RESULTS OF MEASUREMENTS. 39 



that there is little basis for such an opinion, for, while all trees, and 

 especially some of the hardwoods, require a certain amount of soluble 

 salts, almost all of this material is returned to the soil when the leaves 

 fall. 



The matter of soil fertility has received veiy little attention in this 

 study. A few samples of the surface soil (depth 1 foot) were taken, 

 however, to determine the effect of the tree roots upon the quantity 

 of the essential salts at various distances from windbreaks. 



No single element is more essential to all forms of plant growth 

 than nitrogen, though phosphorus and potassium are very essential to 

 the formation of seeds. Nitrogen is an important component of salt- 

 peter, of manure, and of many commercial fertilizers. Nitrogen is 

 present in the soil in many chemical combinations, but is available to 

 plants only in the form of nitrates. The quantity of nitrates available 

 in any soil depends not only upon the total amount of nitrogen which 

 may be present, but also upon the physical properties of the soil, its 

 moisture, warmth, and porosity, all of which limit the activity of the 

 microorganisms . 



ANALYSES FOR NITROGEN. 



Soils were analyzed for their total nitrogen and their available 

 nitrates in three fields adjacent to cottonwood windbreaks and in 

 three adjacent to honey locust. Cottonwood is usually considered 

 the most severe drainer of the fertility of the soil. Honey locust, 

 being a legume, should have a decidedly different effect upon the 

 supply of nitrates. The results of analyses for both trees show a 

 smaller amount of available nitrates in the zone permeated by their 

 roots, but do not show any decrease in the total amount of nitrogen in 

 the soil. In other words, there has been no drain upon the store of 

 those substances which go to make up the nitrates for plant food. 



On the north side of a belt of cottonwood trees, the smallest pro- 

 portion of nitrates and the lowest amount of water are found in the 

 same position relative to the windbreak. The conclusion is that the 

 smaller quantity of nitrates is not an indication of their use by the 

 trees, but is due to a deficiency of the moisture on which the activity 

 of nitrifying bacteria is dependent. Exactly the same relation holds 

 in the case of honey locust. In each instance the amount of nitrates 

 is least at the point where the activity of the roots has most 

 markedly reduced the amount of moisture. It can not be said, then, 

 that forest trees reduce the fertility of the soil in nitrogen, although 

 their use of soil moisture may reduce the activity of the nitrifying 

 bacteria and cause temporary sterility in the zone of root influence. 



