Textarea POSTing NULL when using jQuery

Question

I am saving a load of information to a database, and using jQuery to alert either a success or fail message, followed by clearing the form so the user can input a new set of data without leaving the page. My textarea posts a NULL value when I use the jQuery script, but works perfectly when I don't use the script. All other inputs in the form work just fine, and changing the text area to an input type = text also works.

<form name = "usemarkscheme" id = "usemarkscheme" method ="POST" action ="submitmarks.php">
<a few inputs here that all work>
<textarea name = "add" placeholder = "Enter additional comments here"></textarea>
</form>

This is the jQuery file:

$(document).ready(function () {
    $("input#submit").click(function () {
        $.post($("#usemarkscheme").attr("action"), $("#usemarkscheme: input").serializeArray(), function (data) {
            if (data === "This student has already received a mark for this mark scheme!") {
                alert("This student has already received a mark for this mark scheme!");
            } else {
                alert("Marks entered");
            }
        }
        //close $.post                
        );

        $("#usemarkscheme").submit(function () {
            return false;
        })

        //close onclick       function

        //clear form after every student has been marked
        $("#usemarkscheme").each(function () {
            this.reset();
        });
    });
});

The error messages work well

Answer 1

Maybe

$("#usemarkscheme: input").serializeArray()

is fetching only the input fields. Try adding the textarea selector too. Try using

$("#usemarkscheme input, #usemarkscheme textarea").serializeArray()

or even better

$("#usemarkscheme").serializeArray()

Answer 2

The simply problem is you use $("#usemarkscheme: input") and this will give you only input fealds not the textarea.

If im not wrong you have to try $("#usemarkscheme: input, #usemarkscheme textarea")