The Estimative Index
by Thomas Barr

I was responding to someone about dosing recently and came up with a clearer (I hope!) way to explain this idea and method. It's long but I try to cover most of the bases.

The Problem

#1 - Dosing

  This can be very tricky when dealing with many variables. Often the suggestion is "buy a test kit" and test to see what your nutrient levels are. This works well for CO₂ (but folks should double-check to be sure before proceeding) and GH but the other nutrients like NO₃, K, PO₄, Fe as a proxy for the traces are more problematic. Chuck Gadd's dosing calculator works well for the chemistry challenged.

#2 - Testing

  This is huge issue for most folks. Test kits cost as much as a filter, or much more in some cases. Some folks can afford nice Lamott/Hach kits, most cannot nor wish to invest $300 in this. Cheaper kits are not offered for K. NO₃ kits are very problematic and color reading scales are difficult to assess with cheaper kits. Some folks are color blind. Many folks don't ever want to test or feel there's no need to test. I fell into that group for many years. I did as well as I do today but I am much more consistent now and I also know why it works! Iron (Fe) tests have been a very contentious issue for me for about 3 years now. I believe I have a relatively simple methodology to sidestep much of the drudgery, especially with testing iron and NO₃.

Color reading scales are difficult to assess.

  At issue here is maintenance of nutrient levels. The focus will be on four groups, nitrate (NO₃), phosphate (PO₄), potassium (K) and the trace elements represented by iron (Fe) in a mixture with the other trace elements. Perhaps a better question is how close to a good range of nutrients do we have to be in to have excellent plant growth and no algae?

Using an "estimative index" the accuracy can be as follows:

(+ or -) 5ppm of CO₂ is fine in a 20-30ppm range.
(+ or -) 1ppm or so of NO₃ is pretty reasonable.
(+ or -) 2ppm of K+ is pretty reasonable.
(+ or -) 0.2ppm of PO₄ is pretty reasonable(?).
(+ or -) 0.1ppm of Fe is reasonable(?).

CO₂ range 20-30ppm
NO₃ range 5-10ppm
K+ range 20-30ppm
PO₄ range 0.4-1.0ppm
Fe .5ppm or higher(?)

  PO₄ and Fe are two nutrients that are difficult to assess without first assessing the other nutrients. If the NO₃, K, and CO₂ are in good shape, you can add a fair amount of these within a wide range. I have added up to almost 2ppm of PO₄ consistently week after week. The plant's response is incredible. Adding traces has been a focus for me lately. Many have stuck with the old standby of a residual of 0.1ppm of iron. Well what does this residual tell us? Does it tell us what is available to the plants? Is this enough? Does higher dose when the other nutrients (NO₃ and PO₄) are present cause algae?