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):
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.