RESPIRATION AND OXYGEN. 105 



of Hunter on the relation of soil-texture to aeration, as well as his 

 own work on the effect of potsherds and sand upon aeration and 

 consequent growth. A somewhat fuller account is given of the 

 studies of Russell and Appleyard on the composition of the soil-air, 

 and the relation of irrigation and water-saving to proper aeration 

 and growth is discussed. The importance of air-content as a limit- 

 ing factor is indicated, and the relation of quality in barley, tobacco, 

 and cotton to aeration suggested. 



Hole (1918 : 202) reviews his study of the relation of aeration to 

 the growth of sal seedlings, and gives the experimental evidence to 

 show that soil organisms greatly decrease the oxygen and increase 

 the carbon dioxid in soils which bear no green plants. He deals with 

 the poisonous effect of carbon dioxid in various quantities, and 

 concludes that injury in badly aerated soils is due to an excess of 

 carbon dioxid as well as to the deficiency of oxygen, while admitting 

 that further investigation of this point is needed. He states that 

 soil aeration depends chiefly upon the amount of water and organic 

 matter in the soil, the number and kind of soil organisms, and the 

 rate at which air or water with oxygen in solution penetrates the 

 soil. Finally, he suggests, as Howard has earlier, that the injurious 

 effect of grass on fruit trees may be due to poor aeration. 



Howard and Howard (1918 : 36) have described a new method of 

 pit cultures for the study of air and water relations under essentially 

 natural conditions, and have employed it for determining the effect 

 of mixing the Pusa soil with potsherds and sand on the growth of 

 Java indigo. The average length in soil only was 36.7 cm., in equal 

 parts of soil and sand, 51.6 cm., in soil with one-tenth of potsherds, 

 48.3 cm., and in soil with three-tenths, 50.9 cm., the respective per- 

 centages of increase being 40, 31, and 38. Potsherds at the rate of 

 1 inch per acre increased the yield of oats 18 per cent, wheat 20 per 

 cent, and tobacco 10 per cent. In the case of alfalfa, one-third 

 potsherds gave an increase of 24 per cent, and one-half windblown 

 sand an increase of 42 per cent (19 18^). 



Sen (1918) has shown that the addition of 30 per cent of potsherds 

 to soil greatly increases nitrification. The dissolved oxygen is much 

 greater with 10 to 30 per cent of potsherds than with none, and the 

 oxidation of organic matter is correspondingly hastened. A fall of 

 rain leaches out some of the nitrates and is apt to give rise to denitri- 

 fication, but the oxygen of the rain-water increases the amount 

 in the soil-air and hence tends to stimulate nitrification. The down- 

 ward movement of rain, and especially of the water-table, causes 

 greater aeration of the soil, and results in more active nitrification. 



Howard and Howard (1919) have given a further account of the 

 effect of water-logging on the development of roots and of the influ- 

 ence of drainage upon crop production, and have summarized the 

 various experiments upon the saving of irrigation water (1919^). 



