60 THE VEGETABLE MATTER OF THE SOIL. 



In certain extreme cases, as in those of plants growing in the air and 

 in soils perfectly void of organic matter, this conclusion must be abso 

 lutely true. The phenomena admit of no other interpretation. But is 

 it as strictly true of the more usual forms of vegetable life, or in the or- 

 dinary circumstances in which plants grow spontaneously or are culti- 

 vated by the art of man ? Has the vegetable matter of the soil no 

 connection with the growth of the trees or herbage ? — does it yield them 

 no regular supplies of nourishment ? Does nature every where form a 

 vegetable mould on which her wild flowers may blossom and her pri- 

 meval forests raise their lofty heads ? Has the agricultural experience 

 of all ages and of all countries led the practical farmer to imitate nature 

 in preparing such a soil ? Does nature work in vain ? — is all this ex- 

 perience to be at once rejected ? 



While we draw conclusions, legitimate in kind, we must be cautious 

 how, in degree, we extend them beyond our premises. 



The consideration of one or two facts will shew that our general con- 

 clusion must either be modified or more cautiously expressed. 



1°. It is true that plants will, in certain circumstances, grow in a soil 

 containing no sensible quantity of organic matter — but it is also true, 

 generally, that they do not luxuriate or readily ripen their seed in such a 

 soil. 



2°. It is consistent with almost universal observation, that the same 

 soil is more productive when organic matter is present, than when it is 

 wholly absent. 



3°. That if the crop be carried off a field, less organic matter is left 

 in the soil than it contained when the crop began to grow, and that by 

 constant cropping the soil is gradually exhausted of organic matter. 



Now it must be granted that tillage alone, without cropping, would 

 gradually lessen the amount of organic matter in the soil, by continually 

 exposing it to the air and hastening its decay and resolution into gaseous 

 substances, which escape into the atmosphere. But two years' open 

 fallow, with constant stirring of the land, will not rob it of vegetable 

 matter so effectually as a year of fallow succeeded by a crop of wheat. 

 Some of the vegetable matter, therefore, which the soil contained when 

 the seed was sown, must be carried off the field in the crop. 



The conclusion therefore seems to be reasonable and legitimate, that 

 the crop which we remove from a field has not derived all its carbon di- 

 rectly from the air — but has extracted a portion of it immediately from 

 the soil. It is to supply this supposed loss, that the practical farmer 

 finds it necessary to restore to the land in the form of manure — among 

 other substances — the carbon also of which the straw or hay had robbed 

 the soil. 



But how is this reconcileable with our previous conclusion, that the 

 whole of the carbon is derived from the air ? The difficulty Ls of easy 

 solution. 



A seed germinates in a soil in which no vegetable matter exists; it 

 sprouts vigorously, increases then slowly, grows languidly at the expense 

 of the air, and. the plant dies stunted or immature. But in dying it im- 

 parts vegetable matter to the soil, on which the next seed thrives better 

 —drawing support not only from the air, but b3^ its roots from the soU 

 also. The death of this second plant enriches tk; soil further, and thus, 



