86 KINDS OF INDICATORS. 



acid soil either neutralizes the acid or affects the colloidal relations in such 

 fashion as to make the soil agriculturally productive. It is significant, how- 

 ever, that lime is not the only substance that has this effect, since it is also 

 produced by other materials which improve aeration. An acid soil is regarded 

 as unfavorable to plant growth primarily because of the deficit in oxygen, and 

 consequently because of the poor development of the micro-organisms that 

 reconvert organic nitrogen into available form (plate 15, b). 



The current assumption that bog water contains acids or toxins which are 

 in themselves unfavorable to absorption seems disproved by the experiments 

 of Bergman (1919). This investigator submerged pots containing plants of 

 Phaseolus in bog water and tap water respectively until the tops were covered. 

 In both the leaves wilted and turned yellow within 3 days. Both the bog 

 water and tap water were then oxygenated night and morning, and by the 

 following day the leaves had regained their normal turgor, and remained so for 

 several days while oxygen was supplied. Similar results were obtained with 

 Geranium and Impatiens. With the former, bubbling carbon dioxid through 

 the water containing turgid plants produced wilting on the second day, and 

 led to final chlorosis and fall of the leaves. When pots of Impatiens were 

 submerged in water with and without Philotria, the ones remained turgid, 

 while the others wilted within 3 days. Plants of Coleus and Fuchsia were 

 grown in ordinary pots and in submerged ones, and the root pressure was 

 found to be two or three times as great in the former. When the plants in the 

 submerged pots were aerated by bubbling air, or by placing Philotria or 

 Spirogyra in the water, the root pressure was nearly as great and as well 

 maintained as in the normal conditions. Hydroid species, such as Salix sp., 

 Cyperus alternifolius, and Ranunculus sceleratus, grew about equally well in 

 bog water and tap water, whether aerated or not. 



The studies of Hole and Singh (1914 : 10) upon aeration in forest soils 

 indicate that the lack of oxygen is a factor of greater importance and wider 

 extent than has been supposed. The general summary of their results is as 

 follows (101) : 



"1. The present experiments have confirmed the results previously ob- 

 tained regarding the very injurious effect of bad aeration on the growth of Sal 

 seedlings in the local forest soil. 



" 2. When water is long held in contact with this soil, which is the case under 

 conditions of bad aeration, it becomes heavily charged with carbon dioxid and 

 impoverished as regards its supply of oxygen. 



"3. The bad growth of Sal seedlings in this soil is correlated with an accumu- 

 lation of carbon dioxid in the soil-solution and a low oxygen content, and this 

 possibly explains the evil effects of bad aeration. Further work however is 

 required to prove this and also to decide the relative importance of carbon 

 dioxid and oxygen, respectively. 



"4. Liming this soil, immediately before sowing, has an injurious effect upon 

 Sal seedlings, and, during the rains, soil which has been thus limed appears to 

 contain more carbon dioxid and less oxygen than the unlimed soil. It seems 

 possible that this may be due to accelerated bacterial activity. 



"5. As carbon dioxid is rapidly dissipated and a deficiency of oxygen made 

 good under the ordinary conditions of water cultures, it is not easy to prove 

 the effect of varying quantities of these gases on plants grown in cultures. For 

 the same reason, artificial aeration of such cultures may not show any bene- 

 ficial result. 



