Clover yields in a factorial fertilizer experiment

Usage

data("sinclair.clover")

Format

A data frame with 25 observations on the following 3 variables.

yield: yield t/ha
P: phosphorous fertilizer kg/ha
S: sulfur fertilizer kg/ha

Details

A phosphorous by sulfur factorial experiment at Dipton in Southland, New Zealand. There were 3 reps. Plots were harvested repeatedly from Dec 1992 to Mar 1994. Yields reported are the total dry matter across all cuttings.

Source

Sinclair AG, Risk WH, Smith LC, Morrison JD & Dodds KG (1994) Sulphur and phosphorus in balanced pasture nutrition. Proc N Z Grass Assoc, 56, 13-16.

References

Dodds, KG and Sinclair, AG and Morrison, JD. (1995). A bivariate response surface for growth data. Fertilizer research, 45, 117-122. https://doi.org/10.1007/BF00790661

Examples

if (FALSE) { # \dontrun{

library(agridat)

data(sinclair.clover)
dat <- sinclair.clover

libs(lattice)
xyplot(yield~P|factor(S), dat, layout=c(5,1),
       main="sinclair.clover - Yield by sulfur levels",
       xlab="Phosphorous")

# Dodds fits a two-dimensional Mitscherlich-like model:
# z = a*(1+b*{(s+t*x)/(x+1)}^y) * (1+d*{(th+r*y)/(y+1)}^x)

# First, re-scale the problem to a more stable part of the parameter space
dat <- transform(dat, x=P/10, y=S/10)

# Response value for (x=0, y=maximal), (x=maximal, y=0), (x=max, y=max)
z0m <- 5
zm0 <- 5
zmm <- 10.5
# The parameters are somewhat sensitive to starting values.
# I had to try a couple different initial values to match the paper by Dodds
m1 <- nls(yield ~ alpha*(1 + beta*{(sig+tau*x)/(x+1)}^y) * (1 + del*{(th+rho*y)/(y+1)}^x),
          data=dat, # trace=TRUE,
          start=list(alpha=zmm, beta=(zm0/zmm)-1, del=(z0m/zmm)-1,
                     sig=.51, tau=.6, th=.5, rho=.7))
summary(m1) # Match Dodds Table 2
## Parameters:
##       Estimate Std. Error t value Pr(>|t|)
## alpha 11.15148    0.66484  16.773 1.96e-12 ***
## beta  -0.61223    0.03759 -16.286 3.23e-12 ***
## del   -0.48781    0.04046 -12.057 4.68e-10 ***
## sig    0.26783    0.16985   1.577  0.13224
## tau    0.68030    0.06333  10.741 2.94e-09 ***
## th     0.59656    0.16716   3.569  0.00219 **
## rho    0.83273    0.06204  13.421 8.16e-11 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## Residual standard error: 0.5298 on 18 degrees of freedom

pred <- expand.grid(x=0:17, y=0:9)
pred$z <- predict(m1, pred)

# 3D plot of data with fitted surface.  Matches Dodds figure 2.
libs(rgl)
bg3d(color = "white")
clear3d()
spheres3d(dat$x, dat$y, dat$yield,
         radius=.2, col = rep("navy", nrow(dat)))
surface3d(seq(0, 17, by = 1), seq(0, 9, by = 1), pred$z,
          alpha=0.9, col="wheat",
          front="fill", back="fill")
axes3d()
title3d("sinclair.clover - yield","", xlab="Phosphorous/10",
        ylab="Sulfur/10", zlab="", line=3, cex=1.5)
view3d(userMatrix=matrix(c(.7,.2,-.7,0, -.7,.2,-.6,0, 0,.9,.3,0, 0,0,0,1),ncol=4))
  # snapshot3d(file, "png")

close3d()
} # }