140 



parts of the roots. Potassium oxalate was eliminated by hyacinth roots. 

 Carbon dioxid, however, must be considered primarily and causes the rock 

 etchings, as it occurs dissolved either in the water of the root-hair cells or of 

 the soil interstices. Monopotassium phosphate and carbon dioxid among 

 the root secretions must be especially considered. In pot cultures the latter 

 is of especial importance. It is retained in the root balls in great quantities, 

 the more thickly matted they are and the wetter they are kept by the grower. 

 The production of carbon dioxid is greatly increased by the respiration of 

 the soil micro-organisms which in their metabolism decompose the carbo- 

 hydrates and other organic substances. For instance Stocklasa^ found 

 alcohol, acetic acid and formic acid in forest soil and finally carbon dioxid 

 and hydrogen. The hydrogen often unites with oxygen to form water. 

 Lack of oxygen and the excess of carbon dioxid kill part of the roots and 

 the process is gradually evidenced when plants are grown in small pots, 

 even if over-abundant foodstuffs be given them by fertilizing. However, if 

 fertile earth alone is used, without subsequent additions of fertilizers, the 

 roots, becoming thickly matted on the walls of the pot, do not touch the ball 

 of earth actually as they develop on top of older roots. In such cases, they 

 cannot further draw from the soil the food materials needed in growth. 



Early investigations by Hellriegel'- prove that excessively limited soil 

 space in itself limits production. To perform these experiments many 

 annual and perennial agricultural plants (barley, peas, buckwheat^ clover, 

 etc.) were sown in glass containers of different heights in as uniform gar- 

 den soil as possible and were grown with an observance of all the precautions 

 used in sand and water cultures. In order to prevent any question as to the 

 exactness of the results obtained due to a different amount of soluble nutri- 

 tive elements control experiments were made with an abundant addition of 

 fertilizers, under otherwise similar conditions. The result was, that no 

 difference in production whatever was shown in favor of the fertilized 

 plants, that those not fertilized must thus have found in the unfertilized 

 garden soil all the nutritive substances that they needed for their production. 

 An indirect proof lay also in the results of the experiments given by the 

 unfertilized plants when compared with one another. The yield showed in 

 fact, that clover in the first year had produced about as much dry material 

 as the other varieties of plants. This did not prevent the clover, however, 

 from producing in the second year on the same soil a second crop and in 

 fact a crop two or three times as great, and even in the third year it produced 

 as much as in the first year. From this it is evident that the amount of 

 nutritive substances could not play a role in any of the experimental pots, 

 since they were everywhere present in excess. 



If now, however, the amount of dry substance increased with the size 

 of the container, this result could be ascribed only to the influence of the 



1 Stocklasa and Ernest, Ueber den Ursprung, die Menge und die Bedeutung 

 des Kohlendioxj'ds im Boden. Centralbl. f. Bakteriologie etc. Section II, Vol. XIV. 

 1905, p. 723. 



2 Hellriegel, Beitrage zu den naturwissenschaftlichen Grundlagen des Acker- 

 baues. Braunschweig. Vieweg, 1883. pp. 184-224. 



