RED CLOVER 399 



long been recognized in Europe, where numerous explana- 

 tions as to its cause have been advanced. The principal 

 theories are : 1. The exhaustion of some necessary element 

 from the soil, in particular lime, potash or phosphorus ; 

 2. The formation or excretion by the clover plant of some 

 deleterious substance ; 3. Unfavorable physical condition 

 of the soil, especially the subsoil ; 4. Presence of disease- 

 forming fungi or bacteria; 5. Injurious insects and other 

 animals ; 5. Depletion of humus content of the soil. 



None of these theories has been proven, but it is not 

 unlikely that there may be some truth in each of them. 



Experience in Europe has shown that good clover may 

 be grown on clover-sick soil if a sufficient interval of time 

 elapse. In Germany this is usually four to six years, 

 but on some soils a period of nine or even twelve years 

 seems necessary. 



It is not certain that the increasing difficulty in secur- 

 ing a stand of red clover in various parts of the United 

 States is the same as the European clover sickness, but 

 this seems highly probable. The evidence indicates that 

 the trouble first became prominent in the Atlantic States 

 and has been slowly extending westward. Even in regions 

 where clover sickness is common, land that has long been 

 uncultivated will often produce good crops of red clover 

 for a few years. Alsike clover, however, grows readily on 

 land " sick " to red clover, and in many places is now 

 substituted for the red. 



Soil acidity has recently been considered to be a cause 

 of failure with red clover, but lime has not proven to be 

 a remedy for the trouble. It has not yet been demon- 

 strated that the European practice of planting red clover 

 at long intervals will be equally successful in this country. 



In England the question has been raised as to whether 



