18 Diseases of Truck Crops 



Besides oxygen, the nitrifying organisms demand, 

 as an indispensable condition for work, a sufficient 

 moisture in the soil. In dry soils and during dry 

 weather, nitrification is almost suspended within the 

 upper layers of soil. A third important factor is the 

 chemical reaction of the soil. The nitrifying organ- 

 isms work best when the soil gives a slight alkaline 

 reaction. Too much alkalinity, however, like too 

 much acidity, is detrimental as we shall see further on. 

 Nitrification is further dependent on soil temperature. 

 At 99 degrees Fahrenheit it is at its highest. A de- 

 gree less than 54 F. retards it considerably. At 122 

 degrees F. very little nitrate is produced, and at 131 

 degrees F. nitrification ceases entirely. The physical 

 condition of the soil is another important element to 

 be considered. The highest rate of nitrification is 

 found in truck lands, that is, in the sandy loams. 



Nitrogen Fixation from the Air 



It has been the common knowledge of farmers and 

 truckers that legume plants, such as peas and beans, 

 cause the soil on which they are grown to become 

 more productive. It is not necessary here to enter 

 into an abstract discussion of this phenomenon. 

 Suffice it to say, that science has definitely shown 

 that there is a bacterial soil organism, Pseudomonas 

 radicicola, which is capable of fixing the free nitro- 

 gen from the air. This organism attacks the young 

 rootlets of the legume crops as other parasitic forms 

 also do. Its presence in the root results in a nodule 



