2.4. Statistical analysis
The results were analysed in R using response surface
modelling (RSM) based on a central composite design (Lenth,
2009). The RSM design consisted of a full factorial design for
the two variables (N and P), with five levels for each variable
and three replications for the centre points. The calculated
second order model included the first order terms, the twoway
interaction term and the quadratic terms of the variables
N and P. The obtained quadratic polynomials for the
different response variables were visualised in contour plots.