This is seen in the analyses of the drainage 

 water collected beneath Plot 15. It is generally very rich in 

 nitrates in the autumn as compared with Plot 7 ; whereas in the 

 spring, when the ammonium- salts are aj^plied, a corresponding 

 loss does not happen with Plot 7, because the crop then 

 occupies the land and is able to take up the nitrates as fast as 

 they are formed. 



The diagram Fig. 7 shows the estimated loss of nitrates in 

 lb. per acre on these two plots during the summer and winter 

 respectively, between the spring sowing of manures in 1879 and 

 the corresponding date in 1881. 



