I want Ball to Dont jump while in air,only jump when i press jump button and ball is touching the plane

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {
Rigidbody r=new Rigidbody();
float rotationSpeed;
Vector3 jumpHeight;

void setRotationSpeed(float speed){
	rotationSpeed = speed;
}
Vector3 getJumpHeight(){
	return jumpHeight;
}
void setJumpHeight(float x,float y,float z){
	
	jumpHeight = new Vector3 (x, y, z);
}
// Use this for initialization
void Start () {
	setRotationSpeed (20);
	setJumpHeight (0.0f,5,0.0f);
	r = GetComponent<Rigidbody> ();

}

// Update is called once per frame
void Update () {

	float moveHorizontal = Input.GetAxis ("Horizontal");
	float moveVeritcal = Input.GetAxis ("Vertical");
	Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVeritcal);
	r.AddForce(movement*rotationSpeed);
	if (Input.GetButtonDown ("Jump")) {
		
		r.velocity=jumpHeight;

	}

		


}

}

make a bool that is like _isGrounded or something. then when you jump set it to off, and when its not midair set it true.
Then

if ((Input.GetButtonDown (“Jump”)) && (_isGrounded)) { r.velocity=jumpHeight; }
else return;

somthing like that should help you out