Market Depth

The most useful thing to establish about depth is what it is not, and the nearest neighbour is tightness — the gap between the best bid and the best offer. Tightness is highly visible and easily reported, which is why it is so often taken as the measure of a market’s tradability. But tightness describes the cost of transacting a small quantity immediately, and says nothing at all about size.

The two properties are logically independent, and all four combinations occur. A market can be tight and deep, which is what a well-functioning liquid market looks like. It can be wide and shallow, which is unambiguously difficult. It can be wide and deep — expensive to enter but able to absorb size. And it can be tight and shallow, which is the dangerous combination, because it looks like the first and behaves like the second as soon as any quantity is involved.

The consequence for reading market information is direct: a quoted price and a narrow bid–offer gap describe a market at the smallest transactable size. Neither is evidence about what a meaningful quantity would cost, and treating them as though they were is one of the more common ways to be surprised by a market.

Depth describes what is available now. It contains no information about what would be available in a minute, after a large transaction, or under different conditions. This makes it a genuinely useful observation about the present and a poor basis for any inference about anything else.

The dimension it cannot see is resilience — whether the depth consumed by a transaction would be replaced. Two markets with identical depth can behave completely differently: in one, a large transaction takes out the available interest and fresh interest arrives promptly, so the market recovers; in the other, the same transaction takes out the interest and nothing replaces it, so the price simply stays where the transaction left it. Depth is identical in both, and the outcome is not.

• Observed depth is what is visible at a moment, which is not necessarily all the interest that exists — parties may hold interest back rather than display it
• Displayed interest can be withdrawn, and it can be withdrawn faster than a transaction can reach it, particularly when conditions change
• Depth at the best price and depth a little away from it are different measures, and a market can have one without the other
• Depth in a nearby delivery period says nothing about depth in a deferred one, and thin deferred periods are common in agricultural markets
• In bilateral physical trade there is no observable depth at all, because interest is private — which does not mean the concept does not apply, only that it cannot be measured

The vocabulary of depth comes from organised markets with visible interest, but the property it names exists everywhere, including in physical trade where nothing is visible at all. A producer selling into a local market faces a depth question every time they sell: how much can this set of buyers absorb before the price they are offered starts to fall? The question is identical; only the observability differs.

In physical agricultural markets, depth is bounded by real things rather than by displayed interest — by how much storage exists locally, how much processing capacity is running, how much transport is available to move surplus out, and how many buyers are within reach. A local market’s depth is therefore a physical and infrastructural fact as much as a commercial one, and it can be exhausted absolutely: when storage is full and no transport is available, no price finds a buyer, and the market has no depth at any level.

This is also why harvest is when depth matters most and is least available. Every producer in a region wants to sell the same commodity in the same weeks, into a fixed local capacity. The quantity seeking a buyer rises sharply while the capacity to absorb it does not, and the market’s depth is tested exactly when the greatest quantity is pressing against it.