AIANUEING IN THEOEY AND PRACTICE. 



255 



little from that which existed above it, heoame highly- 

 charged -with carbonio acid, which decomposed the 

 mineral substances contained in the soil; and in this 

 manner year by year more and more of the nitrogen, 

 collected by each generation of plants, became avail- 

 able for the generation that succeeded it. 



In order to start with definite notions on the in- 

 herent quality of 

 soils, let us take, 

 as an pxample, 

 some ordinary 

 arable soil, of a 

 clayey nature, in 

 fair cultivable 

 condition. Such 

 a sou, when all 

 roots and veget- 

 able debris hAve 

 been removed, 

 ^vill contain in the 

 first nine inches 

 of the surface 

 mould a quantity 

 of organic matter 

 containing about 

 3,000 lbs. of ni- 

 trogen and 30,000 

 lbs. of carbon per 

 acre. This nitro- 

 genous organic 

 matter of the soil 

 hijs been derived 

 •either entirely 

 from the decay 

 of vegetable 

 growth, left in 

 the land by pre- 

 teding genera- 

 tions of plants, or 

 possibly, to some 

 extent, also from 

 past applications 

 •of organic ma- 

 nure. In a fertile 

 soil the formation 

 of nitrates is 

 always in pro- 

 gress, and it is 



very important for gardeners to bear in mind that 

 the nitrogenous capital of a soil, which represents to 

 a considerable extent its fertility, depends, as a rule, 

 •on the bulk and composition of the previous plant 

 residues. The present condition of a soil is thus, 

 in great measure, a consequence of its past fertility, 

 which fact may be exemplified by another of the 

 Hothamsted exneriments. 



Accumulation of Plant-food In Soils In 



one field wheat has been grown continuously for 

 forty-one years {1844 — 1884), and during the last 

 thirty years the manuring has not been changed on 

 any of the plots to which reference is about to bo 

 made. The range of produce has been very con- 

 siderable, rising from an average of 14 bushels of 

 dressed wheat and 1 2 cwt. of straw without manuio 

 (Plot 4), to an average of 32J bushels of grain and 

 32f cwt. of straw on the highly-manured Plot 7. 



In October, 1881, samples of soil were carefully 

 taken from each plot in the field and analysed, the 

 results being given in the following table : — 



Troduce of Zand continuously cropped with Wheat 

 during Thirty-eight Years. Also the Fereentage of 

 Kltrogen and Carbon, and the quantity of Nitrogen ua 

 Nitrates per Acre, found in the Soil at the end oj 

 that period. 



Pig. 7.— SdSPLOWEE PlAMTS, showing effects of POTASSIDM NlTPATE. 



Reference to the results shows that the percentage 

 both of nitrogen and carbon in the soil is greater 

 according to the weight of crop annually produced ; 

 the quantity of nitrogen and carbon in the first nine 

 inches of soU is more than one-fomth larger on Plot 7, 

 yielding the maximum crop, than on Plot 4, where 

 the minimum produce was obtained. 



It might be thought that the excess of nitrogen 



