350 ANNUAL REPORT OF THE Off. Doc. 



The preceding table shows from 50 parts of nitrogen in the drain- 

 age as compared with 61.3 parts in the crop up to 67 parts in the 

 drainage as compared with 37.2 parts in the crop. 



The table also shows that in the case of wheat the greater pro- 

 portion of the nitrates in the soil are either beyond the reach of the 

 roots, or what is more probable, that they are washed out of the 

 soil at that season, fall or winter, when the wheat has but limited 

 capacity of absorbing them. For this reason wheat is shown to be 

 one of the most exhaustive of all farm crops as regards nitrogen sup- 

 ply. That no such proportionate loss of nitrates occurs with other 

 crops, like grass and clover, which cover the ground especially during 

 the fall and winter months is shown by numerous figures, some of 

 which will be considered under the head of loss of nitrates. 



(e.) Losses of Nitrates. 

 Relation to Rainfall and Percolation. 



titrates are easily leached out of the soil and carried into the 

 drainage. In a highly porous, sandy soil free of vegetation, the 

 greater proportion of the annual rainfall, 80 per cent, more or less, 

 will pass downward through the soil and appear as drainage. 



In land covered with a sod a much less percentage, about one- 

 third of the rainfall, may pass off as drainage, the remainder being 

 evaporated or thrown off by the plants in transpiration. 



In a plot of fallow ground at Kothamsted^^ for a period of 28 years 

 the annual average outflow of drainage was 11.76 inches and the 

 average rainfall for the same period 28.3 inches, making the average 

 percentage of drainage 41.5 per cent, of that of the rainfall. 



The action of vegetation in preventing an excessive outflow of 

 drainage is most evident during the months of maximum growth. 

 May to September, as the following figures by Greav(>s and Evans" 

 indicate: 



