If I understand/assume correctly, the POINIT is posted when retrieving the data when populating the grid, but not when editing or adding the data? “postData” property will not post it when editing, adding or deleting via the navGrid CRUD functions. For that you need to append it to the editUrl as in editUrl: myUrl + '?POINIT=' +$('#POINIT').val();

When you say it works outside of the grid, I am not sure how you or where you tested this, so try and do a console.log with Firebug or alert to show the value of POINIT just before the grid is defined, something like:

console.log($('#POINIT').val());

$('#list').jqGrid……