Can't detect isgrounded? Help

I am working on movement animations and my walk animation still plays if i am falling and i press w or s. How can i make a var isgrounded be true when i'm on the floor? I am not using the character controller.. Here is my animation script with my attempt at a isgrounded detection.

 var isMoving : boolean;
 var isgrounded : boolean = false;
 var isSprinting : boolean;
 
 function OnCollisionEnter(col : Collision) {
     if (col.gameObject.tag == "floor") {
         isgrounded = true;
     }
 }
 
 
 function Update () {
 
 if (Input.GetKey("w") || Input.GetKey("s"))
 {
  
     isMoving = true;
  
  animation.CrossFade ("walk");
     } else {
     
  isMoving = false;
  animation.CrossFade("idle");
        }
 
 
 if (Input.GetKey("a"))
 {
  
 
  
  animation.CrossFade ("strafeLeft");
    }
 
 if (Input.GetKey("d"))
 {
  
 
  
  animation.CrossFade ("strafeRight");
    }
 
 
 if (Input.GetKey("left shift") && isMoving)
 {
  
     isSprinting = true;
  
  animation.CrossFade ("run");
    }
 }