MANURINa IN THEORY AND PRACTICE. 



255 



Httle from that which existed above it, hecame highly 

 charg-ecl -with, carbonic 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, lot us take, 

 as an example, 

 some ordinary 

 arable soil, of a 

 clayey nature, in 

 fair cultivable 

 condition. Such 

 a soil, when all 

 roots and veget- 

 able debris have 

 been removed, 

 will contain in the 

 first nine inches 

 of the surface 

 mould a quantity 

 of organic matter 

 containing about 

 ;j,000 lbs. of ni- 

 trogen and 30,000 

 lbs. of carbon per 

 acre. This nitro- 

 genous organic 

 matter of the soil 

 has been derived 

 either entirely 

 from the decay 

 of vegetable 

 growth, left in 

 the land by pre- 

 ceding 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- 

 gi'ess, 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 

 Rothamsted 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 be 

 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 manure 

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

 32|- 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 : — ■ 



Froduce of Land continuously cropped %i-lth Wheat 

 during Ihirty-eight Years. jLlso tlie Fercentage of 

 Nitrogen and Carbon, and the quantity of Nitrogen as 

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

 that period. 



Plot 

 4 



PJOt 



5 



Plot 

 10 



Plot 



6 



Plot 



Plot 

 2 



Average Annual Produce {Grain and Straiv) per Acre, 1852 — 81. 



lbs. 

 2,227 



lbs. 



2,394 



lbs. 



3,450 



lbs. 

 3,954 



lbs. 

 5,710 



lbs. 



5,695 



Nitrogen per Cent, in First Nine Inches of Soil, Oct., 1881. 



0-092 



0-098 



1 0-103 



1 0-111 



1 0-121 



1 0-184 



Carhon -per Cent, in First Nine Indies of Soil. 



I'OlO 



1-033 



1-095 



1-205 



1-267 



2-132 



Nitrogen as Nii 



rates per Acre in First Twenty- 

 Inches of Soil. 



seven 



3bs. 

 16-3 



lbs. 

 25-1 



lbs. 



33-8 



lbs. 

 29-4 



lbs. 

 401 



lbs. 

 5L-8 



B A 

 Sunflower Plants, showing effects of Potassium Nitrate. 



Reference to the results shows that the percentn-je 

 both of nitrogen- and carbon in the soil is greater 

 according- to the weight of croj) annually produced : 

 the quantity of nitrogen and carbon in the first nine 

 inches of soil is more than one-fom-th 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 



