Player loses momentum when landing

Hey! C# and Unity amateur here, happy to post my first question on the forum.

I'm making a small 2D sidescroller game and I have an issue with my player movement. After a lot of tinkering, everything seems to work well other than one thing: When the player lands after a jump, they lose their momentum instead of keeping it.

How can I preserve the player's momentum after they land, so that they immediately continue moving forward if they hold the directional key?

Here's my code. Thanks for your help! using UnityEngine; using System.Collections;

 public class playerController : MonoBehaviour {
 
     public Rigidbody2D player;
     public GameObject playerObj;
     public Animator playerAnim;
 
     //For collision checking
     public BoxCollider2D floorColl;

     // Use this for initialization
     void Start () {
         
     }

     // Update is called once per frame
     void Update () {
 
         //Move right
         if (Input.GetKey("right") && !InAirChecker())
         {
             gameObject.transform.localScale = new Vector3(1, 1, 1);
             playerAnim.Play("Run");
             if (player.velocity.magnitude < 2.5f)
             {
                 player.AddForce(new Vector2(15, 0), ForceMode2D.Force);
             }
         }
         if (Input.GetKeyUp("right") && !InAirChecker())
         {
             playerAnim.Play("Idle");
             player.velocity = new Vector2(0f, 0f);
         }
         //Move left
         if (Input.GetKey("left") && !InAirChecker())
         {
             gameObject.transform.localScale = new Vector3(-1, 1, 1);
             playerAnim.Play("Run");
             if (player.velocity.magnitude < 2.5f)
             {
                     player.AddForce(new Vector2(-15, 0), ForceMode2D.Force);
             }
         }
         if (Input.GetKeyUp("left") && !InAirChecker())
         {
             playerAnim.Play("Idle");
             player.velocity = new Vector2(0f, 0f);
         }
         //Jump!
         if (Input.GetKeyDown("up") && !InAirChecker())
         {
             player.AddForce(new Vector2(0, 5), ForceMode2D.Impulse);
             playerAnim.Play("Jump");
         }
 
     }
     bool InAirChecker()
     {
         bool inAir;
         if (player.IsTouchingLayers())
         {
             inAir = false;
         }
         else
         {
             inAir = true;
         }
         return inAir;
     }
 }