Codeigniter HTML Form is Null on form submit

I am not able to pass the form values to codeigniter controller. I can't predict what I am doing wrong here. My View code is here:

                    <form method="POST" accept-charset="utf-8" action="<?php echo site_url();?>/Admin/admin_login">
                        <div class="form-group">
                            <label>Email address</label>
                            <input type="email" class="form-control" value="deepak" placeholder="Email">
                        </div>
                            <div class="form-group">
                                <label>Password</label>
                                <input type="password" class="form-control" value="2321" placeholder="Password">
                        </div>
                                <div class="checkbox">
                                    <label>
                                <input type="checkbox"> Remember Me
                            </label>
                                    <label class="pull-right">
                                <a href="#">Forgotten Password?</a>
                            </label>

                                </div>
                                <label><input type="submit" class="btn btn-success btn-flat m-b-30 m-t-30">Sign in</label>
                                <div class="social-login-content">
                                    <div class="social-button">
                                        <button type="button" class="btn social facebook btn-flat btn-addon mb-3"><i class="ti-facebook"></i>Sign in with facebook</button>
                                        <button type="button" class="btn social twitter btn-flat btn-addon mt-2"><i class="ti-twitter"></i>Sign in with twitter</button>
                                    </div>
                                </div>
                                <div class="register-link m-t-15 text-center">
                                    <p>Don't have account ? <a href="#"> Sign Up Here</a></p>
                                </div>
                    </form>

and my controller is :

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Admin extends MY_Controller 
{
    public function index()
    {
        $this->load->view('admin/login');
    }
    public function admin_login()
    {
        //print_r($_POST);
        if($this->input->post('submit') != NULL )
        {
            echo 'deepak is here';
        }
    }
}

I am surprised HTML Form POST is null! Can you suggest a solution?