drawn visiographically (Fig. VI-9). Unfortunately, a
zero-N, zero-P treatment was not included in this trial,
so no value is available for the origin. However, with
care, reasonably smooth contours can be drawn. In this
example, contour levels of 5000, 5500, 6000, 6200, 6400,
6600, 6800, and 7000 were drawn. It appears that the
maximum yield level will approximate 7000 kg/ha.


P205
(kg/ha)


 0 50 100 150 200 250 300
 N (kg/ha)


FIG.VI-9. Response surface estimated visually.



 The contours in Fig. VI-9 can be smoothed by another
simple process (Fig. VI-10). Slices of the response
surface in Fig. VI-9 are taken for several levels of P.
These represent single variable response surfaces for N.
The points estimated from the surface are plotted as in
Fig. VI-10, and then a smooth curve is drawn through the