For forage yields and fertility management.

From the top, there is no such thing as a clean trend line of yield to element percentage or ppm.

There is a scattered effect in all the trend graphs, and they by eye or with math come up with a nice clean looking trend line.

Phosphate is a good element to look at because it has a steep increasing trend line, a limited critical concentration and then a fairly steep deceasing yield trend with the increasing percentages of phosphate.
I suggest the yield changes will be undetected from 0.384% on the high side and 0.256%.
In YOUR case I suggest there is no yield drag from inadequate phosphate but there may be some yield drag due to excessive phosphate in at least two samples.
It is possible that sample S40 may be right on the critical nutrient concentration for phosphate but there may be some yield drag from a lower nitrogen concentration.


Then to add to the confusion potassium has a wide and conditional trend line that varies with climate conditions. For alfalfa from 1.70% K to maybe 4.00% K might not have a yield drag. Include less than optimum moisture conditions and the optimum yield may be more in the 2.50% K to 3.50% K range.


My elementary knowledge level and limited mathematics proficiency precludes looking at the big picture and determining more than one limiting factor.

There is a system, DRIS, which looks at a series of ratios, trying to get around the one at a time deficiency discoveries.

HERE in the 1980's I discovered a copper deficiency. The early 1990's that led to the discovery of a Molybdenum deficiency.
Next the real potassium deficiency became apparent.
The potassium deficiency turned out to be more than a simple soil chemistry situation.
The obscenely high 50 CEC plus the type of clay involved just added to the complexity. 
Soil analysis did not alert us to these fertility problems.

There is more and the discovery processes is on going.
We never did get into variety differences within a crop selection.

It too bad Soil Life was run off, with his different approach he has made many worth while suggestions.