SPECIAL FARM TOPICS 217 



fertilizers and the adoption of more intensive methods. It is a 

 happy circumstance also for all dark types of tobacco and these are 

 types grown on soils most likely to be troubled with these pests 

 that the repellent crop is so desirable and valuable otherwise. 



The feeding value of cowpea hay is very great, and the stubble 

 adds much to the fertility of the soil. Increased quantities of am- 

 monia tend to darken tobacco, and in cases where brightness is an 

 important consideration, as in the bright-tobacco belt of southern 

 Virginia, North Carolina, and South Carolina, the cultivation of 

 cowpeas preparatory to tobacco might be objectionable. 



The Effect on the Soil of Crops Available for Rotation. The 

 selection of standard crops generally regarded as adapted for cultiva- 

 tion on most tobacco farms in the export and manufacturing dis- 

 tricts under consideration may be divided into four main classes: 

 (1) The inter-cultivated crops, corn and tobacco; (2) the small 

 grains, particularly wheat and oats; (3) the grasses, such as redtop 

 and timothy; and (4) the legumes, including the clovers, vetches, 

 and cowpeas. 



In producing the inter-cultivated crops corn and tobacco 

 the soil should be deeply broken and thoroughly aerated and pulver- 

 ized. This is highly desirable, but, by hastening nitrification, oxida- 

 tion, and decay, it serves also to use up the humus supply more rap- 

 idly. In the cultivation of the small grains, like wheat and oats, 

 the soil is not so thoroughly and deeply broken and aerated. These 

 crops are not so hard on the humus supply as tobacco and corn, but 

 they are exhaustive rather than recuperative in their effect. The 

 true grasses are similar to the small grains in that their cultivation 

 is not attended with deep breaking and aerating of the soil and their 

 entire plant-food requirements are extracted from the soil itself. 

 There is this marked difference, however, in their effect upon the 

 fertility of the soil : They occupy the soil continuously for two or 

 more years, have a dense root system, and form a good sod, which, 

 when turned under, adds materially to the soil's supply of humus, 

 and during the period of their occupancy they hold the soil against 

 washing and leaching. For these reasons they are to be classed as 

 distinctively soil improvers. 



The perennial clovers, such as red, sapling, and alsike clover, in 

 certain respects might best be classed with the true grasses, but they 

 have the additional advantage of being leguminous plants and are 

 able to supply their requirements for ammonia from the air through 

 the aid of the colonies of bacteria living symbiotically in the nodules 

 which they form upon the roots. Their deep taproots also have a 

 valuable effect not produced by the grasses in opening up and aerat- 

 ing the subsoil and in bringing up from the subsoil and utilizing 

 plant-food material not accessible to many other classes of plants. 

 The perennial clovers are often grown with the grasses, and when so 

 grown the combination probably has few superiors in building up 

 the fertility of the soil. The annual legumes, such as cowpeas, crim- 

 son clover, and vetch, also fix atmospheric nitrogen through the aid 

 of bacteria on their roots, and the long taproots have a favorable ao- 



