256 



CASSELL'S POPULAE GAEDENIXG. 



iound in soil regularly manured with ammonium 

 salts would be due to a retention of ammonia in the 

 soil, and not, therefore, determined by the amount of 

 crop residue. That this is not the case is provtd by 

 the fact that the excess of nitrogen is always accom- 

 panied by a corresponding excess of carbon. On 

 Plot 10 there is but a small increase either in the 

 nitrogen or in the carbon of the soil, although the 

 same quantity of ammonia was applied as on Plot 7. 

 The reason is this: On Plot 10 no ash consti- 

 tuents have been for many years applied ; the crop 

 and crop residue are therefore small, in spite of the 

 application of ammonium salts, and the amount of 

 nitrogen and carbon in the soil has consequently not 

 been maintained even by considerable annual remains 

 of roots and stubble. 



On Plot 2 fourteen tons of farmyard manure have 

 been annually applied per acre for thirty-eight years, 

 and the produce during the last thirty years has 

 averaged 33^ bushels of dressed wheat and 32 cwt. 

 of straw. Hence, there is as large an annual crop 

 residue contributed to the soil as in the case of 

 Plot 7, and in addition the very considerable quan- 

 tity of nitrogenous organic matter supplied in the 

 manure. Eeferring to the foregoing table, the effect 

 on the composition of the soil is seen to be most 

 striking. The percentage of nitrogen in the firt^t 

 nine inches of mould is sho's\-n to be double, and the 

 percentage of carbon more than double that found 

 on the unmanured plot. 



Dressings of farmyard dung, which is the usual 

 manure of the gardener, may thus considerably 

 increase the quantity of nitrogenous matter in the 

 soil ; while dressings of ammonium salts, or of 

 sodium nitrate, will have no permanent effect in this 

 direction, except their use results in the production 

 of a larger crop residue. 



Experiments with barley also supply some capital 

 examples of the accumulations of organic matter in 

 ihe soil, which may result from the use of organic 

 manures, such as farmyard dung, rape, or cotton- 

 cake, &c. 



It is thus proved that the nitrogenous organic 

 matter of the soil has its origin in the various vege- 

 table substances left in the land, as residues from 

 preceding crops, to which in some cases must be 

 added the residue fi'om dressings of organic manures. 

 The production of nitrates in soil is of the greatest 

 importance to vegetation, nitrates being the form in 

 which nitrogen is chiefly assimilated by plants ; the 

 :abundance or poverty of nitrates in a soil thus 

 determines, to a gTcat extent, the quantity of crop 

 which the land wall produce. A recognition of 

 "this fact is of vital importance to the horticul- 

 turist if he is to have any accm-ate notion as to 

 the influence of different crops, or plants, in main- 



taining or exhausting the fertility of the soil which 

 he cultivates. 



It is evident that the crop which leaves behind it 

 the largest amount of roots, stubble, and foliage will 

 best maintain or increase the nitrogenous capital of 

 a soil ; while the cfop lea%'ing the smallest residue 

 in the land will be the most exhausting in its 

 effects. 



This carbonaceous organic matter of soils, and the 

 black substance found in farmyard manure, under 

 the general term Sianus, was considered by many 

 intelligent men in the early part of the present een- 

 tury to be the main source of fertility. 



Soil Fertility. — The slow and spontaneous de- 

 composition and oxidation of vegetable substances 

 in the earth is continually going on, and is the 

 natural mode by which the soil is em-iched. At the 

 time when Liebig's writings directed so much atten- 

 tion to the subject of agi^iculture, it was assiuned that 

 this oxidation of organic matter in the soil took 

 place by mere contact with the oxygen of the air. 

 The active oxidation was referred to the fact that a 

 soil is a porous substance, and that the oxygen of the 

 air became condensed within its pores, and was hence 

 capable of exerting an increased power. We now 

 know, from experiments of Schloesing, Muntz, and 

 AYaiington, that a porous medium is by no means 

 essential for nitrification, and that the oxidation of 

 organic matter in a soil generally requires something 

 more than the presence of oxygen. 



Oxidation in nature is, in fact, nearly always per- 

 formed by living agents, either by colourless plant- 

 cells or by means of animal organisms. 



Though, howev er, porosity is by no means essen- 

 tial to the nitrifying power of a soil, it is undoubtedly 

 a condition having a very favourable influence on 

 the rapidity of the process. 



Our view of the nature of a fertile soil has thus 

 been considerably enlarged, and instead of regarding 

 the soil simply as a porous mass of clay, sand, and 

 humus, we now look on it as a medium full of life. 

 " The soil beneath oiu' feet," says Mr. Warington. 

 " is, in fact, not dead, but thickly populated witli 

 a variety of organisms, with the particular function of 

 which we are only gradually becoming acquainted." 



The general result of this nitrifying influence in 

 soils is the conversion of the organic matter into 

 three simple substances— water, carbonic acid, and 

 nitric acid. Humus thus becomes the source of a 

 gradual, but uninterrupted, formation in the soil of 

 carbonic acid, which is probably less useful on ac- 

 count of the carbon it furnishes to vegetation than 

 for the solvent power w-hich it exercises with regard 

 to certain minerals, and especially potash and phos- 

 phoric acid. 



