Clay content is a laboratory number applied to a surface that keeps changing

A percentage measured on a dried sample tells you about the soil and much less about the pitch it turned into.

Forty per cent clay. Now tell me how it will play.

You cannot, and the gap between those two things is where most pitch analysis goes wrong. Clay content is determined by taking a sample, drying it, dispersing it and measuring particle size fractions. It is a property of the material. What a batter faces is a property of a structure: how that material was rolled, at what moisture, how many times, over how many seasons, with what organic matter left in it and what root mass running through it.

Same clay percentage, two curators, two entirely different surfaces. This is not a marginal effect. Rolling schedule and moisture at rolling probably do more to decide bounce than the fraction of fine particles does.

So we have a measurable input standing in for an unmeasurable outcome, which is the classic proxy trap. Proxies are adopted because they are available, then they get treated as though they were the thing itself, and eventually people start setting targets on them. Groundstaff have been handed clay content specifications by governing bodies. That is a target on a proxy, and it will produce compliance with the number rather than the quality the number was supposed to represent.

The honest version is that clay content constrains what a pitch can become without determining it. High clay makes a hard, cracking surface possible. It does not make it happen. Low clay makes that surface very difficult to produce regardless of skill. Constraint, not prediction.

What would actually describe playing character is a direct measurement of the finished article, and those exist. Hardness readings taken with a drop hammer, surface friction, ball rebound tests from a fixed height. They measure the structure rather than its ingredients, and they can be taken on the morning of a match rather than months earlier from a laboratory.

They are also unpopular, because a rebound figure can be checked against what happens, and a soil composition report cannot really be checked against anything at all.

I would rather see three ball rebound heights and a hardness number in a match preview than any amount of soil chemistry. One of those describes the pitch. The other describes a bag of dirt that a skilled person subsequently turned into a pitch, and the skilled person is doing more of the work than the analysis admits.