Soil science

The ninth entry is la réserve utile en eau, and the adjective in the middle of it is one English cannot copy. « Useful » says that a thing comes in handy. Utile, inside a technical compound, says something narrower and harder: the part of a quantity that can actually be got at and put to work. That is precisely the measurement here — not all the water a soil holds, but the share a root can still pull out, between the wet limit and the point where the plant gives up. The same adjective does the same job right across the language. La charge utile of a lorry or a rocket is its payload, what is left once the vehicle itself has been weighed. La surface utile of a flat is the floor you can stand on. La puissance utile of a motor is the power that leaves the shaft, not the power drawn. And en temps utile is in good time, while there is still time for the thing to be of use. « Useful reserve » would be almost right and exactly wrong: utile names what is reachable, not what is welcome.