196 THE LIVING ORGANISMS OF THE SOIL [chap. 



obtained on well-tilled land carrying maize and potatoes. 

 On the cultivated land the proportion of nitrates rises 

 rapidly until the end of June, when the crop begins to 

 draw freely upon them, and reduces them to a minimum 

 throughout August and September. 



One of the best examples of the manner in which 

 the thorough working and aeration of a warm soil 

 promotes nitrification is seen in the management of 

 the turnip crop as usually grown in this country. 

 Though shallow-rooted, and taking away large quan- 

 tities of nitrogen per acre, it is usually grown with 

 but little nitrogenous manure ; phosphates with a little 

 dung or with a comparatively small nitrogenous dress- 

 ing, being sufficient. The rest of the nitrogen is derived 

 from the rapid production of nitrates, due to the very 

 thorough working of the soil in the warm season of the 

 year that is characteristic of the cultivation of the turnip 

 crop. The production of nitrates by cultivation for 

 the benefit of a succeeding crop by bare fallowing, 

 or of an adjoining crop as in the Lois-Weedon system 

 of alternate husbandry, has been already alluded to. 

 At Rothamsted, nearly 60 lbs. per acre of nitric nitrogen 

 were found in October in the top 27 inches of soil 

 that had been fallowed, as against about half that 

 amount in land which had been under crop. The un- 

 manured alternate wheat and fallow plots showed in 

 September 1878 to a depth of 18 inches 33-7 lbs. of 

 nitric nitrogen per acre after fallow, and only 2-6 lbs. 

 after wheat. In land occupied by cereal crops the 

 drainage waters show that there is practically no nitrate 

 left in the soil by May, or, at the latest, June; they 

 reappear again towards the end of July or in August, 

 and after harvest, if rain falls, and especially if the land 

 be ploughed, nitrification becomes very active. It 

 depends upon the rainfall of the autumn and winter 



