148 BIOLOGY OF PLANTS. 



crops, which have been taken from the soil." Hence it would 

 seem, that these substances are the sources of a part at least 

 of the carbon, and that it enters mostly in the form of humates 

 {geates), crenates and apocrenates. 



6. This theory is inconsistent with itself and with facts. 

 If, as Liebig contends, plants give out to the soil effete matter, 

 mostly composed of carbon, this matter cannot affect th e 

 succeeding crop ; for if all the carbon is derived from the at- 

 mosphere, the excretions will not be absorbed in larger quan- 

 tities than other matters, especially as they are nearly insolu- 

 ble. But Liebig and others admit, and experience seems to 

 prove, that the effete matters of one family, are injurious to 

 succeeding crops of the same family, but useful to those of a 

 different race. 



On this theory the fertility of soils does not depend, in the 

 slightest degree, upon the quantity of vegetable matter, the 

 humus or geine ; a conclusion which is opposed to all experi- 

 ence on the subject. For it has been observed by every far- 

 mer, that vegetable substances are highly promotive of fertil- 

 ity, so much so, that it has long been the effort of farmers to 

 convert their soils into loams by the addition of these sub- 

 stances ; that is, by increasing the quantity of vegetable mould 

 or humus. A soil rich in humus or geine is generally fertile, 

 one destitute of it is wholly barren ; and the degree of fertility, 

 as will be fully shown in a future section, is very much in the 

 ratio of the soluble geine which the soil contains. 



According to this theory, a continual course of cropping 

 ought to increase the quantity of carbon in the soil, especially 

 as the soil may be so constituted as to contain but little de- 

 caying humus to supply carbonic acid to the roots. But we 

 know that when afield has been cultivated for a long period, and 

 the crops all removed, it will, in the end, be reduced to abso- 

 lute barrenness ; and when we look for the cause, we find 

 the vegetable matter is mostly or quite removed. The effect 

 of manures, therefore, in keeping up the quantity of vegetable 



