Etymology of "plough back" meaning to reinvest

Plough

A related phrase is to plough money into an investment.

The OED has to plough into meaning "To embed or bury in soil, etc.; fig. to invest (money, esp. a large amount) into an enterprise or business", their first three uses are:

1854 H. D. Thoreau Walden 8 The better part of the man is soon ploughed into the soil for compost.
1895 B. Sedgwick in Westm. Gaz. 12 Sept. 4/3 He ploughed his capital into the land, and it never came out.
1945 N.Y. Times 16 Jan. 12/1 L. P. Sharples..declared that it was important for the municipalities to plow money into airparks and small strips near the center of their communities.

Ploughing back is simply re-investing in a similar way that ploughing was investing in the first place.

Here's a plowed money from The Financial World of August 29, 1921 (p.328):

Plough back

The OED has a literal use of plough back from 1864 ("To plough plant material into (an area of ground) to enrich the soil; to plough in (plant material)") from 1864 and a figurative one ("To invest (income or profit) back into the enterprise producing it") from 1912:

1912 N.Y. Times 10 July 12/3 The management did not embrace the first opportunity to increase the payment on its shares. Instead, the surplus was plowed back into the property, as railroad men say.

Plough back earnings/profits ( or US spelling is plow back):

• to plough back profits into the company to invest the profits in the business (and not pay them out as dividends to the shareholders) by using them to buy new equipment or to create new products

As noted the origin of the expression is mutated from the agriculture world, According to Ngram the expression was used mainly from the 30s ( apparently following the Great Depression), but its usage has decreased from the 80s/90s probably replaced by the more common 'reinvest'.