You are talking of soil fertility limit which I don't think..you can generalise without considering nature of soils and crops in addition to attainable crop yield at a given soil test value.A soil test value designated as optimum at one yield level will become suboptimal at higher yield level.

Assuming that the query is about available forms of the nutrients, have not come across such rating for upper limits of available N, P and K in soils. As plants take up nutrients, they are deposited in the cells of leaves prior to being used by the plant, and ammonium accumulation can damage plant cells. Increasing concentrations of ammonium ions in soil solution may cause ammonium toxicity in some crops (viz., cucumber, foliage ornamentals etc), especially at low temperature. Higher concentrations of NH4, NO3 in soil are reported to cause imbalance in uptake of Cu (wheat, barley), Fe, Zn etc. On the other hand, high K concentration can alleviate NH4 toxicity. Available K in the range of 1000 kg/ha or more had been estimated in field samples without any reported toxicity in plants. However, level of available potassium as high as that mentioned may affect the uptake of Ca and Mg in crops (the link below). In case of P, excess availability in soil is reported to impede Fe and Zn uptake by crop. Would like to have response from others regarding any reported upper limits for the nutrients. The following links may be seen:

http://www.pthorticulture.com/en/training-center/ammonium-toxicity/

http://plantprobs.net/plant/nutrientImbalances/phosphorus.html

https://link.springer.com/article/10.1023/A%3A1020432512980

http://articles.extension.org/pages/74053/high-soil-test-phosphorus-and-potassium-levels-on-a-long-term-organic-farm:-trends-causes-and-solution

This content depends of the cropping system and also of the crop type (trees or annual...) in order to compare the concentration with the crop needs and  also  nutrient uptake. it can be expressed as nutrient use efficiency if you want to give more sense to those values. 

You are talking of soil fertility limit which I don't think..you can generalise without considering nature of soils and crops in addition to attainable crop yield at a given soil test value.A soil test value designated as optimum at one yield level will become suboptimal at higher yield level.

To answer this question, beside cropping system and cultivated crops, we also need the methods you have used, as there are countless methods and most of them gave different results and more general information about your soil, as guideline values might be e.g. soil type specific.

We need a soil test-crop response -based  robust calibration models to deliver such type of information to be effectively utilised in recommending fertilizer doses , instead of   recommending such doses without considering the crop/cropping sequence or the soil as such..

The minimun soil nutrient reserves should be related to the nutrient uptake of the plants you would like to crop.The most important relation is betwen C/Mg/K  in meq/100 gr of 6-3-1

Interesting response.Can we characterize any crop based such type of nutrient ratio being maintained at various critical growth stages? Or ionome composition could be used as s nutritional behavior of a crop?

I wonder about the usefulness of measuring snapshots samples  for plant nutrient concentrations for fertilisation purposes. Instead analysis of crop leafs seems the way to go. For some reasons, especially N and P see attached.

Article Nitrogen and phosphorus limited production of cereals and se...

As an example , i am enclosing two  reviews , how these limits change when same crop is grown under varying agro-pedological conditions.

Thank you all for your wonderful suggestions and data.

 Mr.Ishmael,what methods/extractants you used to estimate the N,P and K concentrations in your soil?What is the soil type/ common name  of soil used and for which crop raising you want to use the soil test data?