Barak offers a good explaination of uptake measurements and the math associated with the theory behind EI.

http://www.soils.wisc.edu/~barak/soilscience626/mm3.htm

Plants can take up and store these nutrients in their vacoules over time. This allows plants to grow well in non stable systems in terms of nutrient supply.

Many aquarist assume that aquatic plants prefer and exist in stable systems.
Even if these systems are provided and great care is spent on maintaining a residual level of a nutrient, the levels can vary greatly over a short time frame in an aquarium.

These levels are moving targets.
When aquarist test these relationships, they need to test at small time intervals to see what type of curve is expressed, such as in the above example. Seldom are these relations a linear line in uptake.

One or two measurements do not tell you much and do not allow you to predict future uptake, thus predict or estimate the future dosing routine.

Plants also possess dual and multiple enzymes that are very good at taking in larger high concentrations of nutrients when abundant, and have very efficicent low level uptake enzymes when the nutrients in limiting and at a low concentration.

This is why we can see plants doing well in a wide variety of systems.

But the issue for EI is that it mitigates the need for testing by estimating the needs of the plants. Given plants ability to modulate their uptake capacity and rate, little is gained by attempting to finely focus on one particular level or uptake rate.

The rate changes through time and with respect to the storage of the luxury consumption.

Regards,
Tom Barr