Table 2. ANOVA Results for Analyses by Response Surface Quadratic Models for the Slow-Release Nitrogen Fertilizer^a.

source	sum of squares	mean square	F	Df	p	significance
model (Y1)b	4820.06	344.29	34.27	14	<0.0001	*
X1	1766.41	1766.41	175.83	1	<0.0001	*
X2	539.69	539.69	53.72	1	<0.0001	*
X3	354.99	354.99	35.34	1	<0.0001	*
X4	305.22	305.22	30.38	1	<0.0001	*
X1X2	93.57	93.57	9.31	1	0.0081	*
X1X3	362.39	362.39	36.07	1	<0.0001	*
X1X4	388.23	388.23	38.65	1	<0.0001	*
X2X3	763.9	763.9	76.04	1	<0.0001	*
X2X4	201.41	201.41	20.05	1	0.0004	*
X3X4	9	9	0.9	1	0.359	NS
X12	23.61	23.61	2.35	1	0.1461	NS
X22	1.48	1.48	0.15	1	0.7069	NS
X32	12.67	12.67	1.26	1	0.2791	NS
X42	0.47	0.47	0.047	1	0.8318	NS
residual	150.69	10.05	----	15	----	----
lack of fit	118.44	11.84	1.84	10	0.2606	NS
pure error	32.25	6.45	----	5	----	----
cor. total	4970.74	----	----	29	----	----

^aSignificant (*), not significant (NS), not available or applicable (----).

^bY₁: r² = 0.9697, Adj-r² = 0.9414, pre-r² = 0.8534.