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Garden with Insight v1.0 Help: Plant next day functions: calculate general growth constraint

The general growth constraints are: temperature, aeration, water, nitrogen, and phosphorus. The temperature and aeration growth constraints were calculated earlier in the next-day routine because they were needed by other functions. The remaining constraints are calculated here.

The water stress growth constraint (or water stress factor) is simply the ratio of actual plant water use today to the potential plant transpiration, which was the first estimate of plant water uptake. The ratio is limited to the range from zero to one.

The nitrogen and phosphorus growth constraints are derived from a common S curve that describes the effect of N or P deficit on new growth. The X value for the S curve is the ratio of the amount of N or P in the plant to the amount of N or P that would be in the plant if it had the optimal concentration of N or P for this point in the growing season. (Remember that the optimal concentrations of N and P change over the growing season using a S curve based on heat unit index.)

The combined general growth constraint is the smallest (worst) of the five general growth constraints. Using the worst of two or more growth factors is called "Leibig's law of the minimum" after Justus von Leibig who observed in 1862 that the worst factor affecting growth tends to limit growth regardless of the other factors. This approach is also called the "barrel stave concept" because if the staves of a barrel (the vertical boards) are all of different heights, only the height of the lowest stave determines whether water runs out. And for yet another metaphor, "a chain is only as strong as its weakest link."

calculation of general temperature growth constraint, aeration growth constraint
EPIC Constraints
Model contents

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Updated: March 10, 1999. Questions/comments on site to
Copyright © 1998, 1999 Paul D. Fernhout & Cynthia F. Kurtz.