Jan. 1908.] 



SCIENTIFIC AGRICULTURE. 



Some Factors Influencing Soil Fertility. 



By C Driebbrg. 



This is the title of a recent Bulletin issued by the United States Department 

 of Agriculture. 



The Excretory Theory of de Candolle is one which every student of Agri- 

 culture is familiar with, and its temporary acceptance and final rejection by the 

 .•cientific and practical agriculturists of the day is a matter of history. At intervals, 

 since that time, attempts have been made to revive de Candolle's theory, and those 

 who remember the time when coffee-leaf disease spread devastation in the planting 

 districts, and the efforts made to fight the fungus, will recall how the excretory 

 theory was conjured up to explain the failure of the then staple industry of the 

 Island. 



It is somewhat astounding, therefore, in this twentieth ceutury to find that 

 the conclusions of scientific workers is tending to confirm the fiudiugs of an old- 

 time scientist whose theory was supposed to be as dead as Queen Anne ! But such 

 is the case, and the researches of Messrs. Schreiuer and Reed have shown a definite 

 tendency in that direction. There is, however, this to be said, viz., that de Candolle 

 and his followers failed to recognise the agencies responsible for the destruction of 

 such organic substances as corresponded with the " Excreta " of the theory, and 

 inclined to the belief that once a crop had made the soil noxious through its 

 excretions, it would always remain so as regards that particular crop. 



The infertility of soils is, according to our experimenters, mainly attributable 

 to toxic substances, organic in their nature, in the soil, transmitted to it — partly if 

 not wholly — by growing plants. It is a common experience to find that the continual 

 growth of a particular crop on the same laud results in a diminished yield. The 

 results of research now goes to prove that the diminished yields are not due 

 primarily to the exhaustion of plant food, but to the toxic substances given off by 

 the plants. This has been demonstrated even in the case of highly-enriched garden 

 9oils that failed to grow crops which at one time flourished on them. 



It has also been found that not only is there a deleterious influence exerted by 

 a growing plant upon its own kind, but, in certain cases, upon a totally different kind 

 of plant. (This would point to the necessity of avoiding such successions in a 

 rotation). To put it another way, some forms of vegetation are antagonistic to 

 others — a condition directly opposed to the symbiotic relation characteristic of 

 certain forms of vegetation. Such conditions are calculated to well nigh bewilder the 

 practical agriculturist and impress him with the complexity of the study of the plant 

 and the soil which are his care. One instance may be given in illustration of this 

 antagonism in vegetable life. In the Woburn Experimental Fruit Farm it was 

 found that young apple trees suffered seriously when grass was allowed to grow 

 about them. After ruling out that such injury was due to removal of plant food, 

 loss of moisture, exclusion of oxygen from the soil, &c, it was proved to satisfaction, 

 as the result of seven years' research, that there was no evidence of root parasitism, 

 but that the deleterious action of the grass was of an actually malignant character 

 akin to that of direct poisoning. We are here reminded of the late Mr. William 

 Jardine's contention, viz., that coconuts, especially young coconuts, did twice as well 

 on bare land as on sward. Instances of converse deleterious action, viz., of tree 

 roots on herbaceous plants are also cited, and go to confirm our own gardening 

 experience in similar conditions. 



