A preliminary survey of topsoil depth was carried out
at about 40 points in the ®eld in mid-July to see if this
parameter might have in¯uenced crop production and N
yield. The results (not given) indicated that the areas of
highest DM and N yields were more or less coincident
with those of deepest (>300 mm in depth) surface ÔAÕ
horizons, whereas areas of lowest production were co