172 



POMOLOGY 



versely, when tilled trees were gradually grassed over, they 

 showed the deleterious effect so common to this fonn of 

 treatment. In one block of standard apples, the sod was 

 replaced immediately after planting, and when they were 

 compared with a block of similar trees under tillage but 

 which were gradually allowed to grass over, the following 

 significant results were obtained: 



Table L 

 annual growth op tilled trees = 100 



These results are explained entirely on the ground that 

 some toxic influence is interfering with the normal func- 

 tioning of the plant and preventing it from utilizing the food 

 present. Somewhat similar results in this country have 

 been explained largely by the low rate of nitrification and 

 often the reduction in soil-moisture, but toxic substances, 

 perhaps due to lack of oxidation, may be involved in the 

 low nitrification which occurs. 



In general, these various experiments would seem to bear 

 out fully the results that might be anticipated from a study 

 of the effects of culture on the soil. Sod has practically 

 always inhibited the growth, size, color, and amount of fo- 

 liage on the trees. Toxicity and lack of available nitrogen 

 and moisture may all play important parts in the effects 

 secured. Tillage has universally increased growth and gen- 

 eral vigor of the trees and the same is often true when a 

 mulch of litter is placed about the trees to such a depth as 

 will destroy the growth of grass. Even partial tillage or 



