sequences are common. When typical farmer practices are
to be included, a superimposed field trial may be
appropriate. On the other hand, when alternatives are
dramatically different from typical practices,
conventional field trials should be defined. A split-plot
arrangement can be appropriate when working with more than
one variable. For example, when one variable requires
different row arrangements, or if there is a large border
effect and the experimental unit size is large, it can be
assigned to the main plot. The other variables, such as
planting distance, varieties, or secondary crop
alternatives can then be assigned to the subplots.
Precision will be greater for the variables in the
subplots because more degrees of freedom are associated
with subplot than with main plot error. An economic
interpretation of these types of trials is mandatory since
the crops involved generally have different market values,
making yield relatively less important (see Chapter VII).

Plant Nutrition

 Fertilizer trials are commonly conducted as
site-specific experiments. Information on soil
characteristics, previous management, and soil analysis
should be determined before locating the experiment.
Generally, at least three levels of each factor should be
considered in order to estimate a response curve.
Experimental designs should allow for measurement of
interaction effects which are common in fertilizer trials.
Factorial designs arranged in randomized complete blocks
(RCB) offer a better estimate of interactions among
factors than split-plot arrangements. The reason is that,
in analysis of variance for the RCB design, the error mean
square (MSE) is estimated with more degrees of freedom.
The split-plot design has the same number of degrees of
freedom for interactions as the RCB, but the residual
degrees of freedom have to be distributed between the main
plot error and the subplot error.
 Special care must be used in field design to avoid
fertilizer runoff effects from adjoining plots. Border
rows or ample distance should be considered between
experimental units. When the local practice is not to use
fertilizer, the check plot should reflect that practice.
When farmers' practices include some fertilizer use, the