Swift: How to make my character in my game only be able to jump once he hits the ground?

I'm developing a game where my character jumps from land to land. I have everything down and done except my remaining problem is if you keep tapping the screen he can keep jumping forever. I want it so he has to hit the ground before he can jump again.

import SpriteKit
import GameplayKit

class GameScene: SKScene, SKPhysicsContactDelegate {
    struct PhysicsCatagory {
        static let Knight : UInt32 = 0x1 << 1
        static let land : UInt32 = 0x1 << 2
    }

    var Knight = SKSpriteNode()
    var land = SKSpriteNode(imageNamed: "CJLand4")

This is my current code that makes him jump:

Knight.physicsBody?.velocity = CGVector(dx: 0, dy: 0)
Knight.physicsBody?.applyImpulse(CGVector(dx: 0, dy: 325))

I have this under touchesBegan and it adds gravity and on impulse it allows him to jump. But I only want him to be able to jump once every time he hits land. Please help if you can.