WINDBREAKS 237 



The shading effect expressed in percentages is recorded as though it 

 were concentrated on a strip equal in width to the height of the trees, and 

 since the shaded area is often wider than the trees are high, it often happens 

 that the figures for shade exceed 100 per cent. This method of calculation 

 is likely to confuse the reader, and yet it is difficult to suggest a better one. 

 Shading effects were measured by exposing solio paper in the shadow and 

 comparing its change of color with exposures made in direct sunlight. 



METHODS OF OBSERVATION 



Soil moisture determinations were made at depths of 10 to 20 inches and 

 at intervals from each windbreak of 10 or 20 feet to the limit of the activity 

 of the roots; and from these measurements it was easy to calculate the 

 distance and intensity of the sapping effect of the tree roots. The greatest 

 benefits arising from windbreaks are believed to result from reduced evapora- 

 tion, due to reduced motion of air currents. To measure the intensity of 

 evaporation, evaporometers were set up at distances of one, two, and five 

 times the height of the trees on both sides of the windbreaks and in addition 

 at ten and twenty times the height on the leeward side. 



The study of the shading effect revealed the fact that cotton wood is least 

 damaging to crops and honey locust most, with boxelder, willow, mulberry 

 and Osage orange following clase behind the honey locust. Cottonwood also 

 has least extent of roots in proportion to its height, and hence has least 

 sapping effect on the soil. It was found that all kinds of trees planted in 

 rows oriented east and west do less damage to crops than when planted in 

 rows oriented north and south and on the other hand trees planted in rows 

 north and south grow faster than in rows east and west. Rows running 

 north and south absorb more light than rows oriented east and west. Alfalfa, 

 corn, and kaffir corn are damaged less by shading than other crops. Damages 

 from shading may be lessened by planting in the shaded area forage crops 

 whose value does not depend upon ripening of their seeds and by planting 

 in the windbreak narrow crowned trees. Sapping effect can be reduced by 

 cultivation to retard evaporation, by deep plowing to cut off the side roots 

 of the trees and by improvements in the fertility and permeability of the 

 soil. Uncultivated Osage orange hedges extend their roots 60 per cent farther 

 than cultivated. 



The theory of the farmers that trees impoverish the soil as far as their 

 roots extend was tested by a number of soil analyses. The analyses showed 

 that there was less available nitrogen in the zone of greatest root activity 

 than in the open field, but this reduction corresponded in position with the 

 zone of least moisture in the soil. Since the total nitrogen in the soil per- 

 meated by the most active tree roots did not appear to be any less than that 

 found out beyond the influence of the trees, it was concluded that the reduced 

 moisture content had retarded bacterial action and prevented the trans- 

 formation into available nitrogen of the nitrogen compounds of the soil. The 

 deficiency of available nitrogen probably results measurably in temporary 

 sterility of the soil. 



