vii.] NITRATES IN SOIL 225 



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 in 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 

 whether these nitrates, formed after harvest, are retained 

 for the succeeding crop or are washed out of the soil. 

 To sum up, increased nitrification, together with the 

 conservation of soil moisture and the warming of the 

 surface soil, are among the chief benefits derived from 

 all forms of surface cultivation. 



Attempts have been made to estimate the fertility 

 of soil by determining the rate at which it will induce 

 nitrification in suitable media, this being taken as 

 a measure of the number of nitrifying organisms 

 present. But, as has been seen, the activity of the 

 ammonia-making organisms is the limiting factor in 



Q 



