7000


Rice
kg/ha P
 / -P=80
 P= 20

 6000
 0=0






 5000







 4000
 0 50 100 150 200 250 300

 N kg/ha



FIG.VI-10. Smoothing the surface with levels of
 phosphorus.

points, as illustrated for P = 40 in Fig. VI-10. The
contours in Fig. VI-9 are then smoothed by taking the N
and P values from Fig. VI-10 for each contour level. Fig.
VI-11 shows the smoothed contours.
 The graphic response surface in Fig. VI-11 can be
used for economic interpretation. Consider the price of P
as $2.00 and the price of N as $4.00. Budget lines can be
constructed as in Fig. VI-11. The first budget line (the
furthest to the left) represents a cost of $200. If a
farmer has only $200 to spend for fertilizer, he can
purchase any combination of N and P along the $200 budget
line. The highest yield which can be achieved with $200
of fertilizer is approximately 4950 kg/ha. This would be