154 AERATION AND AIR-CONTENT. 



organic matter. Strong catalytic power in a soil may be taken as 

 evidence that the many factors of soil fertility will be prominent and 

 the soil will be productive. 



Skinner (1913 : 342) concluded that soil which had grown sesame 

 contained substances that were harmful to cabbage plants, but not 

 to wheat seedlings. Field observations showed that the soil in which 

 sesame had grown was injurious to cabbage, while in the same soil 

 without sesame, cabbage grew well. It is assumed that plants are 

 affected by the remains of previous vegetation or plant growth, and 

 that the effects are more or less specific, injuring one species and not 

 another. Since the plants grew much better in soil solutions shaken 

 up with carbon black, it seems possible that a lack of oxygen or an 

 abundance of CO 2 was responsible. 



Other researches. Jones and Morse (1903) have observed an ap- 

 parent antagonism between the butternut and Potentilla fruticosa, 

 and attribute this to the root relations rather than to the shade. The 

 invasion of the soil by the vigorous roots of the butternut near the 

 surface is thought to interfere with the nutrition of the cinquefoil 

 in some manner. It seems probable that this is a combined light and 

 water relation, since "it is stated that the shrubby cinquefoil is 

 quickly killed by tree-growth of any kind," especially since it is more 

 or less hydrophytic. "If the stock is fenced out of a field the trees 

 will soon come in and the cinquefoil weaken and die out as the trees 

 overshadow it." 



Hedrick (1905) observed that young peach trees shed their leaves 

 and matured quickly when oats were planted in pots with them. 

 Potatoes or tomatoes wrought less injury to the trees, mustard and 

 rape had but slight effect, and beans and crimson clover none at all. 

 The leaves turned yellow before falling, indicating drought resulting 

 from the competition. The effect of a grass sod upon apple trees 

 was later investigated (1910). The grass was cut once or twice each 

 year during the 5 years, while the tilled plot was plowed each spring, 

 cultivated 4 to 7 times until late July, and then planted to a cover- 

 crop. The average yield on the sod plot for 5 years was 72.9 barrels 

 and on the tilled plot 109.2 barrels per acre, while the average weight 

 per apple was 5 and 7 ounces, respectively. The average gain in trunk 

 diameter was 1.1 inches in sod and 2.1 inches under tillage, and the 

 average leaf weight was 9.7 gm. and 11.5 gm., respectively. The 

 average annual growth of branches for sodded trees was 1.9 inches 

 and 4.4 inches for tilled trees, while the average number of laterals per 

 branch was 3.4 for one and 6.7 for the other. These differences are 

 ascribed to water-content, the latter being highest in the tilled plot. 

 Aeration and bacterial activity are regarded as playing some part in 

 the results. 



