154 Mineral Nutrition of Plants 



nitrate, phosphate, calcium, and magnesium was taken up from aerated 

 solutions as from nonaerated solutions. The size of plants and yield of 

 fruits were correspondingly high. Arrington and Shive (4) found that 

 absorption of ammonia nitrogen was 1.34, and of nitrate nitrogen 1.30, 

 times greater from aerated than from nonaerated solutions. Pepkowitz 

 and Shive (56) found that absorption of calcium and phosphorus was 

 directly dependent upon dissolved oxygen supply, whereas absorption 

 of potassium was not so materially influenced. Chang and Loomis {27), 

 on the other hand, found that aeration increased absorption of potas- 

 sium most, followed, in order, by nitrogen, phosphorus, calcium, and 



magnesium. 



Direct evidence for the effect of soil aeration on nutrient availability 

 is scarce, but the evidence available is significant and highly suggestive. 

 Probably the main reason why there are few data on this subject is that 

 there is no good way of measuring or characterizing soil aeration; thus, 

 it has been difficult to prove that certain conditions observed in the field 

 have been caused by poor soil aeration. Many attempts have been made 

 to measure the partial pressure of oxygen in the soil atmosphere di- 

 rectly, but little success has been attained. One difficulty has been that 

 the analytical methods available required a rather large gas sample. 

 This was not easy to extract from the soil in a condition representative 

 of the atmosphere surrounding plant roots. Another difficulty is that 

 measurement of oxygen in the soil air may not indicate the amount of 

 oxygen dissolved in the water bathing individual roots, for normal soil 

 changes in the oxygen concentration of the soil atmosphere may occur 

 too rapidly for equilibrium between dissolved and gaseous oxygen to 

 be maintained. With the recent introduction of the Pauling oxygen 

 analyzei, which requires only a few cubic centimeters of gas and which 

 indicates partial pressure of oxygen directly and automatically, a new 

 tool is available which should give valuable information concerning the 

 mechanism of gas interchange in the soil. 



Even without data on the actual aeration status of the soil, a few ex- 

 periments and observations have been reported which emphasize the 

 importance of aeration to normal plant growth and the effect of inade- 

 quate soil aeration upon nutrient availability. Page and Willard (54) 

 observed in an experiment which compared different tillage methods 

 for corn, that marked potassium deficiency symptoms occurred in corn 



