Commentary for Berakhot 49:23

מיתיבי מי רגלים כל זמן שמטפיחין אסורין נבלעו או יבשו מותרים מאי לאו נבלעו דומיא דיבשו מה יבשו דאין רשומן ניכר אף נבלעו דאין רשומן ניכר הא רשומן ניכר אסור אע"ג דאין מטפיחין

It is quoted in objection : With urine, so long as the ground is wet enough to moisten it is forbidden ; but if it has been absorbed [in the soil] or dried up, it is permitted. But is not soaking into the soil like drying up by the sun? [No ;] for as when it dries up, no trace can be recognised, so when it has penetrated into the soil, there must be no trace visible; but if there be such a trace, it is forbidden [to read the Shema' thereby] even though the ground be not sufficiently wet to moisten !

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot