Snow—especially a late snow in the spring—has long been called the “poor man’s manure” or “poor man’s fertilizer.” The snow contains nitrogen that helps plant growth.
   
“The snow is emphatically and truly called the poor man’s manure” was cited in print in 1810.
 
“Snow is the poor man’s fertilizer” has been cited since at least 1869
 
   
Google Books
A Guide to the Wilderness:
Or, The History of the First Settlement in the Western Counties of New York
By William Cooper and James Fenimore Cooper
Dublin, NY: Printed by Gilbert & Hodges
1810
Pg. 24:
The snow is emphatically and truly called the poor man’s manure. The ground is seldom frozen when the snow is deep, and vegetation is nourished and protected by its covering. The wheat takes a much stronger and deeper root than where the earth is drenched during the winter by heavy rains.
     
8 May 1840, Maine Cultivator and Weekly Gazette (Hallowell, ME), pg. 2, col. 3:
Poor Man’s Manure.
The earth, hereabouts, was well dressed by a generous deposit from the clouds of poor man’s manure. The snow fell as the finale of a protracted N. E. rain storm.
 
Google Books
May 1842, The Farmer’s Magazine, “Prepared Gypsum as a Manure” by the Rev. T. Skipworth, pg. 342, col. 1:
Adverting, indeed, to the cheapness of the prepared gypsum as combined with its efficiency, it may not inappropriately be termed the poor man’s manure; and I wish it no other title.
 
13 April 1844, The Indiana Palladium (Vevay, IN), pg. 1, col. 5:
It is an ancient maxim that, snows in the spring are the poor man’s manure; and the notion is quite prevalent that there is a virtue in snow that cannot be found in water.
   
16 January 1845, Pittsfield (MA) Sun, pg. 4, col. 1: 
“SNOW IS THE POOR MAN’S MANURE.”—This is an old saying, and the following extract from one of Prof. Johnson’s lectures, shows that it is not destitute of truth: ...
 
Google Books
June 1854, Harper’s New Monthly Magazine, “Poor Man’s Pudding and Rich Man’s Crumbs” by Herman Melville, pg. 96, col. 1:
“So, you see, the winter’s snow Itself, is beneficent under the pretense of forst—a sort of gruff philanthropist—actually warming the earth, which afterward is to be fertilisingly moistened by these gentle flakes in March.”

“I like to hear you talk, dear Blandmour; and, guided by your benevolent heart, can only wish to poor Coulter plenty of this ‘Poor Man’s Manure.’”
 
Old Fulton NY Post Cards
19 March 1881, Jacksonville (AL) Republican, “Agriculture,” pg. 4, col. 2:
A Scotch proverb says, “snow is the poor man’s manure.”
 
Twitter
Mary-Ann Hornbaker
‏@mahornbaker
@metmikewcvb Spring snowfall, like tomorrow’s, we New Englanders call poor man’s manure, but only when it covers soil not ice.
4:57 PM - 19 Mar 2015