If you need to add a new Column (a SPField object) into a SharePoint List programmatically you wil notice that using this snippet
SPList list = SPContext.Current.Site.OpenWeb().Lists["SampleList"];
SPField newField = list.Fields.CreateNewField(SPFieldType.URL.ToString(), "FieldName");
list.Fields.Add(newField);
list.Update();
will only add a new SPField to the default Content Type.
If you need to add this field to all Content Types, consider using the following code snippet:
SPList list = SPContext.Current.Site.OpenWeb().Lists["SampleList"];
SPField newField = list.Fields.CreateNewField(SPFieldType.URL.ToString(), "FieldName");
list.Fields.AddFieldAsXml(newField.SchemaXml, true, SPAddFieldOptions.AddToAllContentTypes);
list.Update();
Now your new SPField named FieldName is visible in all your content types.