stated previously, this value of nitrogen can be found by
taking the derivative of the response surface and equating
it to zero. The derivative of the quadratic function is

 dY/dN = 12.25 0.0455 N

This equation is then set equal to zero and solved for N.
In this case, N = 270 kg/ha, which will result in a


 7500-



 7000- 0 .



 6500-
 Rice
 kg/ha .


75 150


225


300


Nitrogen kg/ha


FIG.VI-5. Estimated quadratic surface.