8 SOIL CONDITIONS AND PLANT GROWTH 



hydrogen they contain ; soot is valuable, because carbon is " in a state 

 in which it is capable of being rendered soluble by the action of oxygen 

 and water ". Lime is useful because it dissolves hard vegetable matter. 

 Once the organic matter has dissolved there is no advantage in letting 

 it decompose further, putrid urine is less useful as manure than fresh 

 urine, whilst to make the soil conditions approach those of a nitre bed, 

 as Home had suggested, is quite wrong. All these ideas have long 

 been given up, and indeed there never was any sound experimental 

 evidence to support them. It is even arguable that they would not 

 have persisted so long as they did had it not been for Davy's high repu- 

 tation. His insistence on the importance of the physical properties of 

 soils their relationship to heat and to water was more fortunate and 

 marks the beginning of soil physics, afterwards developed considerably 

 by Schiibler (253). On the Continent, to an even greater extent than 

 in England, it was held that plants drew their carbon from the soil 

 and lived on humus, a view supported by the very high authority of 

 Berzelius (36). 



Hitherto experiments had been conducted either in the laboratory 

 or in small pots: about 1834, however, Boussingault, who was already 

 known as an adventurous traveller in South America, began a series of 

 field experiments on his farm at Bechelbronn in Alsace. He reintro- 

 duced the quantitative methods of de Saussure, weighed and analysed 

 the manures used and the crops obtained, and at the end of the 

 rotation drew up a balance sheet, showing how far the manure had 

 satisfied the needs of the crop and how far other sources of supply 

 air, rain, and soil had been drawn upon. The results of one 

 experiment are given in Table I. on the opposite page. At the 

 end of the period the soil had returned to its original state of 

 productiveness, hence the dry matter, carbon, hydrogen, and oxygen 

 not accounted for by the manure must have been supplied by the air 

 and rain, and not by the soil. On the other hand, the manure afforded 

 more mineral matter than the crop took off, the balance remaining in 

 the soil. Other things being equal, he argued that the best rotation is 

 one which yields the greatest amount of organic matter over and 

 above what is present in the manure. No fewer than five rotations 

 were studied, but it will suffice to set out only the nitrogen statistics 

 (Table II. on the opposite page) which show a marked gain of nitrogen 

 when the newer rotations are adopted, but not where wheat only is 

 grown. 



Now the rotation has not impoverished the soil, hence he concludes 

 that " 1'azote peut entrer directement dans 1'organisme des plantes, si 



