Busscher W., Lipiec J., Bauer P., Carter T.:         
    
        Improved root penetration of soil hard layers by a selected genotype.  (2000 r.)   
        
        Communications in Soil Science and Plant Analysis 
                
                
            2000, 31, (19&20), 3089-3101 
                                 
    
     
            
         Busscher W., Lipiec J., Bauer P., Carter T.:         
    
        Root penetration of artificially compacted hard layers.  (1997 r.)   
        
        Bibliotheca Fragmenta Agronomica 
                
                
            1997, 2A/97, 119-122