114 FUNDAMENTALS OF FRUIT PRODUCTION 



The likelihood of trees under one of the two standard systems of 

 orchard culture suffering from lack of available nitrogen and, on the other 

 hand, the nearly absolute certainty that under the other system the 

 soil will have its total nitrogen I'eserve seriously depleted, suggest 

 that a combination of the two methods possibly might afford a means of 

 maintaining permanently the nitrogen supply of the soil and at the same 

 time obviate the necessity of supplying the trees artificially with readily 

 available nitrates. Such a combination might consist in alternating 

 sod and cultivation each in 2-year periods or, better still, in maintaining 

 alternate tree rows, the "middles," under the two respective systems 

 and then occasionally reversing the treatments on these alternate strips. 

 The marked success that frequently has attended such a combination is 

 evidence of its practicability under many conditions. Such a combina- 

 tion treatment is a compromise also in its influence upon soil moisture 

 supply and soil erosion. In some instances it might prove undesirable 

 because of the increased difficulty in controlling certain orchard pests 

 which are best held in check by cultivation. 



Few, if any, of the plant nutrients obtained from the soil are subject 

 to such great variation from season to season and even from week to 

 week as is nitrogen; likewise few are so completely under the control of 

 the grower through methods of soil management. It is largely because of 

 the first two facts that the problem of maintaining fertility in the orchard 

 generally centers around the nitrogen supply. The discussion that has 

 preceded serves also to bring out clearly the fact that proper treatment 

 of the soil may reduce or altogether remove the necessity for nitrogen 

 fertilization and that, on the other hand, there are instances where it may 

 be true economy not to employ those practices that will lead to greatest 

 nitrate formation but deliberately to limit this process and supply the 

 deficiency by artificial means. 



Nitrogen Fixation. — Nitrogen gas is not available to the higher plants, 

 but it is acted upon by nitrogen-fixing bacteria which convert it either 

 to nitrates or to other nitrogenous compounds that in due time are con- 

 verted into nitrates. Some of these bacteria are able, independent of any 

 association with the roots of higher plants, to fix this atmospheric nitro- 

 gen and thus effect the first step in rendering it available. ^^' ^^ Indeed 

 there are conditions under which their activity is so great that the resul- 

 tant accumulation of nitrates renders the soil toxic to trees and other 

 plants.^' For the most part, however, nitrogen fixation by bacteria 

 is effected by forms living in colonies on the roots of leguminous plants 

 where they produce nodules or tubercles. 



As very few of the species bearing edible fruits belong to the legume 

 family, nitrogen-fixing bacteria are of comparatively little direct benefit 

 except when they fix nitrogen in the absence of host plants. However, 

 they become of great value indirectly when leguminous cover crops or 



