how to automatically update column based on the second layer, trigger, postgreSQL

I am newbie in postgreSQL. I have 2 tables, observation and landcover, i want to create a trigger which would allowed to update the column observation.land based on the spatial information according to the second layer (table) landcover.code_06.

I mean, If I update point layer observation in QGIS , I want to have in the attribute table same information in column LAND as in the column CODE_06.

I've created this, but i have no idea, what's wrong or why it doesn't work.

ALTER TABLE observation ADD COLUMN land varchar (20)

CREATE OR REPLACE FUNCTION landinfo_land()
  RETURNS trigger AS
$BODY$
  BEGIN
    NEW.land := observation.land

                  FROM observation.land AS b, landcover.code_06 AS  w
          WHERE w.geom && NEW.geom AND
ST_Intersects(ST_PointOnSurface(NEW.geom), b.geom);
    RETURN NEW;
  END;
$BODY$
  LANGUAGE 'plpgsql' VOLATILE
  COST 100;
ALTER FUNCTION landinfo_land() OWNER TO postgres;


CREATE TRIGGER update_land_value
  BEFORE INSERT OR UPDATE
  ON observation
  FOR EACH ROW
  EXECUTE PROCEDURE landinfo_land();