1919] Coit-Hodgson: Abnormal Shedding of Washington Navel Orange 323 



air through the solution. The economic applications of this principle 

 are many, but are of course particularly evident in regions where 

 through special conditions lack of soil aeration is emphasized, as is the 

 case in certain parts of India. The soil is naturally very heavy and 

 easily packed by the torrential rains. Lack of aeration is accentuated 

 during certain portions of the growing season by the occurrence of 

 monsoons and tropical rainstorms of great severity. Howard 53 has 

 shown most conclusively that under these conditions the production 

 of the gram or chick-pea, Cicer arietinum, grown to the extent of over 

 eighteen million acres, is absolutely conditioned by the soil aeration. 

 If the soil is permitted to become packed by summer rains and the 

 air supply cut off, the plants wilt down with water actually stand- 

 ing on the surface of the soil. Absorption is cut down to practically 

 nothing, while transpiration is not reduced in the same ratio, resulting 

 in ultimate wilting. While not extensive, all the experimental data 

 available on the production of this crop in California show this same 

 intolerance of lack of soil air. Howard has shown this same condition 

 affecting fruit trees and other crops, among which is the indigo plant. 

 Free 54 has shown that with Coleus blumei "even a very small decrease 

 of oxygen below that normal to the atmosphere is injurious to the 

 plant. Thus a plant, the roots of which were supplied with gas con- 

 sisting of 75 per cent air and 25 per cent nitrogen, was injured within 

 three days and killed within 45 days. With lower oxygen content in 

 the soil atmosphere injury and death are still more prompt." In 

 many cases the lack of aeration is first evidenced by the shedding of 

 the leaves and flowers. Soils of arid regions in general are well 

 aerated, and especially soils of open structure such as sands and 

 sandy loams. Therefore it is not likely that lack of soil aeration is 

 the factor conditioning absorption of water by citrus trees. However, 

 this problem is now under investigation and will be reported on later. 

 Under most conditions of lack of aeration not only is oxygen 

 deficient but carbon dioxide is present in excess. The experimental 

 data available seem to indicate that while in general lack of oxygen 

 and excess of carbon dioxide in the soil atmosphere are detrimental, 

 there is no set rule. Cannon, 55 and Livingston and Free 56 have shown 



5 3 Soil Aeration in Agriculture, Agr. Ees. Inst. Pusa, Bull. 61, 1916. 



j>4 Cannon, W. A., and Free, E. E., The Ecological Significance of Soil Aera- 

 tion, Science, n.s. vol. 45 (1917), pp. 178-80. 



5 5 On the Eelation between the Eate of Boot-Growth and the Oxygen of the 

 Soil, Ann. Bep. Dir. Dept. Bot. Ees., Carnegie Inst. Washington, Yearbook 15 

 (1916), pp. 74-75. 



56 Eelation of Soil Aeration to Plant-Growth, ibid., p. 78. 



