LOSS OF NITRATES IN WET AUTUMNS 223 



It has already been pointed out in dealing with the wheat 

 crop (p. 63) that the increased crop after fallow is almost 

 wholly dependent on the retention in the soil of the nitrates 

 thus formed in the summer. Should a wet autumn and early 

 winter succeed, the nitrates are washed so far down in the 

 subsoil as to be out of reach of the crop, which then shows a 

 very small return for the previous summer fallowing. 



The rapidity with which nitrification may take place after 

 harvest is also noticeable in the table. For three months 

 or so before the removal of the wheat crop the soil in which 

 it is growing contains only little nitrates, but if rain falls 

 when the ground has been broken up after harvest the con- 

 ditions become extremely favourable to nitrification, because 

 the soil is warm and well aerated by stirring, and possesses 

 a suitable degree of moisture. Hence heavy autumnal rains, 

 before the land is again occupied by a crop to take up the 

 nitrates, may easily result in serious loss to the land, and 

 some quick-growing covering crop like mustard is valuable, 

 because it seizes upon the ready formed nitrates. At a later 

 date the nitrogenous compounds the mustard has formed from 

 the nitrates, which would otherwise have been washed away, 

 are returned to the land, either by being ploughed in or fed 

 off, and become available on their decay for the nutrition of 

 the succeeding crop. Even a free growth of weeds on the 

 stubble will diminish the loss of nitrates to the land. 



IV. NITRATES IN MANURED AND CROPPED SOILS. 



In several instances, as, for example, in the Broadbalk field 

 in 1893, soil samples have been drawn to depths of 9 feet, 

 and the distribution of the nitrates in the successive 9 inches 

 determined. Table LXXVIII. shows the results for certain 

 of the plots sampled in October of that year, after a very dry 

 summer yielding a crop much below the average, and also after 

 a fall of about 4J inches of rain between harvest and the time 

 of sampling. 



