A probe sinks easily into a deck post at the ground line while the wood above remains firm. This indicates:

Wood decay fungi need four things at once: wood, moisture, oxygen and a suitable temperature. The ground line on a deck post is the one location where all four converge reliably — the wood wicks moisture from the soil while still having free access to air, whereas wood fully buried is oxygen-starved and wood well above grade dries out. That is why the classic finding is a post that is firm above and punky right at grade, and why probing at that specific height is standard practice. The pattern in this question is diagnostic on its own: sound wood above, soft wood at the ground line, is decay, and the fact that the deterioration is at a load-bearing post makes it a structural finding rather than a maintenance note.

Decay concentrates at the ground line because moisture from the soil and oxygen from the air are both available there, while wood higher up stays drier and wood fully buried lacks oxygen. A probe sinking easily at exactly that height, with firm wood above, is the signature of that process.

Wood-destroying fungi require moisture content above roughly 20 percent, oxygen and moderate temperature. Ground contact provides moisture while air remains available, making the ground line the highest-risk location on any wood post or column.

Decay Lives at the Waterline — where wet soil meets open air. Fully buried starves it of oxygen; fully exposed starves it of water.

Check whether the post is a pressure-treated species rated for ground contact and whether it bears on a proper footing or directly on soil. Both are commonly tested alongside the decay itself.

• •Dismissing a soft probe result as excessive force
• •Reading ground-line decay as poor lumber quality
• •Assuming any wood deterioration near soil is insect activity