Oracle Apex item Item Field can not non-numeric jQuery.

Create a dynamic action named with initialization.

Event : page load.

now go to the target item .then go to the properties section.

From advance section >CSS clases >give the class name     only-numeric

$(".only-numeric").bind("keypress", function (e) {

var keyCode = e.which ? e.which : e.keyCode               

if (!(keyCode >= 48 && keyCode <= 57)) {

  return false;

}else{

  return true;

}

});