1,800,000 bushels is about 0.00000002 times as big as Halley's Comet
In other words, it's 0.0000000160 times the size of Halley's Comet, and the size of Halley's Comet is 63,000,000 times that amount.
(Comet Halley, officially "1P/Halley") (volume calculated based on 2005 dimensions)
Halley's Comet, the famous comet visible to observers on Earth approximately every 76 years, has a peanut- or potato-shaped nucleus made of rock, dust, ice, and various frozen gasses. The nucleus measures approximately 15 km by 8 km by 8 km, which yields a calculated volume of 110,000,000,000,000.00000000000000000 bushels. The coma of the Comet — the trail of sublimated gasses that give the Comet its visibility — may be up to 100,000 km in length.