What are the subtle differences in meaning and usage between poverty and poorness?
I'll tell you one big difference: the frequency :-)
At Google Books:
About 938,000 results
About 815 results
"Poorness" is quite rare, by comparison.
Also, that "poverty" is used only in financial contexts is not true. Better dictionaries show:
1 a : lack or relative lack of money or material possessions : privation, want
transition from a life of almost the greatest pomp and circumstance … to one just, but only just, above the line of genteel poverty — Geoffrey Gorer>
in poverty, morality and even a touch of happiness was possible, never in destitution — R. A. Schermerhorn>
had roamed the picturesque poor quarters … but this ugly, barren poverty on the Spanish land was his first view of some men's helpless fate — Janet Flanner>
b : renunciation as a member of a religious order of the right as an individual to own, to receive by inheritance or gift, or to dispose of property
2 a : meagerness of supply : scarcity, dearth
biographer … is necessarily embarrassed by the poverty of personal information preserved — John Loftis>
the cold thin atmosphere of his work was due … to a poverty of ideas and sensuous imagery — V. L. Parrington>
b : poorness in kind or quality : inferiority
cannot hide poverty of form under an opulent mask of orchestral color — Hunter Mead>
c : lack of desirable elements or attributes : deficiency
the … poverty of North and Northeastern Africa in river-producing power — Samuel Haughton>
suffered … from a certain poverty in our English critical vocabulary — Irving Babbitt>
3 a : debility due to malnutrition : feebleness, emaciation
produce insufficient fodder … and one or two ranches suffered quite heavy losses from poverty — Report: Northern Rhodesia Veterinary Department>
b : lack of fertility
poverty of the soil>
Merriam-Webster Unabridged Dictionary
Poverty is the word for the concept of "poorness." There is nothing that grammatically prevents "poorness" from being used. "Poorness" is technically correct, but inelegant. Inelegance is something of a flexible concept, but it includes words like "cactuses," "angryness," etc. A creative writer might use poorness in place of poverty to make the reader involuntarily think twice about it, but it isn't a word normally encountered.
Caveat: this is true when talking about "poor" as it relates to scarcity. When used to describe low quality (i.e. poor test scores), "poverty" no longer applies. -Credit for pointing this out: Janus Bahs Jacquet.
Poverty : One meaning :
The state of having little or no money and few or no material possessions.
Poorness : 4 meanings :
The state of having little or no money and few or no material possessions
Less than adequate
The quality of being meagre
The quality of being poorly made or maintained
Summary Of Difference : Poverty is used in financial contexts, while Poorness is used in wider contexts.
Poverty is French/Latin pauvreté from Latin paupertas/tatis. Poor and poorness is the English development from French pauvre, adj, and Latin pauper, pauperis, adj. So both nouns belong to the same word family and basically mean the same. Poorness can be used figuratively as in the poorness of his style (speaking of a writer).
Etymonline's article on poor, adj, is very interesting. http://www.etymonline.com/index.php?allowed_in_frame=0&search=poor&searchmode=none
To me, "poorness" relates to a current state of affairs under which a person has low or no income, which may be by choice (such as farming or a business which produces limited income, or choosing to live off of limited savings for a period as a break, such as backpacking through Europe), or a temporary status such as between jobs or while a college student, whereas "poverty" relates to a prolonged, non-voluntary condition from which one cannot easily extract him/herself or be easily extracted.