There is no law or rule that states that only radiologists can do ultrasonography. The kidney is one of the easiest abdominal organs to image. There is little anatomic variation and the range of pathologies is small. They're your patients. Renal biopsy is a nephrology procedure and since it requires ultrasound guidance, nephrologists need to be performing ultrasonography and have access to equipment.
Training is available.
Sonographic evaluation of renal failure. Although these are distinct components of ultrasonography, they are interelated. One must be a competent scanner in order to interpret studies and the best studies are ones in which interpretation was done during the scanning. The Renal Division at Emory University School of Medicine sponsors a range of hands-on training events, often in conjunction with national nephrology organizations. These are designed specifically for nephrologists, and are the only programs devoted specifically to renal ultrasonography.